The Indoor Environment During Swimming Competitions and Its Impact on Construction Materials: Airborne Trichloramine as a Degradation Factor

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This study provides practical insights for engineers, architects and pool operators when designing and managing indoor swimming facilities exposed to chemically aggressive conditions. In particular, the results emphasize that swimming competitions create extreme scenarios in which airborne trichloramine concentrations can rise well above everyday levels. Recognizing these peak exposures is important for selecting appropriate construction materials, adjusting ventilation strategies, and planning preventive maintenance. Taking such conditions into account at the design and operation stage may significantly increase the durability of structural elements and improve the overall safety.

Abstract

Swimming is one of the most popular forms of recreational sport worldwide, recommended for people of all ages as a healthy activity. While numerous studies have focused on the impact of indoor air quality on the health of pool users, relatively few have addressed how specific airborne parameters in indoor swimming facilities affect the durability of construction materials. This article analyzes the current state of knowledge on the influence of the pool indoor environment on structural reliability, with trichloramine (NCl₃) emphasized as a degradation factor. Indoor pool environments are classified as chemically aggressive, due to elevated air temperature (~30 °C), high humidity (often exceeding 60%), and the presence of volatile chlorine compounds released from disinfected water. Our case study demonstrates that during swimming competitions, the average concentration of airborne NCl₃ reached a value of 900 µg/m³, with peaks up to 1200 µg/m³, i.e., about ten times higher than on typical usage days. The median trichloramine concertation during the competition was 1071 µg/m³. Such exposure conditions accelerate corrosion processes in stainless steels and other building materials, reducing service life and requiring targeted monitoring and preventive maintenance. Based on the findings, recommendations are provided regarding material selection, highlighting the importance of surface texture, ventilation strategies, and protective measures tailored to periods of intensive facility use.

1. Introduction

There are approximately 13 million swimming pools worldwide, of which 29% are located in Europe [1]. Indoor swimming pools are classified as chemically aggressive environments, with recognized health effects on users [2,3]. It should be emphasized that water treatment processes have a direct impact on indoor air quality in swimming pools [4]. Chlorine is the most commonly used disinfectant in pools [4]. However, chlorine use results in the formation of disinfection by-products (DBPs) through reactions with organic compounds introduced by swimmers, such as sweat, urine, and personal care products [5]. More than 100 DBPs have been identified in swimming pools, many of which contain nitrogen likely originating from human inputs (urine and sweat) [6,7]. Some sources report that over 250 DBPs have been identified, but only about 20 have been thoroughly characterized [8]. A major group of DBPs are trihalomethanes (THMs), with chloroform being the most abundant. Another important group are chloramines, among which trichloramine (NCl₃) is the most volatile [9]. Relative humidity, temperature, and chlorine-based DBPs (e.g., chlorides and chloramines, particularly volatile NCl₃) form a mixture that can reduce the durability of construction materials [10].

The generally accepted indoor air parameters in swimming pools are air temperature 2 K above the water temperature, typically ranging from 28 to 34 °C (for recreational and sports pools), and relative humidity of 50–60% [11,12]. More attention is usually given to pool water parameters, which are routinely monitored for temperature, chlorine concentration, trihalomethanes, chloroform, and microbiological indicators. Although certain guidelines specify permissible chlorine compound levels in indoor air, in practice THMs/chloroform and trichloramine (limit value of 500 µg/m³) are not controlled. Swimming pool facilities are not obliged to register these compounds, especially since their measurement is complex and technically demanding.

Nevertheless, studies on trichloramine concentrations and other volatile chlorine compounds in pools are widely reported in the literature. In ref. [2], 119 measurements of NCl₃ concentrations were performed at heights of 30 cm and 150 cm above the water surface. The mean trichloramine concentration at 30 cm above the water surface was 560 µg/m³ (range 130–1340 µg/m³), while at 150 cm it was 570 µg/m³ (range 140–1270 µg/m³). In the study of ref. [13], the concentration of THMs in indoor pool water ranged from 49 to 117 µg/L, and from 50 to 131 µg/L in outdoor pools.

In ref. [14], the authors included 116 non-smoking adults who swam for 40 min in an indoor pool. Chlorine, chloramines, pH and four THMs were measured in the water. The median concentration trichloramine in the air was 472.6 µg/m³. After swimming, exposure biomarkers increased—THMs in exhaled air and TCAA in urine.

The authors of [15] analyzed 20 swimming pools in Taipei and evaluated the impact of airborne trichloramine on human health. The main factors affecting trichloramine concentrations were the number of swimmers and the free chlorine concentration in pool water. The observed levels may have been influenced by the regulated free chlorine range in Taipei (0.3–0.7 ppm), which is lower than the World Health Organization recommendation.

In ref. [9], the authors investigated trichloramine concentrations in 74 swimming pools in Sweden. They determined that the geometric mean concentration of NCl₃ in public training pools ranged from 20 to 90 µg/m³. The mean concentration for educational pools was 180 µg/m³, and for recreational pools 200 µg/m³. They also identified an effect of time of day on measured values in training pools, with higher concentrations observed in the evening. Notably, trichloramine was not assessed as a factor influencing facility durability but rather as a compound present in the swimming pool environment.

The authors of ref. [16] carried out 22-day monitoring, collecting air samples above the water surface and around the pool. They measured air temperature, CO₂, humidity, volatile organic compounds (including THMs), chloroform, and ultrafine particles (UFP). The results showed stable temperature, CO₂, and humidity, but higher VOC levels during evening training sessions. THM concentrations stayed within recommended limits, although daily fluctuations occurred.

In ref. [17], it was demonstrated that the swimming pool environment is a dynamic interplay between thermal comfort, ventilation, energy consumption, and air quality. The study was conducted over nine summer days in a naturally ventilated indoor pool in Greece. The results showed that operative temperature frequently exceeded permissible limits. High values of temperature and radiation led to pronounced thermal discomfort (PMV values above the users’ comfort threshold). However, the analysis did not account for the impact of these conditions on construction materials.

The authors of ref. [1] identified and discussed microbiological, physicochemical, operational, and maintenance-related hazards in six pools in Greece. Only 13.1% of samples met the permissible range for free chlorine (0.4–0.7 mg/L). They emphasized risks related to facility operation and maintenance, with a strong emphasis on the role of pH in disinfection efficiency. Approximately 49% of the tested pools showed pH values outside the acceptable range (7.2–7.8), and 37.8% had values above 8.0. A pH below 7 leads to water corrosivity, which can damage infrastructure.

