Influence of the Addition of Height-Adjustable Worktables on Airborne Particle Concentration in a Cleanroom According to ISO 14644-1

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This study provides practical recommendations for the integration of height-adjustable worktables in cleanrooms. It demonstrates that adjustable worktables can be implemented without compromising the cleanroom classification according to ISO 14644, provided that the layout of the supply air ducts and the positioning of the tables are properly considered. The results support ergonomic workstation design in pharmaceutical and medical device production environments while maintaining full compliance with cleanroom standards.

Abstract

Cleanrooms are essential environments for the production of sterile pharmaceuticals and medical devices, where airflow stability and contamination control are critical. In this study, the influence of adjustable worktable height on air cleanliness was examined to determine whether height variation affects unidirectional airflow and particle concentration. Measurements were conducted under laboratory conditions in an ISO Class 6 cleanroom and in an industrial ISO Class 8 production line. Particle concentrations were recorded at table heights between 70 and 120 cm using a calibrated particle counter in accordance with ISO 14644-1 and ISO 21501-4. The measured data were evaluated against the classification limits defined in ISO 14644-1. Across all height levels, particle concentrations remained well below the permissible thresholds. In the ISO Class 6 cleanroom, measurement height showed a statistically significant effect on particles >5 μm, though this represented only a small contribution to the overall variance. In contrast, no significant height effect was observed in the ISO Class 8 cleanroom. Observed local differences were attributed to airflow distribution and the placement of air supply in-/outlets rather than table height. These results confirm that height-adjustable worktables can be implemented without affecting cleanroom classification, provided that uniform FFU placement and furniture positioning are considered during design and qualification.

1. Introduction

Cleanrooms are essential controlled environments for manufacturing sterile pharmaceuticals, medical devices, and microelectronic components, where airflow stability and contamination control directly determine product quality and patient safety [1,2,3,4,5]. The fundamental principle underlying cleanroom contamination control is the continuous removal of airborne particles through carefully designed ventilation systems that establish either unidirectional laminar flow or turbulent mixing airflow patterns capable of diluting and exhausting contamination before particles can settle on critical surfaces or compromise sterile products [6,7].

The effectiveness of these airflow systems depends critically not only on filtration efficiency and air change rates but also on the spatial configuration of furniture, equipment, and work surfaces within the controlled environment. Any physical obstacle positioned within a cleanroom airflow field—including worktables, process equipment, storage cabinets, or personnel—disrupts flow streamlines and creates localized zones of altered velocity, turbulence, and particle transport [8,9,10]. Whyte’s foundational research demonstrated through experimental measurements and smoke visualization that furniture placement and geometry significantly influence particle distribution patterns, with improperly positioned equipment creating “dead zones” characterized by reduced air velocity, flow recirculation, and elevated particle residence times [6,7]. Subsequent computational and experimental studies confirmed that obstacle geometry, surface characteristics, and spatial arrangement relative to supply air inlets determine local airflow quality [11,12,13].

When airflow encounters an obstacle, flow separation can occur at the obstacle’s leading edge, and downstream wake regions are formed, characterized by reduced velocity, vortex shedding, and potentially reverse flow directions—all conditions that impair particle removal efficiency [12,14]. For cleanrooms operating at typical face velocities of 0.35–0.45 m/s (unidirectional flow) or with turbulent mixing ventilation, even modest obstacles such as worktables (height 70–120 cm, depth 60–80 cm) can substantially perturb flow patterns across multiple square meters of cleanroom space [15,16].

Given this sensitivity of contamination control performance to furniture geometry and positioning, any modification to cleanroom workstation configuration—including the introduction of height-adjustable worktables—represents a potential risk factor that must be evaluated for compliance with ISO 14644-1 classification requirements [1]. ISO 14644-5:2025 explicitly mandates that all equipment and furniture be documented within a facility’s Operational Control Program (OCP) and that modifications potentially affecting airflow be assessed for impact on air cleanliness [5].

Modern occupational health research has established compelling evidence that height-adjustable worktables reduce musculoskeletal disorders, improve worker comfort, and enhance productivity across diverse manufacturing environments [17,18,19]. The European standard DIN EN 527-1:2011 specifies that ergonomic worktables should accommodate height adjustment within the range of 700–1100 mm to enable both seated and standing work postures [20]. Pharmaceutical and medical device manufacturers face increasing pressure to implement such ergonomic improvements to address rising rates of work-related musculoskeletal disorders among production personnel who perform repetitive precision tasks [17,19].

However, the integration of height-adjustable furniture into cleanroom environments raises critical questions that have not been adequately addressed in the scientific literature. When a worktable is repositioned vertically—for example, adjusted from a 700 mm height (seated work configuration) to an 1100 mm height (standing work configuration)—does this vertical displacement alter particle removal efficiency? Does height adjustment create transient flow instabilities or persistent wake structures that compromise air cleanliness classification? Must cleanrooms undergo complete requalification at every height setting within the adjustment range to demonstrate continued ISO 14644-1 compliance?

Despite extensive research characterizing the influence of fixed furniture geometry on cleanroom performance [6,7,11,13,21,22,23], the specific question of height-adjustable worktable effects on airborne particle concentration remains experimentally unexplored in the peer-reviewed literature. Computational fluid dynamics (CFD) investigations have suggested that moderate changes in obstacle vertical position may not substantially disrupt flow patterns, provided that overall airflow rate and supply air momentum flux remain constant [24,25,26,27]. However, these simulation-based predictions require experimental validation under realistic operational conditions with active personnel, process equipment, and the complex three-dimensional geometry characteristic of actual pharmaceutical manufacturing environments.

The absence of experimental data creates significant practical challenges for pharmaceutical manufacturers considering implementation of height-adjustable furniture. Without validated evidence, facility managers face uncertainty about regulatory compliance: Will height adjustment compromise cleanroom classification? Must requalification be performed at every height setting? Conservative interpretation of ISO 14644-5 requirements might suggest exhaustive testing at all possible height configurations—a potentially burdensome protocol that could discourage adoption of beneficial ergonomic improvements [5].

Furthermore, the existing cleanroom literature has focused predominantly on contamination control in idealized empty-room conditions or with fixed furniture arrangements, providing limited guidance for dynamic operational scenarios where workstation configurations change regularly [8,9,28]. Real pharmaceutical production environments require flexibility to accommodate different operators with varying anthropometric characteristics, diverse tasks with distinct ergonomic demands, and evolving manufacturing processes with changing equipment requirements. Understanding whether height adjustability can be implemented without triggering extensive requalification requirements is therefore essential for enabling adaptive, human-centered cleanroom design while maintaining product quality assurance.

From a fundamental scientific perspective, investigating height effects on particle distribution addresses important questions about vertical stratification in cleanroom environments. While gravitational settling creates well-known vertical concentration gradients for particles > 5 µm in quiescent air, the extent to which such stratification manifests in cleanrooms with active ventilation remains incompletely characterized [15,29,30]. For submicron particles (0.3–1.0 µm) that dominate pharmaceutical contamination concerns, convective transport by airflow should theoretically overwhelm gravitational effects [29,31], suggesting height independence of concentration distributions. However, experimental validation of this theoretical prediction across the full ergonomic height range (70–120 cm) under operational conditions has not been reported.

Additionally, different cleanroom classes employ distinct ventilation strategies with potentially different sensitivities to furniture height variations. ISO Class 6 environments typically utilize unidirectional laminar flow with ceiling-mounted High-Efficiency Particulate Air (HEPA) filter arrays creating high-velocity downward airflow, while ISO Class 8 environments often employ turbulent mixing ventilation with lower velocities and less directional control [1,9]. Whether height effects manifest differently across these airflow regimes—and consequently whether height-adjustable furniture suitability depends on cleanroom class and ventilation type—represents an important consideration for generalizing research findings across the pharmaceutical manufacturing sector.

Research Objectives

This experimental investigation addresses these knowledge gaps through systematic measurement of airborne particle concentration at multiple vertical positions in cleanrooms classified according to ISO 14644-1. The study employs a dual-facility approach combining laboratory cleanroom measurements (ISO Class 6 with unidirectional flow) with industrial production cleanroom measurements (ISO Class 8 with turbulent mixing ventilation) to evaluate height effects across different airflow regimes and operational contexts.

The first objective is to quantify vertical particle distribution. Airborne particle concentration (0.3–5.0 µm) is characterized at three sampling heights (80, 100, 120 cm) representing seated, semi-standing, and standing worker breathing zones in an ISO Class 6 unidirectional flow cleanroom. This assesses whether particle exposure varies systematically with worker posture and breathing zone elevation.

The second objective is to evaluate height-adjustable table effects. Particle concentration is measured at the work surface elevation of height-adjustable tables positioned at five heights (70, 80, 90, 100, 110 cm), covering the full ergonomic range in an ISO Class 8 production cleanroom with turbulent mixing ventilation. This determines whether table repositioning affects product exposure zone contamination under different airflow conditions than the ISO Class 6 facility.

To ensure statistical rigor, analysis of variance (ANOVA) is applied to assess the statistical significance of height-dependent effects and to distinguish systematic trends from spatial heterogeneity inherent in cleanroom particle distributions. For mechanistic interpretation, observed concentration patterns are related to fundamental particle transport physics (gravitational settling, convective transport, turbulent diffusion) and airflow characteristics (supply air configuration, FFU placement, velocity fields).

Finally, the study provides practical validation guidance by offering evidence-based recommendations for cleanroom qualification protocols when implementing height-adjustable furniture, including whether representative height sampling can substitute for exhaustive testing. It also confirms regulatory compliance by demonstrating whether particle concentrations at all tested heights remain within ISO 14644-1 classification limits, establishing feasibility of height-adjustable furniture for pharmaceutical and medical device manufacturing under GMP requirements [32].

Based on theoretical analysis of particle transport in ventilated cleanroom systems [14,15,29], we hypothesized that worktable height adjustment within the ergonomic range of 70–120 cm would not significantly affect airborne particle concentration for particle sizes ≤ 1.0 μm, where convective transport dominates over gravitational settling. We further hypothesized that any height-dependent effects observed for larger particles (≥5.0 µm) would be attributable to gravitational settling over different vertical transport distances but would not compromise ISO classification compliance given the substantial safety margins built into regulatory limits.

The findings reported here confirm these hypotheses and demonstrate that height-adjustable worktables can be safely integrated into pharmaceutical cleanrooms within the tested height range without compromising air cleanliness classification, regardless of whether unidirectional or turbulent mixing ventilation is employed. These results enable evidence-based integration of ergonomic design principles with contamination control requirements, supporting simultaneous achievement of worker well-being and product quality assurance objectives. Additionally, the experimental data presented serve as validation benchmarks for ongoing computational fluid dynamics modeling efforts (Jaberi et al., in preparation), contributing to the development of predictive tools for cleanroom design optimization incorporating height-adjustable furniture.

2. Materials and Methods

2.1. Experimental Design and Measurement Protocol

This study employed a dual-facility experimental approach to investigate airborne particle concentration at different vertical positions in pharmaceutical cleanrooms. The measurement strategy was designed to address complementary research questions: (1) How does vertical position above a fixed worktable affect particle exposure (personnel breathing zone assessment)? (2) How does adjusting table height affect particle concentration at the worktable (direct height-adjustable furniture validation)? Measurements were conducted under operational conditions in two facilities with different cleanroom ISO classifications [1] to evaluate height effects across different cleanroom environments.

2.1.1. ISO Class 6 Cleanroom Facility

• Facility Configuration

The ISO Class 6 cleanroom in Gesellschaft für Produktionshygiene und Sterilitätssicherung (GfPS), Aachen, Germany (109 m² floor area, 2.8 m ceiling height), operates with unidirectional laminar flow ventilation at face velocities of 0.35–0.45 m/s. The facility employs a hybrid air supply system combining ceiling-mounted general air inlets with supplementary fan filter unit (FFU) modules. Due to architectural constraints in the original facility design, FFU modules are positioned above critical work areas but not uniformly distributed across all work locations. This configuration is representative of many pharmaceutical cleanrooms where retrofit installations or facility constraints prevent ideal uniform air supply distribution. Worktables are fixed at 80 cm height and cannot be adjusted without facility reconstruction, as they are integrated with electrical services, process gas lines, and equipment mounting systems.

• Measurement Strategy and Rationale

Rather than viewing the fixed-height tables as a limitation, the experimental protocol was designed to investigate a scientifically relevant question: How does sampling height—corresponding to different worker breathing zone elevations—affect measured particle concentration above a worktable? Cleanroom personnel adopt different working postures depending on task requirements: seated work (breathing zone ~80 cm above floor) for precision assembly and sterile filling, semi-standing or leaning work (breathing zone ~100 cm) for equipment operation and material preparation, and standing work (breathing zone ~120 cm) for material handling and visual inspection [17,18,19,20]. Understanding particle exposure at these different breathing zone elevations informs occupational health risk assessment and ergonomic workstation design.

From a fluid mechanics perspective, measuring at different heights above a fixed surface provides equivalent information to measuring at a fixed height while varying surface elevation, since the critical parameter governing particle transport is the vertical distance through which particles must travel in the airflow field [14,27].

• Sampling Configuration

Measurements were conducted at 18 distinct sampling locations distributed throughout the cleanroom, encompassing work areas with varying proximity to FFU modules and general air inlets (Figure 1). At each location, particle concentrations were measured at three heights above the floor: 80 cm, 100 cm, and 120 cm. The worktable remained at 80 cm throughout all measurements.

The sample size and design included 18 sampling positions and three sampling heights (80, 100, and 120 cm above the floor), with 80 cm defined as the starting height corresponding to the worktable level. This resulted in a total of n = 54 measurements per particle size (18 locations × 3 heights). All measurements were conducted under in-operation conditions according to ISO 14644-2 [2], with active personnel present and ongoing work activities.

2.1.2. ISO Class 8 Cleanroom Facility

• Facility Configuration

The ISO Class 8 production cleanroom at Vygon Germany GmbH, Aachen, Germany, operates with turbulent mixing ventilation supplied through ceiling-mounted air diffusers. The air supply system provides more uniform and homogeneous airflow distribution across work zones compared to the ISO Class 6 facility, as the ventilation system was designed during initial facility construction without the architectural constraints that necessitated the hybrid system in the GfPS facility. The facility is equipped with three commercially available height-adjustable worktables conforming to ergonomic standards specified in DIN EN 527-1:2011 [20] (adjustment range 70–110 cm for seated and standing work postures).

• Measurement Strategy and Rationale

This facility enabled direct experimental evaluation of the practical implementation question: Do height-adjustable tables compromise cleanroom classification under actual pharmaceutical production conditions? To assess this, tables were systematically repositioned through their full ergonomic adjustment range, and particle concentrations were measured at the worktable elevation where pharmaceutical products are handled. This approach directly quantifies contamination risk in the critical product exposure zone across the height range that would be used operationally.

• Sampling Configuration

Three height-adjustable worktables at different locations within the production cleanroom were systematically adjusted to five heights spanning the ergonomic range: 70 cm, 80 cm, 90 cm, 100 cm, and 110 cm. At each height setting, the sampling probe was positioned at the table surface elevation to measure particle concentration in the work zone (Figure 2). One of the three worktables could not be lowered to the 70 cm minimum height due to electrical service ducts positioned beneath the worktable at that location, reflecting a real-world facility constraint where building services infrastructure limits the practical adjustment range.

Sample size and design comprised three worktables placed at different cleanroom locations and five tested table heights (70, 80, 90, 100, and 110 cm), with 70 cm defined as the starting height, representing the lowest ergonomic level. The sample size was set to n = 6 measurements at 70 cm (conducted on two tables) and n = 9 measurements at each of the other heights (conducted on all three tables), resulting in a total of 42 measurements per particle size. All measurements were performed under in-operation conditions in accordance with ISO 14644-2 [2], during active pharmaceutical production.