The corrosion resistance of stainless steel is determined both by environmental exposure conditions and by alloy composition and surface finish [10]. Corrosion has a significant impact on safety and reliability across a wide range of applications [18]. The degradation of stainless steel elements in swimming pools mainly arises from stress corrosion cracking, intergranular corrosion, and pitting [10]. It should be emphasized that corrosion plays a crucial role in material life cycle, safety, and costs [1]. Chlorine, in the form of liquid or aerosols, can form deposits that, under high relative humidity, promote pitting corrosion of stainless steel and damage to other construction materials in pool halls [10].

In ref. [10], failures of floor drains in an indoor pool were investigated. The study determined that the cause was pitting corrosion due to chloride deposition and insufficient cleaning. The resistance of austenitic stainless steel was shown to depend on alloy composition and environmental conditions. The use of chlorine-based disinfectants in pools leads to high concentrations of chlorides, acidic or strongly oxidizing agents in water, and chloramines in the air. The contents of Mo, Cr, and Ni in the floor drains corresponded to the lower end of the permissible range for X2CrNiMo17-12-2 (1.4404) steel, according to PN-EN 10088-2 [19]. The authors recommended the use of stainless steels with higher contents of Mo, Cr, N, and Ni. Proper alloy selection and maintenance can ensure adequate corrosion resistance.

The authors of ref. [20] analyzed pitting corrosion in ventilation ducts in a school swimming pool. They concluded that the corrosion resistance of austenitic stainless steel sheet X10CrNi18-8 (PN-EN 10088-2) was insufficient for ventilation installations in chlorinated indoor pools. The humid environment, with elevated chloride content, requires steels with lower carbon content, higher nickel content, and molybdenum alloying. The presence of chlorides and high humidity was identified as the main cause of corrosion. Chloride ions locally dissolve the passive protective layer, leading to pit initiation. Subsequent reactions between chloride ions, metal ions, and water deepen the corrosion process. For stainless steel with 18% Cr–8% Ni, pitting corrosion occurs at chloride concentrations as low as 100 ppm and pH values between 4 and 8.

Material degradation in pool halls is not exclusively related to indoor air conditions. The authors of ref. [21] reported structural damage in a swimming pool roof caused by moisture ingress through the roofing system, rather than by hall humidity alone. Leaks occurred via screws fixing aluminum profiles to the wooden sandwich panels. In addition to wood decay, corrosion of steel connectors embedded in the wood was observed, caused by elevated ambient humidity.

In ref. [22], corrosion of metallic fasteners was examined in 150 indoor pools in Switzerland. Approximately 80% of the facilities exhibited elevated chloride content. The transition from ozonation to chlorination was identified as a potential risk factor. Construction materials used included: stainless steel (15%, types 1.4301, 1.4571, 1.4541, 1.4401), galvanized steel (80%, of which 10% had additional coating), and others such as copper-bearing steel (5%). In one facility, highly alloyed steel 1.4529 was reported. Among stainless steel elements, 87% showed corrosion damage, whereas no corrosion was observed on 1.4529 after five years of operation. In galvanized steel elements, 14% exhibited corrosion, with the main damage occurring on coatings thinner than 10 µm. Duplex systems showed no signs of deterioration. The study concluded that standard austenitic stainless steels with Mo contents up to 3% should not be used in critical elements such as ceiling fasteners or railings. Galvanized steel is suitable, especially when duplex systems are applied. Highly alloyed steels (1.4529 and 1.4565) provide sufficient resistance for critical load-bearing components in splash zones not subjected to regular cleaning.

German guidelines [23] for the design and manufacture of stainless steel structures classify ventilation installations in chlorinated indoor pools as belonging to the highest corrosion risk category. They recommend the use of austenitic stainless steels such as X1CrNiMoCuN20-18-7, X1NiCrMoCuN25-20-7 (1.4529), or X1NiCrMoCu25-20-5 (1.4539), according to PN-EN 10088-1:1998 [24]. The sanitary inspectorate guidelines [25] on water quality and hygiene requirements for swimming pools specify that materials in contact with water must be resistant to disinfectants and must not adversely affect the physicochemical or microbiological quality of the water.

In ref. [26], it was emphasized that the application context determines the required resistance of stainless steel in swimming pool halls. The standard grade 1.4301 (304) is typically used for pool linings, while highly alloyed steels such as 1.4529 (≈20% Cr, 25% Ni, 6% Mo) are required for suspended ceiling fasteners in indoor pools. According to EN 13451-1 [27], only highly alloyed grades such as 1.4565 (X2CrNiMnMoNb25-18-5-4), 1.4529 (X1NiCrMoCuN25-20-7), and 1.4547 (X1CrNiMoCuN20-18-7) are suitable for stainless steel elements not subjected to regular cleaning. Where chloride concentrations in pool water are <250 mg/L, steel 1.4539 (904 L) may also be used. For regularly cleaned and accessible elements, less demanding grades such as 1.4401 (316), 1.4404 (316 L), 1.4578, 1.4571 (316Ti), 1.4439 (317LMN), or 1.4462 (2205) may be applied. Outdoor pools present milder conditions, permitting the use of moderately corrosion-resistant steels such as 1.4301 (304), 1.4307 (304 L), 1.4567, 1.4541 (321), and 1.4318 (301LN).

Improper material selection in swimming pool infrastructure poses a safety hazard for users [28]. A tragic example occurred on 9 May 1985, when the suspended ceiling of the Uster indoor swimming pool collapsed, killing twelve people. The ceiling was supported by rods made of austenitic chromium-nickel stainless steel (1.4301). Of 207 rods, 108 failed—94 in a brittle mode and 14 in a ductile mode. The degradation resulted from the aggressive indoor pool atmosphere, leading to localized corrosion initiation. Under tensile stress, stress corrosion cracking propagated until the rods lost their load-bearing capacity, causing ceiling collapse. This event underscores the necessity of careful material selection at the design stage. While highly alloyed steels may increase initial investment costs, they reduce long-term maintenance and repair expenses.