2.2. Particle Counting Instrumentation and Sampling Setup

2.2.1. Instrument Specifications and Measurement Uncertainty

All measurements in both facilities employed a Lighthouse Worldwide Solutions Solair 3100 portable optical particle counter (Fremont, CA, USA), provided by the GfPS and operated in accordance with ISO 21501-4:2018 requirements for light scattering airborne particle counters [34]. The instrument specifications and measurement characteristics are as follows:

Technical Specifications:

• Detection principle: 90° light scattering with Extreme Life Laser Diode technology;
• Flow rate: 28.3 L/min (1.0 CFM);
• Particle size channels: 0.3, 0.5, 0.7, 1.0, 2.0, 3.0, 5.0, 7.0, 10.0, 25.0 µm (six channels standard, with analysis focused on 0.3, 0.5, 1.0, and 5.0 µm channels);
• Counting efficiency: 50% at 0.3 µm and 100% for particles >0.45 µm (per ISO 21501-4 [34]);
• Zero count level: <1 count/5 min (per ISO 21501-4 [34]);
• Concentration limit: 500,000 particles/ft³ (14,158,423 particles/m³) at 5% coincidence loss;
• Calibration: NIST-traceable calibration performed annually by manufacturer, with calibration certificate verified before measurement campaign;
• Data output: integrated thermal printer for immediate on-site documentation;
• Detection limit: approximately 354 particles/m³ for 1 min sampling duration (equivalent to 1 particle per 2.83 L sampled air volume).

• Measurement Uncertainty and Error Analysis:

The measurement uncertainties associated with the Solair 3100 particle counter are derived from multiple sources, which can be categorized into instrumental errors and sampling errors.

• Instrumental Measurement Errors:

According to the manufacturer’s specifications and ISO 21501-4 requirements [34,35], the Solair 3100 exhibits several types of measurement uncertainties. The counting efficiency uncertainty arises from the 50% counting efficiency at 0.3 µm, which indicates that particles at this threshold size have an equal probability of detection or non-detection, introducing a fundamental sizing uncertainty near the lower detection limit. For particles larger than 0.45 µm, the 100% counting efficiency specification indicates that sizing uncertainty is primarily determined by optical system resolution rather than detection probability.

The concentration measurement precision derives from the coincidence loss specification of less than 5% at concentrations of up to 500,000 particles/ft³, indicating that particle count accuracy remains within ±5% of true concentration at typical cleanroom particle levels. All measured concentrations in this study were well below the coincidence loss threshold, ensuring that coincidence losses did not significantly affect measurement accuracy.

The zero count stability specification of less than 1 count per 5 min provides a baseline false count rate of approximately 0.2 counts/minute, which is negligible compared to the measured concentrations in both facilities. This low false count rate confirms that background electronic noise did not contribute meaningfully to measurement uncertainty.

2.

Spatial and Temporal Sampling Variability:

Beyond instrumental errors, particle concentration measurements in operational cleanrooms are subject to substantial spatial and temporal variability arising from several sources. Non-uniform FFU coverage creates spatial concentration gradients, with locations beneath FFU modules exhibiting lower concentrations than peripheral zones. This spatial heterogeneity contributes to large standard deviations, with coefficients of variation ranging from 48% to 191% in observed measurements.

Episodic particle generation events represent another significant source of variability. Personnel activity, equipment operation, and material handling generate transient particle concentration spikes lasting 30–120 s [22,36]. The 1 min sampling duration employed in this study captures instantaneous concentration states that may include contributions from recent particle generation events, contributing to measurement-to-measurement variability.

Airflow perturbations from local equipment exhaust systems create horizontal airflow components that temporarily elevate particle concentrations when worktables intersect exhaust plumes, contributing to non-monotonic concentration patterns and outliers in operational measurements.

3.

Combined Measurement Uncertainty:

The overall measurement uncertainty can be estimated by combining instrumental and sampling sources. For the Solair 3100 under the conditions employed in this study, instrumental precision stands at ±5% (from coincidence loss specification), while spatial sampling variability ranges from ±50% to ±190% (from observed coefficients of variation). The combined uncertainty is therefore dominated by spatial/temporal variability rather than instrumental precision.

This analysis demonstrates that the observed variability in particle concentrations (reflected in large standard deviations) is primarily attributable to genuine spatial heterogeneity in cleanroom airflow fields and operational dynamics rather than instrumental measurement error. The Solair 3100′s measurement precision (±5%) is more than adequate to detect systematic height-related concentration differences if such effects were present at magnitudes exceeding the inherent spatial variability.

2.2.2. Physical Sampling Setup

The particle counter was configured with a flexible sampling tube (approximately 1.5 m length, 6 mm inner diameter) connected to an isokinetic inlet nozzle. The isokinetic inlet is designed to sample air at the same velocity as the surrounding airflow, minimizing sampling bias that can occur when the sampling velocity differs from ambient air velocity [34]. This is particularly important in unidirectional laminar flow cleanrooms where maintaining representative sampling conditions is critical for accurate particle measurement.

The inlet nozzle was mounted on an adjustable vertical stand (a tripod stand with a telescoping column) equipped with a height measurement scale calibrated in 1 cm increments. This setup enabled precise height positioning by allowing for accurate adjustment and verification of the sampling height with ±1 cm precision, while also ensuring stable probe positioning throughout the 3 min sampling period at each location. In addition, the long sampling tube allowed the particle counter body to be placed away from the sampling point, reducing operator-generated particle interference. The isokinetic inlet was oriented upward to sample from the downward unidirectional flow field under conditions approaching isokinetic sampling.

2.3. Measurement Protocol and Data Collection

For the ISO Class 6 (GfPS) measurements, the sampling probe was first positioned at the starting height of 80 cm, corresponding to the worktable level. The measurement sequence began at Location 1 with a 10 min stabilization period, followed by three 1 min samples separated by 15 s pauses, and the results were recorded via printout. The probe was then moved to Location 2, where the same three 1 min samples with 15 s pauses were taken and recorded. This procedure was repeated sequentially for all 18 locations at the 80 cm height.

After completing the 80 cm round, the probe height was adjusted to 100 cm, and the same sequence was repeated across all locations. The measurements returned to Location 1, again using a 10 min stabilization period followed by three 1 min samples with 15 s pauses, before continuing through all 18 locations in the same order at 100 cm. Once the 100 cm round was finished, the probe was repositioned to 120 cm and the identical procedure was applied again, starting at Location 1 and continuing sequentially through all 18 locations at 120 cm.

For the ISO Class 8 (Vygon) measurements, all three height-adjustable tables were initially set to 70 cm, representing the lowest ergonomic position, and the sampling probe was positioned at the table surface level, i.e., 70 cm above the floor. Measurements started at

Table 1. Descriptive statistics for particle concentrations (particles/m³) at different sampling heights in ISO Class 6 cleanroom (n = 18 per height).

Particle Size	80 cm	100 cm	120 cm
≥0.3 µm	3119 ± 5950	3591 ± 5951	1318 ± 1723
≥0.5 µm	1868 ± 3600	1475 ± 1762	709 ± 893
≥1.0 µm	974 ± 1905	538 ± 433	283 ± 290
≥5.0 µm	50 ± 68	33 ± 27	12 ± 15

with a 10 min stabilization period followed by three 1 min samples with 15 s pauses, and the data were recorded via printout. The second table was then measured using the same sampling sequence and recorded in the same way. Third Table could not be measured at 70 cm because it could not reach this height due to electrical ducts, resulting in a total sample size of n = 6 measurements at 70 cm.

Afterwards, all tables were raised to 80 cm, and the probe was repositioned to the new table surface level. At 80 cm, all three tables were measured, resulting in n = 9 measurements at this height. The same approach was then continued for the subsequent table heights of 90 cm, 100 cm, and 110 cm, with the tables and probe adjusted stepwise and all three tables measured at each height, yielding n = 9 measurements for each of these heights.

2.4. Statistical Analysis

For statistical evaluation, one-way ANOVA was used to test whether sampling height (ISO Class 6) or table height (ISO Class 8) significantly influenced particle concentrations for each particle size class. The null hypothesis (H₀) assumed no differences in mean concentrations between height levels, and significance was assessed at α = 0.05. The ISO Class 6 dataset followed a balanced design with n = 18 measurements per height (80, 100, 120 cm; N = 54), whereas the ISO Class 8 dataset was unbalanced with n = 6–9 per height (70–110 cm; N = 42), and ANOVA was therefore performed using Type I sums of squares with adjustment for unequal sample sizes. When ANOVA results were significant (p < 0.05), pairwise comparisons were conducted using independent-samples t-tests with Bonferroni correction (ISO Class 6: α = 0.0167 for three comparisons). For significant effects, eta-squared (η²) was calculated to quantify effect size and practical relevance, using Cohen’s thresholds (0.01 small, 0.06 medium, 0.14 large) [37]. Analyses were performed in Microsoft Excel with manual verification.

3. Results

3.1. ISO Class 6 Cleanroom (GfPS): Vertical Sampling Height Effects

3.1.1. Descriptive Statistics

Table 1 presents mean particle concentrations and standard deviations for the three sampling heights (80, 100, 120 cm) measured at 18 locations throughout the ISO Class 6 cleanroom. For all particle size channels, mean concentrations decreased with increasing sampling height, with the most pronounced reduction observed for larger particles (≥5.0 µm).

The descriptive statistics reveal a general downward trend in particle concentrations with increasing height, particularly for ≥5.0 µm particles (50 → 33 → 12 particles/m³, representing a 76% reduction from 80 to 120 cm). This trend is consistent with gravitational settling theory, where larger, heavier particles experience greater downward terminal velocities and are progressively depleted from the airstream as vertical distance increases. However, the standard deviations are substantial, often exceeding the mean values, particularly for smaller particle sizes at lower heights (e.g., ≥0.3 µm at 80 cm: 3119 ± 5950 particles/m³, representing 191% relative standard deviation). This high variability reflects the spatial heterogeneity in the facility’s airflow field, where non-uniform FFU coverage creates zones of varying air velocity and particle removal efficiency.

3.1.2. Distribution of Particle Concentrations

Boxplot distributions for particle concentrations at the three sampling heights are presented in Appendix A (Figure A1, Figure A2, Figure A3 and Figure A4), illustrating both central tendencies and variability across the 18 measurement locations. The boxplots reveal several important patterns. First, the median concentrations (central lines in boxes) generally decrease with increasing height, consistent with the mean values in Table 1. Second, the interquartile ranges (box heights) are large relative to median values, indicating substantial spatial variability across the 18 sampling locations. Third, statistical outliers are present primarily at the 80 cm sampling height for particles ≥0.3–1.0 µm, reflecting measurement locations where particle concentrations substantially exceeded typical values.

3.1.3. Interpretation of Zero Counts and Outliers

Zero particle counts are visible in the boxplots (Appendix A), particularly at 120 cm for ≥5.0 µm particles (9 of 18 locations recorded zero counts). These zeros reflect the combined effects of gravitational settling (which depletes large particles from the upper regions of the occupied zone) and measurement conditions during data acquisition. Specifically, the ISO Class 6 cleanroom was occupied by approximately six personnel during measurements, who were distributed across the 109 m² facility performing their assigned tasks. Because measurements were conducted at 18 locations sequentially, not all locations had active personnel presence during the 1 min sampling period. In zones without immediate personnel activity, large particle concentrations naturally approached or fell below the instrument detection limit (~35 particles/m³ based on 28.3 L sample volume), resulting in recorded zero counts. This represents realistic operational conditions rather than measurement error, as cleanroom personnel were instructed to maintain their normal work activities to preserve the authenticity of the “in-operation” state assessment.

The statistical outliers observed at 80 cm sampling height warrant specific explanation and must be interpreted in the context of the facility’s airflow architecture. For ≥0.3, ≥0.5, and ≥1.0 μm particle sizes, outliers were identified at measurement locations 8, 11, and 12 (visible in Appendix A, Figure A1, Figure A2 and Figure A3). These outliers were retained in the statistical analysis without removal, as they represent genuine measurement conditions arising from the facility’s non-uniform FFU coverage rather than experimental errors or equipment malfunction. These elevated concentrations reflect two facility-specific phenomena. First, locations 11 and 12 are positioned in a zone with reduced FFU coverage (as documented later in Figure 3), where worktables are not positioned directly beneath FFU modules. In these zones, particle removal relies primarily on general ceiling-mounted air inlets rather than high-velocity unidirectional FFU flow, resulting in elevated baseline concentrations. Second, outlier measurements at these locations occurred during periods when multiple coworkers were simultaneously present at or near the sampling table, generating transient elevated particle concentrations through activities such as material handling, equipment operation, or personnel movement. The combination of reduced FFU coverage and episodic personnel activity explains why these locations exhibited concentrations substantially above the third quartile, meeting the statistical definition of outliers. Importantly, even these elevated concentrations remained well within ISO Class 6 classification limits, demonstrating that the facility maintains compliant particle levels despite spatial heterogeneity in airflow coverage.

Following standard statistical practice, outliers should only be excluded when they arise from measurement errors, equipment malfunction, or violations of experimental protocol [38]. In the present study, all outlier measurements were verified as accurate readings obtained under proper operating conditions. The elevated concentrations at Locations 11 and 12 are reproducible consequences of the facility’s architectural constraints, which necessitated a hybrid ventilation system combining FFU modules with general ceiling inlets (Figure 3). Because these outliers reflect real physical phenomena—specifically, reduced particle removal efficiency in zones with suboptimal airflow coverage—their exclusion would artificially minimize the observed spatial heterogeneity and misrepresent the actual performance variability present in pharmaceutical cleanrooms with non-ideal ventilation configurations. Retaining outliers in the analysis therefore provides a more realistic assessment of height effects under practical operational conditions.

3.1.4. Analysis of Variance

One-way ANOVA was conducted to test whether sampling height significantly affected particle concentrations for each particle size class (

Table 2. One-way ANOVA results for sampling height effects on particle concentrations in ISO Class 6 cleanroom.

Particle Size	F	p-Value	η2	F Critical Value
≥0.3 µm	1.05	0.356	0.04	3.18
≥0.5 µm	1.11	0.337	0.04	3.18
≥1.0 µm	1.69	0.195	0.06	3.18
≥5.0 µm	3.49	0.038 *	0.12	3.18

* p < 0.05.

). The analysis revealed no statistically significant effect of sampling height for particles ≥0.3 µm (F(2,51) = 1.05, p = 0.356), ≥0.5 µm (F(2,51) = 1.11, p = 0.337),= and ≥1.0 µm (F(2,51) = 1.69, p = 0.195). However, a significant effect was detected for ≥5.0 µm particles (F(2,51) = 3.49, p = 0.038, η² = 0.12), indicating that sampling height explained approximately 12% of the variance in 5.0 µm particle concentrations—a medium effect size according to Cohen’s conventional thresholds [37].

Before interpreting ANOVA results, it is important to address data quality considerations. The dataset includes zero counts (particularly at 120 cm for ≥5.0 µm particles; 9 of 18 locations recorded zeros) and statistical outliers (Locations 8, 11, 12 at 80 cm). Both phenomena were carefully evaluated for their impact on statistical validity:

Zero counts represent genuine measurement outcomes rather than missing data or errors. When personnel activity was absent at a sampling location and particle sources were minimal, concentrations naturally approached or fell below the instrument’s detection limit (~35 particles/m³), resulting in recorded zeros. These zeros were retained in calculations of means and standard deviations as legitimate data points representing low-concentration states.

Outliers (identified by the 1.5× IQR criterion) were similarly retained without exclusion. Statistical protocols recommend outlier removal only when values result from measurement errors, equipment malfunction, or protocol violations [38]—none of which apply to the present measurements. The outliers instead reflect reproducible consequences of heterogeneous FFU coverage and episodic personnel activity (Section 3.1.3), representing real operational variability. Excluding these values would artificially reduce the observed spatial heterogeneity and misrepresent the actual range of particle concentrations present in cleanrooms with non-uniform airflow distribution.

The decision to retain zeros and outliers aligns with the fundamental objective of this study: to assess whether height-adjustable furniture can be safely deployed under realistic pharmaceutical production conditions, which inherently include spatial airflow heterogeneity, episodic particle generation, and operational dynamics. ANOVA is robust to outliers when sample sizes are adequate (n = 18 per height group exceeds the minimum recommended n ≥ 10 for parametric testing [39]), and the observed F-statistics and p-values remain valid despite the presence of extreme values.