The authors [29] investigated the degradation of materials used in swimming pool environments (stainless steels AISI 316L, AISI 321, and duplex steel 1.4462). The materials were subjected to an immersion corrosion test under stagnant conditions, using NaClO solutions with concentrations ranging from 2% to 100%. Stainless steel AISI 316L released small amounts of corrosion products, maintaining mechanical strength but losing ductility. AISI 321 was prone to corrosion and lost both strength and ductility. Duplex steel 1.4462 showed almost no signs of corrosion, retaining strength and impact resistance. Despite manufacturing limitations, its use could reduce swimming pool investment and maintenance costs. The most commonly used material in ventilation installations is steel sheet; however, an alternative is ducts made of plastics such as PE and PVC [30]. Plastic ducts are characterized by lower hydraulic losses and easier processing, but they are sensitive to solar radiation. Article [31] discusses pitting corrosion observed in a ventilation grille (made of Cr-Ni-Mo steel) in a swimming pool. It was determined that pitting corrosion is the main corrosion mechanism occurring in swimming pool environments. Welded joints showed lower corrosion resistance than the steel itself, due to their microstructure and surface finish quality. Concrete is also used in swimming pools [32]. The main issue was the uneven stress distribution caused by irregular concrete surface finishing.

Material selection, as indicated by numerous publications, is important in increasing the corrosion resistance of swimming pool structures. In practice, topographical irregularities of material surfaces also have a significant impact on a wide range of surface functional properties, including corrosion [33]. An increase in surface roughness results in a larger effective contact area with the corrosive environment, thereby intensifying corrosion and consequently accelerating material degradation [34]. Roughness and surface defects, such as cracks, dents, and pores, determine the initiation and intensity of corrosion processes. Topographic valleys and crevices can accumulate moisture and aggressive chloride ions, found in swimming pools, and consequently contribute to the initiation of local corrosion [35]. The simultaneous presence of an aggressive corrosive environment and stress concentrations, primarily around micro-geometric surface irregularities, result in the development of stress corrosion cracking. Furthermore, topographic irregularities can prevent the formation of uniform, protective passive layers, thus impairing corrosion resistance [36]. Surface topography is one of the most important factors determining corrosion resistance, but equally important are the material type and chemical composition, microstructure, and residual stresses. Surface corrosion is also determined by external factors, including the type of corrosive environment, temperature, and atmospheric pressure [37].

Most existing studies on indoor air quality in swimming pools have focused on the health impacts of DBPs on swimmers and staff [2,9], while others highlight the importance of ventilation and energy balance for thermal comfort and stable conditions [1]. Much less attention has been devoted to the impact of volatile chlorine compounds, particularly trichloramine, on the durability of construction materials and installations [8,10,18,20,22]. Existing design and sanitary guidelines classify the pool environment as highly corrosive and recommend the use of high-alloy stainless steels and protective systems [23,25,26]. However, research shows that even materials compliant with European standards can undergo accelerated corrosion under elevated trichloramine concentrations [10,20,22]. Current regulations do not account for specific operational scenarios such as swimming competitions, which may generate extreme levels of volatile chlorine compounds [2,9,16]. This research gap justifies the need to investigate air quality during swimming competitions, which may represent a critical scenario both for user health and for the durability of materials in indoor pools. This article focuses on the analysis of indoor air quality parameters in Poland during the winter period (November 2024), comparing standard and intensified pool use, in order to demonstrate the environmental conditions to which construction materials in indoor pools are exposed.

2. Materials and Methods

2.1. Studied Pool

A series of studies on indoor air quality parameters was conducted in an indoor swimming pool located in Poland. The case study concerns a swimming facility consisting of two water bodies: a competition pool A (51.5 m × 26 m × 2.5 m) and a second pool B (30 m × 25 m with a movable floor ranging from 0 to 5 m). The competition pool can be divided into two 25 m basins. One side of the competition pool A is used for events where swimming races take place, while the other side is designated for warm-up activities. Next to pool B, there are four diving platforms (3 m, 5 m, 7 m, and 10 m).

The water treatment systems for both pools include UV disinfection and disinfection with calcium hypochlorite. The characteristics of the swimming pool facility are presented in

Table 1. Characteristics of swimming pool facility.

Parameter
Location	Poland
Years of construction	2014–2017
Stands	1700 people
Ventilation system	mechanical
Disinfection	UV and chlorine
Volume	45,400 m3
Number of pools	2 (A and B)
Length	A: 50 m B: 25 m
Water area	A: 1339 m2 B: 750 m2
Water temperature	26.5 ± 0.18 °C
Free chlorine	0.8 ± 0.17
Combined chlorine	0.24 ± 0.06

.

The water temperature was stable, with an average value of 26.5 ± 0.18 °C, indicating only minor fluctuations during the measurements. Such conditions are typical for indoor swimming pools and help maintain both bathing comfort and the stability of chemical processes occurring in the pool water. The concentrations of free and combined chlorine were 0.8 ± 0.17 mg/L and 0.24 ± 0.06 mg/L, respectively. These values were maintained throughout the entire measurement period, and no significant differences between the competition mode and the rest of the time were observed. The obtained values fall within the recommended range for pool water, ensuring effective disinfection.

The analyzed facility is equipped with a mechanical ventilation system with four air-handling units dedicated to the pool hall (Figure 1). The units supply air through floor diffusers and under selected spectator seats, while four exhaust outlets—one per unit—are located near the roof. The maximum possible air capacity of the units is 25,000 m³/h. During the measurement period, the instantaneous supply airflows ranged from 22,000 m³/h to 24,200 m³/h, while the exhaust airflows ranged from 22,800 m³/h to 24,650 m³/h.

2.2. Methods

Air quality parameter monitoring was carried out using ETHERA NEMo TC devices (jak u bachleya). The NEMo device measures air parameters using a proprietary nanoporous Sol–Gel material on the sensor substrate, along with an electronic/optical system to determine the concentration of target compounds. For the measurement of trichloramine concentration, the Sol–Gel material was impregnated with potassium iodide. The sensors in the device were replaced every 24 h. The measurement ranges of the analyzed parameters of the device are shown in

Table 2. Measurement ranges of the analyzed parameters.

Parameter	Measuring Range	Resolution	Accuracy
Trichloramine	0–250 ppb	-	Between 0 and 40 ppb: ±10 ppb ±10%; Between 40 and 100 ppb: ±20 ppb ±10% (average in one day)
CO2	0 to 5000 ppm	1 ppm	± 50 ppm ± 3 % of reading value
Humidity	0 to 95 %	0.08 %	±3 % between 11% and 89% (±7 % for the rest of the range)
Temperature	−55 °C to +125 °C	0.08 °C	±2 °C between −25 °C and 100 °C (±0.5 °C after calibration)

.