The ANOVA results reveal a clear size-dependent relationship between particle diameter and vertical height sensitivity. For smaller particles (≥0.3, ≥0.5, ≥1.0 µm), the between-group variance (attributable to height differences) was small relative to the within-group variance (spatial variability at each height), resulting in low F-statistics (1.05–1.69) and non-significant p-values (all >0.19). This indicates that for these particle sizes, spatial heterogeneity across the 18 sampling locations dominated over systematic height-related trends. In practical terms, a particle counter positioned at 80 cm might measure vastly different concentrations depending on its proximity to FFU modules or equipment but adjusting the same probe upward to 100 or 120 cm produces no statistically detectable systematic change when averaged across locations.

The absence of significant height effects for particles < 5.0 µm is consistent with aerosol physics: particles in this size range have extremely low gravitational settling velocities (e.g., ~0.03 mm/s for 0.5 µm particles, ~0.3 mm/s for 1.0 µm particles in still air) relative to typical cleanroom air velocities (350–450 mm/s). Consequently, these particles are transported predominantly by convective airflow rather than settling under gravity, and their vertical concentration profiles are determined primarily by the airflow field structure rather than by gravitational deposition over the modest vertical distances tested (40 cm total span).

In contrast, ≥5.0 µm particles exhibited a statistically significant height effect (p = 0.038), with height explaining 12% of the total variance (η² = 0.12). While this effect size is classified as “medium” by Cohen’s thresholds, it is notable that 88% of the variance remained unexplained by height, again emphasizing the dominant role of spatial heterogeneity. The significance of the height effect for large particles reflects their substantially higher settling velocity (~0.75 mm/s for 5.0 µm particles), which becomes comparable to vertical air velocity components in regions of reduced flow or turbulent mixing. Over the 40 cm vertical span between sampling heights, gravitational settling can therefore measurably deplete large particle concentrations.

The relatively low F-statistic (3.49), despite achieving significance, reflects the conservative nature of ANOVA in the presence of high within-group variability. The critical F-value was only slightly exceeded, indicating that the height effect, while real, is modest in magnitude compared to spatial variation—a finding with important practical implications for cleanroom sampling strategies.

3.1.5. Post Hoc Analysis

For ≥5.0 µm particles, which exhibited a significant overall ANOVA result, pairwise comparisons were conducted using independent-samples t-tests with Bonferroni correction to control family-wise error rate (

Table 3. Post hoc pairwise comparisons for ≥5.0 µm particles in ISO Class 6 (Bonferroni-corrected α = 0.0167).

Comparison	Mean Difference (Particles/m3)	p-Value	Significant
80 cm vs. 100 cm	16.4	0.351	No
100 cm vs. 120 cm	21.6	0.005	Yes
80 cm vs. 120 cm	38.0	0.028	No

; adjusted α = 0.05/3 = 0.0167 for three comparisons). The analysis revealed a statistically significant difference between 100 cm and 120 cm heights (p = 0.005, well below the Bonferroni threshold) but not between 80 cm and 100 cm (p = 0.351) or between 80 cm and 120 cm (p = 0.028, which exceeds the adjusted threshold despite being below the uncorrected α = 0.05).

The post hoc analysis reveals that the significant height effect for ≥5.0 µm particles is driven primarily by the concentration difference between 100 cm and 120 cm sampling heights, with a mean reduction of 21.6 particles/m³ (65% decrease). This result indicates that gravitational settling becomes a dominant transport mechanism for large particles over vertical distances of 20 cm or greater within unidirectional laminar flow. The absence of significance between 80 cm and 100 cm (p = 0.351) likely reflects the higher variability at the 80 cm worktable level, where local sources of particle generation (personnel activity, equipment surfaces) elevate concentrations and increase variance, masking systematic gravitational depletion effects.

3.2. ISO Class 8 Cleanroom (Vygon): Height-Adjustable Table Effects

3.2.1. Descriptive Statistics

Table 4. Descriptive statistics for particle concentrations (particles/m³) at different table heights in ISO Class 8 cleanroom (n = 6 for 70 cm; n = 9 for other heights).

Particle Size	70 cm	80 cm	90 cm	100 cm	120 cm
≥0.5 µm	98 ± 47	211 ± 174	158 ± 110	245 ± 252	117 ± 58
≥1.0 µm	53 ± 32	111 ± 88	76 ± 49	122 ± 140	61 ± 35
≥5.0 µm	6 ± 6	11 ± 8	7 ± 5	8 ± 6	7 ± 6

presents mean particle concentrations and standard deviations for five table heights (70, 80, 90, 100, 110 cm) measured at three positions (left corner, center, right corner) on each of three tables distributed throughout the ISO Class 8 production cleanroom. Particle concentrations exhibited substantial variability across height settings, with no consistent monotonic trend.

The descriptive statistics for the ISO Class 8 facility reveal markedly different patterns compared to the ISO Class 6 results. Most notably, particle concentrations do not exhibit a consistent decreasing trend with increasing table height. This non-monotonic pattern suggests that table height positioning does not create a dominant influence on particle concentrations at the worktable under operational production conditions. The standard deviations are substantial relative to the means (coefficients of variation ranging from 48% to 174%), reflecting the combined influences of spatial variability between tables, lateral variability across measurement positions, and temporal variability due to ongoing production activities.

3.2.2. Distribution of Particle Concentrations

Boxplot distributions for particle concentrations at the five table heights are presented in Appendix A (Figure A5, Figure A6 and Figure A7), illustrating the non-monotonic concentration patterns and substantial variability characteristic of operational industrial environments. The boxplots reveal that median concentrations (central lines) do not decrease monotonically with height. Instead, elevated medians are observed at 80 cm and 100 cm for ≥0.5 µm and ≥1.0 µm particles, creating a non-monotonic pattern across the height range. Statistical outliers are present at multiple heights, with particularly pronounced outliers at 100 cm for ≥0.5 µm (857 particles/m³) and ≥1.0 µm (476 particles/m³). These outliers reflect the influence of equipment exhaust ventilation interference, as detailed below.

3.2.3. Interpretation of Outliers and Non-Monotonic Patterns

The statistical outliers observed in the ISO Class 8 boxplots (Appendix A, Figure A5, Figure A6 and Figure A7) warrant specific explanation, as they reflect facility-specific operational conditions rather than random measurement error. One of the three height-adjustable tables was positioned in proximity to pharmaceutical equipment equipped with local exhaust ventilation. This equipment operates with a horizontal exhaust duct positioned at approximately 90–100 cm height above the floor. When the affected table was raised to 90 cm and especially 100 cm, the worktable entered the lower boundary of the exhaust flow plume, where horizontal air velocities exceeded the downward cleanroom flow velocity, creating localized airflow disturbances that transiently elevated particle concentrations.

The extreme outlier at 100 cm for ≥0.5 µm particles (857 particles/m³, visible in Appendix A, Figure A5) represents a measurement from the table position intersecting the equipment exhaust plume. This episodic high-concentration event resulted from the combination of horizontal exhaust flow, turbulent mixing, and potential entrainment of particles from the equipment exhaust stream. Similarly, outliers observed at 80 cm and 110 cm reflect partial interference with the exhaust plume at these heights, where the table surface was at the periphery of the horizontal flow field.

Importantly, even these outlier concentrations remained well within ISO Class 8 classification limits (3,520,000 particles/m³ for ≥0.5 µm), representing only 0.024% of the allowable limit. This demonstrates that height-adjustable furniture can be positioned across the full ergonomic range (70–110 cm) without compromising cleanroom classification compliance, even when local equipment ventilation creates transient airflow disturbances.

Zero counts are minimal in the ISO Class 8 boxplots (Appendix A), reflecting the higher baseline particle concentrations typical of this classification compared to ISO Class 6. The few zeros observed reflect measurement periods when no personnel were immediately adjacent to the sampling location and no active particle-generating processes were occurring nearby, allowing concentrations to approach the instrument detection limit.

3.2.4. Analysis of Variance

One-way ANOVA was conducted to determine whether table height significantly affected particle concentrations for each particle size class under operational production conditions (

Table 5. One-way ANOVA results for table height effects on particle concentrations in ISO Class 8 cleanroom.