The device records parameters every 10 min. The position of the NEMo device was determined based on the ventilation layout and technical constraints of the swimming pool facility. The device was installed in a location not directly affected by supply air jets and where it would not interfere with pool users during the long measurement period. For technical reasons, the device could be mounted only at a height of 1.5 m above the water surface and 1.5 m from the pool edge. This height approximately corresponds to the breathing zone of a standing person and to the level where some materials near the pool surface are exposed.

In addition, according to [38], higher concentrations of trichloramine were observed at greater heights (2.5 m and 4.5 m). Therefore, the measurements at 1.5 m can be considered representative and even conservative, as higher levels would likely occur closer to the ceiling.

The air parameter measurements were taken at ten-minute intervals and included the following:

• Temperature [°C];
• Relative humidity [%];
• Carbon dioxide (CO₂) concentration [ppm];
• Trichloramine (NCl₃) concentration [µg/m³].

The number of people present in the pool hall was also monitored during swimming competitions, which primarily involved participants aged 10 to 15 years.

The data were analyzed under three pool operating modes: night, normal, and competition. The night mode covers the period when the facility is closed to both pool users and staff. During this time, the technical infrastructure remains active to maintain proper water and air quality parameters. No water surface disturbances are observed in this mode. The normal mode corresponds to standard pool use, during which regular classes or recreational swimming take place. The competition mode (intensive use) is characterized by a high level of swimming activity, resulting in strong water agitation. This leads to increased moisture emissions due to intensive evaporation, as well as higher release of volatile chlorine compounds. This mode occurs during swimming competitions and other events with high participant attendance.

Swimming competitions held between 22 and 25 November were selected for the analysis. The measurement period also encompassed the days following the event. The relatively short duration of the measurement period was determined by the operational characteristics of swimming pool facilities. Swimming competitions typically last from one to four days, with the exception of the Olympic Games, continental championships, and world championships, which usually extend over approximately one week. Such events are most often held on weekends and at irregular intervals. Consequently, it is challenging to identify a swimming facility where competitions involving a large number of athletes are organized regularly and with sufficient frequency to enable long-term measurements. Therefore, the measurements in this study cover only a short period of time.

The analyzed competition period consisted of four blocks, each beginning with a warm-up:

• A—22 November 8:00–13:00;
• B—23 November, 8:00–12:00;
• C—23 November, 16:00–19:00;
• D—24 November, 8:00–12:00;
• E—25 November, 8:00–12:00.

The night period—without the presence of pool users or staff—covered the hours from 23:00 to 5:30. The measurement period lasted for six consecutive days in November. A detailed breakdown of the usage periods is presented in

Table 3. Periods of pool operation during study period.

Data	Time	Mode
	00:00–05:30	Night
	05:31–07:59	Normal
22.11	08:00–13:00	Competition
	13:01–22:59	Normal
	23:00–23:59	Night
23.11	00:00–05:30	Night
	05:31–07:59	Normal
	08:00–19:00	Competition
	19:01–23:59	Night
24.11	00:00–05:30	Night
	05:31–07:59	Normal
	08:00–12:00	Competition
	12:01–22:59	Normal
	23:00–23:59	Night
25.11	00:00–05:30	Night
	05:31–07:59	Normal
	08:00–12:00	Competition
	12:01–22:59	Normal
	23:00–23:59	Night
26.11	00:00–05:30	Night
	05:31–22:59	Normal
	23:00–23:59	Night
27.11	00:00–05:30	Night
	05:31–10:00	Normal

.

The analysis of the obtained results was carried out in terms of air parameter variability using statistical tests: Pearson’s correlation (to assess relationships between the number of people and air parameters).

The aim of the study was to determine the impact of air parameters during intensive use (competitions) and standard use (normal operation) on the materials used in the pool hall. The variability in air parameters in the indoor swimming pool was determined along with their mean values. The results were compared with required and generally accepted standards.

3. Results

3.1. Users and Swimmers

During the analyzed study period, differences in the number of pool users were observed (

Table 4. Number of people on individual measurement days.

Date	Time of Observation	Swimmers (in the Pool)	Swimmers (Outside the Pool)	Stands	Total Number of People
22 November	10:15–11:30 and 16:40–20:15	26–170	24–93	32–534	142–632
23 November	7:15–19:15	0–104	6–118	0–299	6–470
24 November	7:15–12:30	0–90	4–107	1–246	5–390
25 November	7:15–12:30	22–70	19–81	63–309	125–416
26 November	9:30–12:30	27–58	7–15	2–8	40–73
27 November	-	-	-	-	-

and Figure 2). The highest number occurred on 22 November, with a maximum of 170 people in the water; there was an average of 89 people in the water, 53 near the pool basins, and 181 in the stands. On the remaining days, the number of swimmers ranged from 37 to 46, with 11 to 71 people near the pool area. The maximum total number of people present in the facility during the study period was 632. On 26 November, a decrease was observed, with an average of only 52 people in the entire facility. The difference between the days results from the normal operating mode.

3.2. Air Parameters

The measurements carried out in the pool hall made it possible to assess the variability in air parameters (Figure 3 and Figure 4). The analysis was conducted both in a comprehensive manner and with differentiation by individual days and operating modes. The study considered three modes of operation: competition, normal, and nighttime. Four key air quality parameters were measured: air temperature, relative humidity, carbon dioxide concentration, and trichloramine concentration.

On 22 November, a distinct increase was observed in all air quality parameters associated with user activity. The air temperature ranged from 27.5 °C to 29.0 °C, with a daily mean of 27.7 °C. Relative humidity varied between 45% and 58%, averaging approximately 52%. The mean carbon dioxide (CO₂) concentration was about 640 ppm, within a range of 515–898 ppm, representing the highest level recorded during the observation period.

The concentration of trichloramine (NCl₃) ranged from 0 to 1167.5 µg/m³, which corresponded to the upper detection limit of the measuring device. The mean trichloramine concentration for the day was approximately 430 µg/m³.

On the following day, air quality parameters remained at similar levels. The air temperature ranged from 27.5 °C to 28.5 °C, with a mean value of 28.5 °C, while relative humidity varied between 45% and 59%, averaging 52%. The carbon dioxide (CO₂) concentration fluctuated between 400 and 820 ppm, with an average of 578 ppm. The trichloramine (NCl₃) concentration ranged from 0 to 1167.5 µg/m³, with a mean value of 459 µg/m³.

On 24 November, the highest mean trichloramine concentrations of the entire period were recorded (582 µg/m³). The air temperature ranged from 26.5 °C to 29.0 °C, with a mean of 27.7 °C, while relative humidity varied from 43% to 62%, averaging at 53%. The CO₂ concentration ranged between 404 and 668 ppm, with an average of 545 ppm.

The data collected on 25 November were comparable to previous days. The air temperature ranged from 27.0 °C to 29.5 °C, with a mean value of 27.7 °C. Relative humidity varied between 48% and 63%, averaging approximately 55%. The carbon dioxide (CO₂) concentration ranged from 408 to 679 ppm, with a mean of 550 ppm. The trichloramine (NCl₃) concentration varied from 0 to 1167.5 µg/m³, with an average of 519 µg/m³.

On 26 November, greater variability in air quality parameters was observed. The air temperature ranged between 26.5 °C and 29.5 °C, with a mean of 27.7 °C. Relative humidity varied from 45% to 66%, with an average of 57%. The CO₂ concentration ranged between 438 and 603 ppm, with a mean value of 520 ppm. The mean trichloramine concentration recorded on that day was 532 µg/m³.

On the last day of measurements, a decrease was noted in CO₂ concentration (448–542 ppm) and relative humidity (50–66%) was observed, with mean values of 486 ppm and 60%, respectively. Air temperature ranged from 26.5 °C to 28.0 °C (mean 27.3 °C), and the mean trichloramine (NCl₃) concentration reached 596 µg/m³.

In general, air temperature within the facility varied from 26.5 °C (recorded on 24 November, 26, and 27) to 29.5 °C (on 25 and 26 November), with daily mean values ranging from 27.5 °C (on 23–24 November) to 27.9 °C (on 25 November). The difference between minimum and maximum temperature was 2 °C, while daily means differed by only 0.4 °C. Relative humidity ranged from 43% (on 24 November) to 66% (on 26 and 27 November). These values fall within the typical range for indoor swimming pool environments, with a daily variability of about 10–15%.

More dynamic changes were observed for parameters directly associated with user presence—namely, CO₂ and trichloramine concentrations. CO₂ levels varied between 400 ppm (on 23 November) and 898 ppm (on 22 November). Trichloramine concentrations ranged from 0 to 1167.5 µg/m³ throughout all analyzed days, with the highest daily mean of 582 µg/m³ (on 24 November) and the lowest of 430 µg/m³ (on November 22).

The competition mode included the days of 22, 23, 24, and 25 November, with varying time intervals. During this period, the highest concentrations of both CO₂ and trichloramine were recorded.

During the night period (00:00–05:30) on 22 November, CO₂ concentrations were at their lowest, averaging around 490 ppm, while trichloramine ranged from 120 to 150 µg/m³. The air temperature was approximately 27.5 °C, and relative humidity averaged 52%. In the normal mode (05:31–07:59), corresponding to the early hours of facility operation, increased activity in the pool hall resulted in CO₂ concentrations rising to 550–600 ppm and trichloramine levels to about 400 µg/m³. Air temperature increased to around 28 °C, while relative humidity decreased slightly to 48–50%.

During the competition period (08:00–13:00), the highest values were recorded, with mean CO₂ levels of approximately 760 ppm and mean trichloramine concentrations near 1000 µg/m³ (maximum value: 1167 µg/m³). Air temperature ranged between 28.5 °C and 29 °C, while relative humidity decreased to 45–48%. In the afternoon normal mode (13:01–22:59), air quality gradually improved, with CO₂ decreasing to about 580 ppm and trichloramine to around 450 µg/m³. The temperature was approximately 27.8 °C, and relative humidity increased to about 53%. During the late-night period (23:00–23:59), CO₂ levels decreased to around 500 ppm, and trichloramine concentrations dropped to approximately 200 µg/m³.

On 23 November, during the night period (00:00–05:30), a further decline was noted, with mean CO₂ concentrations around 490 ppm and trichloramine levels below 200 µg/m³ (minimum value: 0 µg/m³). Under normal operation, CO₂ and trichloramine increased to approximately 520 ppm and 350 µg/m³, respectively. During the competition mode (08:00–19:00), a rapid increase in both parameters was observed: mean CO₂ reached 720 ppm, and trichloramine rose to about 950 µg/m³ (maximum 1167.5 µg/m³). Relative humidity decreased to 45–50%. Between 19:01 and 22:59, the facility was closed, and CO₂ and trichloramine concentrations declined to around 480 ppm and 200 µg/m³. During the subsequent night mode, air quality improved again, accompanied by higher relative humidity (~60%).

On 24 November, during the night period (00:00–05:30), CO₂ levels decreased from 589 ppm to approximately 460 ppm, while the mean trichloramine concentration was about 150 µg/m³. Under normal operation (05:31–07:59), CO₂ increased to around 560 ppm and trichloramine to roughly 500 µg/m³. The highest values were recorded during the competition period (08:00–12:00), when CO₂ reached approximately 670 ppm and trichloramine exceeded 1000 µg/m³. After the competitions, in the normal mode (12:01–22:59), CO₂ concentrations moderately decreased to about 540 ppm, while trichloramine remained elevated at around 700 µg/m³. During the night hours (23:00–23:59), both parameters dropped to approximately 480 ppm and 150–200 µg/m³, respectively, with air temperature between 27 °C and 28 °C and relative humidity around 55%.

On 25 November, during the night period (00:00–05:30), the air was characterized by low concentrations of CO₂ (mean ≈ 480 ppm) and trichloramine (mean ≈ 270 µg/m³). In the normal mode (05:31–07:59), a slight increase in CO₂ to around 520 ppm and trichloramine to 350–430 µg/m³ was observed. During the competition phase (08:00–12:00), maximum values were recorded—CO₂ averaged 640 ppm, and trichloramine averaged 750 µg/m³. Following the competitions, during normal operation (12:01–22:59), both concentrations gradually decreased (CO₂ ≈ 520 ppm, trichloramine ≈ 450 µg/m³). During the night period (23:00–23:59), CO₂ and trichloramine levels further declined to about 480 ppm and 200 µg/m³, respectively, while relative humidity reached approximately 60%.

On 26 November, during the night period (00:00–05:30), CO₂ concentration was around 460 ppm, and trichloramine about 130 µg/m³. During normal operation (05:31–22:59), a moderate increase in CO₂ (mean 520 ppm) and trichloramine (mean 540 µg/m³) was recorded. In the afternoon, temporary peaks in trichloramine concentrations exceeding 1000 µg/m³ were noted. During the night period (23:00–23:59), both parameters decreased again to approximately 470 ppm and 200 µg/m³.