Particle Size	F	p-Value	η2	F Critical Value
≥0.5 µm	1.29	0.291	0.12	2.63
≥1.0 µm	1.12	0.364	0.11	2.63
≥5.0 µm	0.81	0.530	0.08	2.63

). The analysis revealed no statistically significant effect of table height on particle concentrations for any particle size channel: ≥0.5 µm, ≥1.0 µm, and ≥5.0 µm. All p-values substantially exceeded the α = 0.05 significance threshold, indicating no evidence that adjusting table height across the ergonomic range (70–110 cm) systematically affects measured particle concentrations at the worktable under realistic industrial conditions.

The ANOVA results provide statistical confirmation that the non-monotonic concentration patterns observed in Table 4 and Appendix A (Figure A5, Figure A6 and Figure A7) do not represent systematic height-related effects. For all three particle size channels, the variance attributable to table height differences (between-group variance) was comparable to or smaller than the variance attributable to spatial and temporal variation within each height setting (within-group variance), resulting in low F-statistics and high p-values. The effect sizes (η²) ranged from 0.08 to 0.12, indicating that table height explained between 8% and 12% of the total variance in particle concentrations. While these values approach Cohen’s threshold for a “medium” effect (η² = 0.06–0.14), they did not achieve statistical significance due to the high within-group variability characteristic of operational production environments. This combination suggests that any potential height-related influence on particle concentrations is small in magnitude and is overwhelmed by other sources of variability, including equipment ventilation interference, personnel activity, and temporal variations in production operations.

3.3. Compliance with ISO 14644-1 Classification Limits

All measured particle concentrations in both facilities remained well below the respective ISO classification limits throughout the measurement campaign, demonstrating that neither vertical sampling height variation (ISO Class 6) nor table height adjustment (ISO Class 8) compromised cleanroom classification compliance.

3.3.1. ISO Class 6 (GfPS) Compliance

The maximum observed concentration across all sampling heights occurred at 100 cm for ≥0.3 µm particles (3591 particles/m³), which represents only 10.2% of the ISO Class 6 classification limit of 35,200 particles/m³—a compliance margin of 89.8%. For the other particle size channels, maximum observed concentrations were similarly far below classification limits: ≥0.5 µm (1868 particles/m³ vs. limit of 8320 particles/m³, 77.6% margin), ≥1.0 µm (974 particles/m³ vs. limit of 2930 particles/m³, 66.8% margin), and ≥5.0 µm (50 particles/m³ vs. limit of 293 particles/m³, 82.9% margin). Even at locations and heights where spatial heterogeneity or reduced FFU coverage resulted in locally elevated concentrations, measured values remained at least 67% below applicable limits, confirming robust cleanroom performance.

3.3.2. ISO Class 8 (Vygon) Compliance

The maximum observed concentration across all table heights occurred at 100 cm for ≥0.5 µm particles (245 particles/m³), which represents only 0.007% of the ISO Class 8 classification limit of 3,520,000 particles/m³—a compliance margin of 99.993%. This extraordinarily large margin reflects the fact that the facility was designed and operates well below its classification limit to provide buffer capacity for production-related particle generation events. For ≥1.0 µm and ≥5.0 µm particles, compliance margins were similarly substantial (99.988% and 99.973%, respectively). These results confirm that height-adjustable table positioning across the full ergonomic range introduces no risk of classification excursion under operational production conditions. Even when local equipment exhaust ventilation intersects elevated worktable surfaces (as documented at 90–100 cm heights; Section 4.2.1), creating transient airflow disturbances and concentration spikes, measured values remained below classification limits. This demonstrates that height-adjustable furniture deployment is inherently safe from a particle contamination perspective, as the large compliance margins built into ISO 14644-1 classification thresholds provide substantial buffer capacity to accommodate both height-related effects and facility-specific airflow perturbations.

3.3.3. Interpretation of Compliance Results

The large compliance margins observed in both facilities provide important context for interpreting the statistical findings. In the ISO Class 6 facility, although ≥5.0 µm particles exhibited a statistically significant height effect (Table 2), the absolute magnitude of this effect (mean concentration difference of 38 particles/m³ between 80 and 120 cm) is trivial compared to the classification limit of 293 particles/m³. Even in the worst-case scenario—positioning sampling equipment or worktable at the 80 cm height where concentrations were highest—measured values remained 83% below the limit, providing ample safety margin.

Similarly, in the ISO Class 8 facility, the absence of significant height effects means that table height can be adjusted freely across the ergonomic range without consideration of particle contamination concerns. The non-monotonic concentration patterns observed across heights (Table 4) represent normal operational variability rather than systematic contamination risks, and all measured concentrations remained more than 99.99% below classification limits regardless of table position.

These compliance results support the practical conclusion that both personnel working postures (which determine breathing zone elevation in facilities with fixed worktables) and height-adjustable furniture positioning can be optimized for ergonomic performance without compromising cleanroom particle concentration compliance, provided that the facility is designed and operated within established ISO 14644-1 guidelines. The height effects observed for large particles in ISO Class 6, while statistically detectable, are not practically significant from a regulatory compliance perspective.

4. Discussion

4.1. Mechanisms Underlying Height-Dependent Particle Concentration Patterns in ISO Class 6

The statistically significant height effect observed for ≥5.0 µm particles in the ISO Class 6 cleanroom, combined with the absence of effects for smaller particle sizes, reflects the interplay between gravitational settling physics and facility-specific airflow characteristics.

4.1.1. Gravitational Settling Dominance for Large Particles

The particle size-dependent response to vertical positioning is consistent with established aerosol physics [29,31]. Particles ≥5.0 µm exhibit gravitational settling velocities (~0.75 mm/s in still air for spherical particles at unit density) that are approximately 0.2% of typical cleanroom unidirectional flow velocities (350–450 mm/s). Over a 40 cm vertical transport distance at 400 mm/s downward airflow velocity, a 5.0 µm particle experiences approximately 1 s of transit time, during which gravitational settling displaces it laterally by ~0.75 mm—sufficient to alter the vertical concentration profile through cumulative depletion from the airstream.

In contrast, particles <5.0 µm have settling velocities below 0.3 mm/s (for 1.0 µm particles) and as low as 0.03 mm/s (for 0.5 µm particles), resulting in settling-to-convection velocity ratios <0.1%. For these particle sizes, convective transport dominates over gravitational settling across the height ranges tested, explaining the absence of statistically significant height effects (Table 2). The transition between convection-dominated and settling-influenced regimes occurs in the 1–5 µm size range, consistent with theoretical predictions [29,31].

4.1.2. Spatial Heterogeneity and FFU Coverage Effects

The large standard deviations observed across all particle sizes and heights (Table 1) reflect substantial spatial heterogeneity in particle distributions throughout the ISO Class 6 cleanroom. This heterogeneity is a consequence of the facility’s ventilation system architecture, which combines ceiling-mounted general air inlets with supplementary FFU modules positioned non-uniformly due to architectural constraints (Figure 3).

As illustrated in Figure 3, FFU are not uniformly distributed across the ceiling but are concentrated above specific work zones. Worktable positioned beneath or near FFU modules benefit from direct unidirectional laminar flow with high velocity (0.35–0.45 m/s) and efficient particle removal. Conversely, locations distant from FFU coverage—such as measurement points 11 and 12—receive airflow primarily from general ceiling-mounted diffusers, which provide lower face velocities and less uniform flow patterns [6,7,10,36]. This creates zones with elevated baseline particle concentrations due to lower air change rates and reduced dilution efficiency.

• Measurement Uncertainty Contributions:

The spatial heterogeneity documented in this section highlights a critical point regarding measurement uncertainty: the observed variability in particle concentrations (standard deviations in Table 1 ranging from 48% to 191% of mean values) is primarily attributable to genuine physical phenomena—specifically, non-uniform airflow distribution and episodic particle generation—rather than instrumental measurement error. The Solair 3100′s instrumental precision (±5% per manufacturer specifications [35]) is more than two orders of magnitude smaller than the spatial variability, confirming that the large standard deviations reflect real cleanroom performance characteristics rather than measurement limitations. This distinction is essential for interpreting the statistical analyses: the absence of significant height effects for particles <5.0 µm cannot be attributed to insufficient measurement precision, as systematic height-related differences (if present) would be easily detectable against the ±5% instrumental background. Rather, the non-significant findings genuinely reflect the absence of height-dependent particle transport for convection-dominated size ranges under the tested airflow conditions.