Finally, on 27 November, the last day of measurements, nighttime CO₂ concentrations remained within 440–460 ppm, while trichloramine reached about 600 µg/m³. Trichloramine concentrations remained at a moderate level overall. Air temperature ranged from 26.5 °C to 28 °C, and relative humidity from 50% to 66%, showing an upward trend.

3.3. Air Parameters and Intensity of Use

The research results indicate a clear relationship between the number of facility users and selected indoor air quality parameters in the swimming pool hall. As the number of swimmers and spectators increased, simultaneous rises in carbon dioxide (CO₂) and trichloramine (NCl₃) concentrations were observed. These parameters showed the strongest dependence on user activity, while air temperature and relative humidity remained relatively stable throughout the measurement period.

The highest CO₂ concentrations were recorded during swimming competitions, when the number of people present in the facility reached its maximum (up to approximately 630 individuals). Under these conditions, average CO₂ levels exceeded 700 ppm, whereas during nighttime and early morning hours—when the facility was unoccupied—they remained at 450–500 ppm. This confirms that the primary source of CO₂ emission and accumulation in the pool hall was human respiration, not necessarily from swimmers in the water alone.

A similar pattern was observed for trichloramine. NCl₃ concentrations exhibited significant fluctuations in direct response to water activity. During periods of intense pool use, increased water agitation and evaporation promoted the release of volatile chlorine compounds into the air. The highest concentrations (up to 1167 µg/m³) were recorded during competition sessions, while nighttime levels dropped below 200 µg/m³. These results confirm a direct relationship between facility occupancy and the emission and volatilization of trichloramine.

Statistical analysis supported these observations. Pearson’s correlation coefficients demonstrated a strong positive relationship between the number of users and the concentrations of CO₂ (r ≈ 0.82, p < 0.05) and trichloramine (r ≈ 0.78, p < 0.05). In contrast, correlations for air temperature and relative humidity were weak and statistically insignificant (|r| < 0.3), indicating the effective operation of the HVAC system in maintaining stable microclimatic conditions regardless of occupancy level.

In summary, CO₂ and trichloramine concentrations can be considered reliable indicators of both occupancy intensity and ventilation efficiency in indoor swimming pool facilities. Periods of increased pool activity—particularly during competitions—require enhanced ventilation rates or air purification methods to prevent the accumulation of chlorinated by-products and maintain appropriate indoor air quality in the pool hall.

4. Discussion

The obtained results clearly confirm the significant impact of user occupancy on indoor air quality in the swimming pool hall and on the variability in volatile disinfection by-product (DBP) concentrations. The simultaneous increase in carbon dioxide (CO₂) and trichloramine (NCl₃) concentrations during swimming competitions indicates two overlapping phenomena: elevated CO₂ emissions due to human respiration and intensified release of chloramines from the water surface into the air as a result of increased mixing and evaporation. Similar relationships were observed by other authors in studies conducted in both sports and recreational facilities [2,9,16]; the same occurs with trihalomethanes (THMs). The authors of [39] also confirmed the influence of the number of swimmers on THM and TCAM levels, with increases up to 110 μg/m³ and 0.52 mg/m³, respectively.

The range of trichloramine concentrations obtained in this study (average 430–582 µg/m³, maximum 1167 µg/m³) falls within the ranges reported in the literature. In [2], NCl₃ concentrations ranged between 130 and 1340 µg/m³, while in [9] the mean values were 180–200 µg/m³. It should be noted, however, that the measurements presented in this paper were conducted during the winter season, when limited air exchange and low infiltration of outdoor air could promote the accumulation of volatile compounds, leading to slightly higher values than under typical conditions [16].

Correlation analysis confirmed a strong relationship between the number of users and the concentrations of CO₂ and NCl₃ (r ≈ 0.8), indicating their mutual coupling with swimmer activity. At the same time, the small variability in air temperature and relative humidity demonstrates the effectiveness of the HVAC system, which maintained stable microclimatic conditions despite changes in occupancy. Similar observations were reported in [17].

It is important to emphasize the impact of all users on the air quality within the swimming pool hall. In addition to swimmers, spectators also influence the air parameters in indoor pools. The authors of [40] investigated the effect of spectator occupancy (number of viewers) on ambient temperature, relative humidity, thermal comfort, and water evaporation rate. Although spectators do not directly affect the rate of water evaporation, the total number of users influences the operation of the HVAC system.

From a material engineering perspective, the results are significant because they indicate the potential influence of volatile chlorine compounds on the durability of structural components. According to the literature [10,18,20,22], trichloramine and other chlorinated disinfection by-products may cause localized damage to stainless steels through pitting and stress corrosion. NCl₃ concentrations exceeding 500 µg/m³, recorded during competitions, may accelerate corrosion processes, especially under conditions of elevated humidity. The combination of chloramine vapors, temperatures around 28 °C, and humidity above 50% creates an environment particularly aggressive to austenitic steels, as confirmed by cases of corrosion in ventilation ducts and structural components of swimming pool halls [10,20,22].

The results also show that the competition mode represents a critical operational scenario for indoor swimming pools. Current design guidelines and standards do not account for short-term but highly intensive loads associated with large numbers of users. As noted in [17,23,25,26], standard ventilation settings, optimized for everyday use, may be insufficient to effectively dilute chloramines during peak occupancy. Other studies conducted during swimming competitions have also reported a strong relationship between pool activity and trichloramine concentration. In [41] the authors observed a several-fold increase in NCl₃ concentrations during national swimming events, attributed to enhanced emission from the water surface due to vigorous mixing and aeration. More recent research [42] confirmed that during competitions, gaseous trichloramine concentrations in air exceeded 930 µg/m³, and their variability was closely linked to the number of swimmers and CO₂ levels. The findings of the present study confirm these relationships under real operating conditions, while maintaining stable microclimatic parameters. The similarity of NCl₃ and CO₂ trends suggests that both compounds undergo comparable mass transfer processes from the water phase to the air, as also emphasized in ref. [42]. Unlike earlier studies, this research included multiple operational modes, confirming that the pool’s ventilation system operated efficiently and that elevated pollutant levels were primarily due to high user activity.

In the present study, the air distribution system corresponds to the configuration described in ref. [43] and ref. [44], where the supply air is introduced from the lower part of the hall and extracted through ceiling outlets. This layout, commonly used in Polish swimming pool facilities, promotes upward air movement and the efficient removal of moist and warm air from the pool surface. Numerical analyses in ref. [44] confirmed that such a system provides a realistic distribution of air temperature, humidity, and velocity fields, with good agreement between CFD predictions and experimental measurements. The moisture emitted from the pool surface rises with the airflow, increasing humidity in the upper zones and potentially reaching building materials where condensation and adsorption can occur.

A similar transport mechanism applies to volatile chlorination by-products such as trichloramine (NCl₃), which is also generated at the air–water interface and moves with the surrounding air masses. Due to its volatility and relatively low density, NCl₃ follows convective air currents and can accumulate near the ceiling or in poorly ventilated zones. Once transported upward, it may interact with building materials through adsorption or surface reactions, contributing to long-term material degradation and odor retention. Understanding this coupled movement of moisture and contaminants is therefore crucial for assessing air quality and surface exposure in swimming pool environments.

The forced ventilation of indoor swimming pools plays a key role in diluting and removing trichloramine (NCl₃), which forms at the air–water interface. Numerical studies for different air change rates (ACHs) have shown that NCl₃ concentration in the breathing zone strongly depends on the local airflow velocity and direction near the water surface [45,46]. Increasing the total airflow rate enhances mass transfer and dilution of NCl₃ in the lower breathing zone, but when airflow exceeds 6 air changes per hour, evaporation from the pool surface rises (by up to 16%) and the thermal comfort of wet bathers deteriorates [45]. Moderate increases in airflow (3.6–5.0 ACH) or the use of deck-level extraction effectively reduce NCl₃ without adverse effects on comfort or evaporation.

CFD analyses confirm that airflow patterns in swimming pool halls are complex. The dispersion of trichloramine results from the interaction of supply air jets, buoyancy, and local recirculation zones [47]. Deck-level extraction has been shown to capture NCl₃ near its emission source, with reductions of up to 25% near the deck and 48% in upper zones, depending on vent position [47].

A direct relationship between airflow velocity and NCl₃ accumulation has also been observed. A mean air velocity of 5.8 cm/s above the water is insufficient to prevent build-up, while increasing the airflow to 8.0 air changes per hour (14.7 cm/s) significantly reduces concentrations [46]. This confirms the dependence of trichloramine dilution on air velocity in the near-surface boundary layer. However, higher airflow also increases energy use and may reduce comfort [45,46].

In the cited studies, air was supplied from the top and exhausted near the floor. In the present research, the system differs—supply air is introduced in the lower part of the pool area and exhausted through corner grilles. This configuration changes the airflow pattern and NCl₃ transport, promoting mixing in the lower zone. Such differences in ventilation geometry should be considered when comparing results, as they directly affect contaminant dispersion and removal efficiency.

Measurements of CO₂ and NCl₃ [38] showed that the dynamics of CO₂ are similar to those of NCl₃. Therefore, measurements of gas-phase CO₂ may be effective for controlling NCl₃ concentrations. However, this statement requires further investigation.

Lee and Blatchley [4] observed that modifications to the water treatment system led to improvements in water chemistry, including a decrease in liquid-phase trichloramine concentrations. These changes, however, did not have a significant impact on gas-phase trichloramine levels, which were mainly dependent on the number of swimmers and the intensity of pool use. Thus, we did not perform separate analysis regarding pool water chemistry and focused on air quality.

The present study lasted six days. In the literature, measurements in ref. [4] were conducted for only four days, while in [42] they lasted one week, and in ref. [16] they covered several consecutive days within a week.

To address the issue of increased pollutant concentrations during intensive use, the application of variable-air-volume (VAV) ventilation systems controlled in real time based on CO₂ concentration appears to be an effective solution. Ideally, ventilation should respond to trichloramine levels, but due to the difficulty of direct measurement, CO₂ monitoring can serve as a practical proxy. Recent studies confirm the effectiveness of such control strategies in terms of both air quality and energy efficiency [16,17,48]. The air distribution system and ventilation configuration are also important. Similar relationships between user activity and DBP emissions have also been reported in other works. In ref. [49] authors noted that the penetration of chloramines into the air can be exacerbated by poorly organized ventilation near the water surface, while proper airflow direction and decentralized exhaust systems in the pool zone can effectively reduce exposure to volatile chlorination by-products and lower their concentrations in the hall air away from the water surface (in the vicinity of structural components). In ref. [50], a CFD analysis showed that malfunctioning ventilation can lead to zones with limited air exchange, promoting condensation and local increases in humidity. The authors highlighted that reduced air supply compared to the design values increases negative pressure, disturbing airflow distribution and causing vapor condensation on roof and façade elements. Under such conditions, as observed in this study, high concentrations of chlorine compounds in air may accelerate material degradation.

The hygiene of swimmers determines the quality of pool water [51], which in turn affects air quality. Contaminants introduced into the water by swimmers react with chlorine and form disinfection by-products (DBPs) [51,52]. The authors of [53] demonstrated that the condition of the ventilation system, the age of the facility, and the quality of tap water are also correlated factors. UV treatment has a positive effect on reducing trichloramine. One way to improve conditions related to the concentration of free chlorine is to reduce the amount of chlorine used for water treatment. In ref. [15], it was shown that regulations in Taiwan influence the level of NCl₃, where the concentrations achieved were lower than in other studies. Apart from ventilation, the parameters of the pool water itself have a significant impact on the indoor air quality of the pool hall. By applying this water treatment method, the amount of chlorine used can be reduced. The reduction in DBPs is also possible through the removal or limitation of their precursors (including urea, ammonium ions, and creatinine). This has been confirmed in ref. [54,55], where urea and uric acid were identified as the main precursors responsible for the formation of trichloramine and cyanogen chloride (CNCl), respectively, in swimming pools. An effective preventive measure is to ensure that swimmers take a shower before entering the pool, which has been shown to reduce the concentration of trihalomethanes (THMs) in pool water by 27% [8,51]. The intensity of pool basin use is also a factor affecting the rate of NCl₃ transfer from water to air. This was confirmed in ref. [54], where the process of water–air exchange was found to be relatively slow, taking approximately 20 h or 5.8 days (for rough and smooth surfaces, respectively) in the case of a still water surface.