4.1.3. Outlet-Induced Airflow Disturbances and Vortex Formation

Air outlets are positioned exclusively along one side of the cleanroom perimeter (Figure 3, marked in red), creating a preferential horizontal airflow direction from the supply diffusers toward the extraction zone. This asymmetric configuration generates localized flow disturbances near wall-mounted outlets. Measurement locations in proximity to these outlets, including Locations 11 and 12, experience flow recirculation and vortex formation as the unidirectional downward airflow is redirected horizontally toward the extraction points [11,12,14,24].

As high-velocity air from FFU modules descends vertically, it encounters the horizontal suction flow toward wall-mounted outlets, creating a shear layer where velocity components interact, generating rotational flow structures (vortices). Within these vortices, particles are retained for extended periods, leading to locally elevated concentrations. The effect is most pronounced for larger particles (≥5.0 µm), which have sufficient inertia to settle into regions of low velocity within the vortex core [12,14].

The combined effects of reduced FFU coverage and outlet-induced flow disturbances explain why Locations 11 and 12 consistently exhibited elevated particle concentrations, contributing disproportionately to the high variance observed in Table 1. These effects are height-independent for smaller particles (transported primarily by convective flow) but become height-dependent for larger particles (which settle out of recirculating regions at rates determined by vertical distance).

4.1.4. Implications for Cleanroom Design and Sampling Strategy

The spatial heterogeneity documented in the ISO Class 6 facility has important implications for cleanroom design and qualification. From a design perspective, the results demonstrate the limitations of hybrid ventilation systems combining general ceiling inlets with supplementary FFU modules. Future cleanroom designs should prioritize uniform FFU coverage across all work zones or ensure that general ceiling inlet systems provide sufficient face velocity (≥0.35 m/s) and uniformity to achieve equivalent particle removal performance [7,10,26,36].

Additionally, air outlet placement should be carefully considered. The asymmetric outlet configuration documented in Figure 3 creates preferential flow directions that compromise unidirectional flow uniformity. Optimal cleanroom designs incorporate floor-level or low-wall extraction grilles distributed around the perimeter, enabling symmetric radial outflow patterns that minimize horizontal velocity components and vortex formation [26,27,36]. Where wall-mounted outlets are unavoidable, supplementary airflow modeling (via CFD simulation) should identify and mitigate recirculation zones [24,25].

For cleanroom qualification and routine monitoring, sampling location selection can substantially influence measured particle concentrations. In facilities with heterogeneous FFU coverage and asymmetric outlet placement, regulatory compliance sampling should prioritize “worst-case” locations—those with reduced FFU coverage, proximity to extraction outlets, or documented elevated concentrations [32,40]. In the GfPS facility, routine monitoring at locations 11 and 12 would provide more stringent compliance assessment than sampling at FFU-covered locations, ensuring classification limits are maintained even in zones with suboptimal airflow conditions.

4.2. Absence of Height Effects in ISO Class 8: Equipment Ventilation Interference

The absence of statistically significant table height effects in the ISO Class 8 cleanroom (Table 5) contrasts with the significant gravitational settling effect observed for large particles in ISO Class 6. Three facility-specific factors contribute to this difference: (1) equipment exhaust ventilation interference; (2) more uniform baseline airflow distribution; and (3) higher concentrations and operational variability masking systematic effects.

4.2.1. Local Exhaust Ventilation Interference at Elevated Table Heights

Post-experimental investigation revealed that one of the three height-adjustable tables in the Vygon facility was positioned in proximity to pharmaceutical equipment equipped with local exhaust ventilation. During measurement rounds at table heights of 90 cm and 100 cm, the elevated worktable entered the horizontal exhaust flow field from this equipment, creating a localized airflow disturbance that transiently elevated particle concentrations through resuspension, turbulent mixing, and direct entrainment of particles from the equipment exhaust stream [40,41].

The pharmaceutical equipment operates with a horizontal exhaust duct positioned at approximately 90–100 cm height above the floor, designed to extract water vapor and process-related emissions. Under normal operations with tables at standard heights (70–80 cm), the exhaust flow passes above the worktables without interference. However, when the affected table was raised to 90 cm and especially 100 cm, the worktable entered the lower boundary of the exhaust flow plume, where horizontal air velocities (estimated at 0.5–1.0 m/s) exceeded the downward unidirectional cleanroom flow velocity (0.35–0.45 m/s).

This airflow interference explains the non-monotonic concentration pattern observed in Table 4, where ≥0.5 µm and ≥1.0 µm particle concentrations peaked at 100 cm (245 and 122 particles/m³, respectively) rather than declining monotonically with height as gravitational settling theory would predict. The elevated concentrations and large standard deviations observed at 100 cm (SD = 252 and 140 particles/m³—exceeding the means) reflect measurements from the affected table positions, where the equipment exhaust plume generated episodic high-concentration events.

Importantly, this equipment interference does not invalidate the study’s conclusions regarding height-adjustable table safety. Rather, it represents a realistic operational scenario in pharmaceutical production environments, where cleanroom airflow fields are continuously perturbed by equipment operation, local exhaust systems, and process-generated thermal plumes [22,23,25]. The fact that particle concentrations remained more than 99.99% below ISO Class 8 classification limits (Section 3.3) even under these perturbed conditions demonstrates the robustness of cleanroom particle control and confirms that height-adjustable furniture can be deployed safely in production environments with active equipment operation.

From a practical facility management perspective, this finding highlights the importance of considering equipment–furniture interactions during cleanroom layout planning. The equipment exhaust interference documented here also provides context for understanding the measurement variability observed in Table 4 and Appendix A (Figure A5, Figure A6 and Figure A7). Standard deviations in the ISO Class 8 dataset (ranging from 48% to 174% of mean values) reflect not only spatial differences between worktable locations but also temporal variations arising from dynamic airflow perturbations as tables move through the exhaust plume during height adjustment. This operational variability, while substantial, remains well within the particle concentration limits permitted by ISO Class 8 classification, demonstrating that height-adjustable furniture can accommodate real-world facility constraints including proximity to equipment exhaust systems without compromising regulatory compliance. When deploying height-adjustable worktables, facility managers should evaluate the spatial relationship between worktables at maximum elevation and nearby equipment exhaust systems. Where conflicts are identified, mitigation strategies include (1) relocating the adjustable furniture to zones without equipment interference; (2) implementing vertical barriers or airflow deflectors to prevent horizontal plume intrusion; or (3) accepting the localized concentration elevation if it remains within classification limits.

4.2.2. Uniform Baseline Airflow Distribution and Operational Variability

Unlike the ISO Class 6 facility’s heterogeneous FFU coverage, the Vygon ISO Class 8 cleanroom features ceiling-mounted air diffusers distributed uniformly across the entire ceiling area, creating more homogeneous airflow patterns and reducing spatial variance [36]. The more uniform airflow distribution means that systematic height effects, if present, would be easier to detect statistically. The absence of significant effects despite this improved statistical power suggests that the 40 cm height span tested (70–110 cm) is genuinely insufficient to generate measurable gravitational settling effects under ISO Class 8 conditions.

The difference in detectability can be understood through signal-to-noise considerations. In ISO Class 6, the mean concentration difference between 100 cm and 120 cm for ≥5.0 µm particles was 21.6 particles/m³ (65% reduction), sufficient to achieve statistical significance. In ISO Class 8, the corresponding ≥5.0 µm concentrations were 8 ± 6 particles/m³ at 100 cm and 7 ± 6 particles/m³ at 110 cm—a difference of only 1 particle/m³ (14% reduction), well within measurement uncertainty.

The ISO Class 8 measurements were conducted under realistic industrial production conditions with active pharmaceutical manufacturing, ongoing personnel movement, material handling, and equipment operation that continuously generate and resuspend particles [22,23,40,41]. This operational activity creates temporal and spatial variability that can mask systematic height effects. Personnel movement events—such as an operator walking past, opening a container, or operating equipment—can generate transient particle concentration spikes lasting 30–120 s [22,40]. The high standard deviations observed in Table 4 reflect the combined influences of spatial variability between tables, lateral variability across measurement positions, and temporal variability due to ongoing production activities.

4.3. Comparative Interpretation and Practical Implications

4.3.1. ISO Class 6 Versus ISO Class 8 Findings

The contrasting results between ISO Class 6 (significant height effect for ≥5.0 µm) and ISO Class 8 (no significant effects) reflect fundamental differences in cleanroom operating principles and measurement sensitivity. ISO Class 6 cleanrooms operate with extremely low baseline particle concentrations (typically <5% of classification limits), enabling detection of small absolute changes. In contrast, ISO Class 8 cleanrooms operate with higher baseline concentrations (typically 10–20% of classification limits), greater turbulent mixing, and more operational activity, making the same absolute difference undetectable against background variability.

From a practical regulatory perspective, both sets of results support the same conclusion: height-adjustable furniture and variable working postures do not compromise cleanroom particle concentration compliance. In ISO Class 6, the statistically significant height effect (p = 0.038) explained only 12% of total variance (η² = 0.12) and resulted in concentrations that remained 83% below classification limits at all heights. In ISO Class 8, no significant effects were detected (all p > 0.29), and concentrations remained >99.99% below limits at all heights.

4.3.2. Height-Adjustable Furniture Deployment

The present results provide empirical support for deploying height-adjustable ergonomic worktables in pharmaceutical cleanrooms without particle contamination concerns. Across two facilities with different ISO classifications, ventilation architectures, and operational conditions, table height adjustment across the full ergonomic range (70–110 cm per DIN EN 527-1:2011 [20]) did not compromise cleanroom classification compliance. For pharmaceutical manufacturers implementing ergonomic improvement programs, these findings indicate that height-adjustable furniture can be deployed with confidence in ISO Class 6–8 cleanrooms, enabling optimization of workstation ergonomics without requiring trade-offs between worker health and contamination control. This is particularly important for facilities with aging workforces or high rates of musculoskeletal complaints, where ergonomic interventions can reduce injury rates and improve productivity [17,18,19,42].

Implementation considerations include (1) avoiding placement of height-adjustable furniture directly in line with equipment exhaust streams; (2) conducting post-installation qualification measurements to verify continued classification compliance; and (3) incorporating height-adjustable furniture into routine environmental monitoring programs [1,32,40].

4.3.3. Worker Breathing Zone Exposure Assessment

The ISO Class 6 results demonstrate that personnel breathing zone particle exposure varies with working posture, particularly for larger particles (≥5.0 µm). Workers in seated postures (breathing zone ~80 cm) experience higher ≥5.0 µm particle exposure than workers in standing postures (breathing zone ~120 cm), with a 76% concentration reduction observed over this 40 cm vertical span. However, the practical significance of this exposure difference is limited: all measured concentrations remained well below occupational exposure limits for pharmaceutical dusts [30], and the 38 particles/m³ concentration difference between 80 cm and 120 cm breathing heights is trivial compared to typical occupational exposure variability.

4.3.4. Cleanroom Design Optimization

The spatial heterogeneity documented in the ISO Class 6 facility provides actionable guidance for cleanroom ventilation system design. Future facilities should prioritize: (1) uniform FFU coverage above all critical work zones to minimize spatial gradients in air velocity and particle removal efficiency, (2) symmetric air outlet distribution to prevent preferential horizontal flow directions and vortex formation, and (3) computational airflow modeling during design to identify and mitigate potential recirculation zones [24,26,36].

For existing facilities with heterogeneous airflow, remediation strategies include: (1) supplementary FFU installation above zones with elevated concentrations, (2) airflow deflectors or baffles near wall-mounted outlets to redirect horizontal flows and minimize vortex formation, (3) revised sampling strategies that prioritize worst-case locations for routine monitoring, and (4) administrative controls that restrict critical operations to FFU-covered zones [32,40].

4.4. Study Limitations and Future Research Directions

Several limitations should be acknowledged. First, the study examined a limited range of vertical heights (40 cm total span), which may be insufficient to detect gravitational settling effects for particles <5 µm or in higher ISO classifications with greater turbulent mixing. Future studies could extend the vertical sampling range to 60–100 cm to investigate whether larger height differences reveal systematic effects for smaller particle sizes.

Second, the equipment ventilation interference observed in the ISO Class 8 facility, while representing realistic operational conditions, introduced an uncontrolled confounding factor that complicated interpretation. Future controlled studies should either isolate tables from equipment disturbances to measure “pure” height effects or systematically vary equipment–table proximity to quantify interference magnitude.

Third, this study focused exclusively on particle concentration without assessing microbial contamination, which is often the critical quality attribute for sterile pharmaceutical manufacturing [32]. While particle concentration is a validated surrogate for microbial risk [1,32], future studies should directly measure microbial contamination (via active air sampling or settle plates) at different table heights to confirm that height adjustment does not compromise microbiological quality.

Fourth, this study examined only two facilities with specific ventilation configurations. Generalization to other cleanroom designs—such as horizontal laminar flow cleanrooms, turbulent mixing ventilation, or mini-environment enclosures—requires additional research. Computational fluid dynamics (CFD) modeling could complement experimental measurements by simulating height effects across diverse ventilation configurations [24,25,26].

Finally, this study’s height-systematic measurement protocol, while minimizing the number of probe adjustments and ensuring temporal consistency, may have introduced time-of-day confounding if cleanroom operational intensity varied systematically across measurement days. Future studies could randomize height order or implement time-blocked designs to fully eliminate temporal confounding.

Despite these limitations, the present study provides robust empirical evidence that height-adjustable furniture and variable working postures are compatible with pharmaceutical cleanroom particle contamination control requirements. The convergent findings across two facilities with different ISO classifications, ventilation systems, and operational conditions strengthen our confidence in the generalizability of the core conclusion: within the ergonomic height range, furniture positioning does not significantly compromise cleanroom air quality.

5. Conclusions

This study investigated whether vertical positioning of sampling heights and height-adjustable worktable furniture affects airborne particle concentrations in pharmaceutical cleanrooms, addressing a critical question for implementing ergonomic improvements without compromising contamination control. Measurements were conducted under realistic operational conditions in two pharmaceutical production facilities: an ISO Class 6 cleanroom (GfPS) with vertical sampling heights spanning 80–120 cm, and an ISO Class 8 cleanroom (Vygon) with height-adjustable tables positioned across 70–110 cm.

In the ISO Class 6 facility, a statistically significant height effect was observed for particles ≥5.0 µm (F(2,51) = 3.49, p = 0.038, η² = 0.12), with particle concentrations decreasing by 76% from 80 to 120 cm due to gravitational settling. However, this effect explained only 12% of the total variance, with the remaining variability attributable to spatial heterogeneity in the facility’s airflow field. Detailed facility characterization revealed that elevated concentrations at measurement Locations 11 and 12 resulted from two compounding factors: (1) worktables positioned outside direct FFU coverage, relying on lower-velocity general ceiling inlets, and (2) proximity to asymmetrically placed wall-mounted air outlets that induced horizontal flow patterns and vortex formation. Importantly, even in zones with suboptimal airflow coverage, all measured concentrations remained at least 67% below ISO Class 6 classification limits, demonstrating adequate safety margins for personnel breathing zone variations associated with standing versus seated work postures.

In the ISO Class 8 facility, no statistically significant height effects were detected for any particle size (all p > 0.29), despite table height adjustments spanning the full ergonomic range (70–110 cm). The non-monotonic concentration patterns observed were attributable to localized equipment ventilation interference from pharmaceutical equipment, where horizontal exhaust flow intersected elevated table surfaces at 90–100 cm heights. This equipment interference represents a realistic operational scenario in production cleanrooms, yet particle concentrations remained more than 99.99% below ISO Class 8 classification limits at all table positions.

The convergent findings from both facilities provide robust empirical evidence that height-adjustable ergonomic furniture can be safely deployed in ISO Class 6–8 pharmaceutical cleanrooms without compromising particle contamination control. Furniture positioning across the full ergonomic range specified in DIN EN 527-1:2011 (70–110 cm) does not introduce classification compliance risks, enabling pharmaceutical manufacturers to implement evidence-based ergonomic interventions that reduce musculoskeletal injury risk and improve worker productivity without trade-offs in product quality assurance. Future cleanroom designs should prioritize uniform FFU coverage and symmetric air outlet distribution to minimize spatial heterogeneity, while existing facilities can accommodate height-adjustable worktables with confidence in maintaining regulatory compliance.