The air quality in the swimming pool hall varies depending on the location within the pool area [40,56]. The authors of ref. [40] demonstrated that the location of air supply devices significantly affects the ability to achieve the desired indoor parameters. The best results under high occupancy conditions were obtained when the air supply nozzles were positioned as close to the spectator stands as possible. In the present study, the influence of spectators was also observed, among other things, on the concentration of carbon dioxide, which increased even when the number of swimmers was low. In the study conducted by the authors [56] the applied ventilation strategy did not ensure uniform air exchange conditions across all analyzed locations within the pool hall. Moreover, the supply air velocity was identified as a factor contributing to the reduction in air exchange rates above the pool surface.

From the standpoint of material durability, the results emphasize the need for proper material selection and regular monitoring of both air quality and the technical condition of metallic components. Long-term exposure to chlorine compounds, even at moderate concentrations, may initiate localized corrosion [8,13,14,15]. As noted in ref. [17], the presence of chlorine and chloramines in pool hall air, combined with high humidity (60–70%), favors the formation of acidic condensates on metallic surfaces, leading to pitting and stress corrosion. In their study, 87% of elements made of steels 1.4301 and 1.4401 exhibited degradation, while steel 1.4529 remained fully resistant after five years of operation. These findings confirm that the durability of pool structures strongly depends on air quality and the presence of chloramines. Although the analyzed facility (four years in operation) showed no visible signs of degradation, the measured NCl₃ levels indicate that such processes may develop over time if ventilation and environmental control are not properly maintained.

Airborne trichloramine is a significant factor contributing to the degradation of construction materials in swimming pool facilities. Corrosion processes are strongly affected by material selection and surface topography, as reported in the literature [33,57]. Most studies investigate the effects of chlorine in aqueous solutions, which, when combined with increased humidity, contributes to pitting corrosion of materials used in swimming pool halls [10]. While the impact of volatile chlorine compounds, including trichloramine, has not been extensively investigated in the literature, it is recognized that these compounds can adsorb onto material surfaces and accelerate corrosive degradation. The studies indicated that trichloramine levels rise significantly in response to pool occupancy, with peak concentrations occurring during swimming competitions. This environment can enhance the aggressive chemical effects on the surfaces of swimming pool materials. The intensification of corrosion processes is influenced by surface topography. As surface roughness increases, the effective contact area between the material and the corrosive environment also increases [58]. However, corrosion processes inherently increase surface roughness, thereby enlarging the effective contact area with the corrosive environment [59]. Furthermore, surface valleys and micro-defects, such as cracks, dents, or pores, enhance the adsorption and retention of trichloramine, especially in the presence of moisture, creating localized environments highly susceptible to corrosion. Additionally, the combined presence of trichloramine, moisture, and mechanical stresses around topographic irregularities can lead to stress corrosion cracking [60]. Therefore, smooth surfaces without defects are recommended for the construction of swimming pool facilities.

Surface topography is closely linked to wettability, with both factors affecting the corrosion behavior of materials used in swimming pool construction [59,61]. Higher topographic complexity combined with low surface free energy promotes hydrophobicity, whereas smooth surfaces with higher surface free energy tend to be hydrophilic [62]. Surfaces with high wettability tend to retain a liquid layer containing chloride ions or chloramines, thereby increasing both the duration and the extent of contact between the corrosive medium and the material, which facilitates corrosion. Conversely, hydrophobic surfaces with low wettability limit the retention of liquids that react with trichloramines, thereby exhibiting improved corrosion resistance [63].

Literature studies indicate that pitting and stress corrosion are the primary degradation mechanisms affecting stainless steels used in swimming pool facilities [10]. Airborne trichloramines, in combination with moisture and chloride ions, create an aggressive oxidizing environment that enhances the degradation of the passive layer on metal surfaces. This degradation is particularly pronounced in austenitic stainless steels, leading to the initiation of pitting corrosion, which predominantly occurs at topographic irregularities. The chemical composition of steel is a key factor influencing corrosion resistance. Increased contents of molybdenum (Mo), chromium (Cr), nickel (Ni), and nitrogen (N) improve corrosion resistance to environments containing trichloramine. In particular, high-alloy austenitic steels (enriched in Mo, Cr, Ni, and N) exhibit high corrosion resistance. Additionally, duplex steels with an austenitic–ferritic structure combine high corrosion and mechanical resistance and provide protection against stress corrosion cracking [64]. Materials characterized by improved corrosion resistance are generally associated with higher economic investment. Therefore, the selection of construction materials for swimming pool facilities should consider not only initial investment costs but also long-term performance and corrosion resistance.

Considering the long-term exposure of swimming pool materials to airborne trichloramines in a humid environment is crucial for the design and assessment of the durability of swimming pool facilities. Selecting appropriate construction materials and optimizing surface topography can markedly enhance corrosion resistance, ultimately improving structural durability and overall safety. In addition to material selection and surface characteristics, designing efficient ventilation systems that minimize the concentration of aggressive volatile compounds in the pool is crucial for reducing trichloramine levels.

In summary, the conducted research demonstrates that while the ventilation system effectively maintained stable thermal conditions, it did not always prevent the accumulation of chlorinated by-products during periods of intensive use. Future studies should couple air quality monitoring with material degradation analyses to develop design and operational recommendations that enhance the long-term durability of indoor swimming pool infrastructure.

Limitations

This study was carried out in one indoor swimming pool facility, which limits the possibility of transferring the results directly to other types of pools or ventilation systems. Measurements were conducted during a specific period (November), so seasonal variations in air quality and ventilation efficiency were not captured.

The monitoring focused on four key air parameters: temperature, relative humidity, CO₂, and trichloramine. Other disinfection by-products, such as trihalomethanes, were not analyzed.

Future studies should include longer monitoring periods covering different seasons and pool types, combined with parallel water and air analyses to better describe the behavior and release of chlorine by-products under varying operating conditions. Future research can additionally be expanded to include a comprehensive Life Cycle Assessment (LCA) approach, which would enable a deeper understanding of the environmental impact of such systems throughout their entire life cycle. The application of LCA methods in the analysis of devices designed to maintain air quality in swimming pools will allow for the assessment of their overall environmental impact. This analysis would consider not only local pollutant emissions but also the carbon footprint associated with their operation [65].

5. Conclusions

• The study confirmed a clear relationship between the number of users and indoor air quality at the pool. Along with increasing occupancy, higher concentrations of carbon dioxide (CO₂) and trichloramine (NCl₃) were observed, while air temperature and relative humidity remained stable throughout the measurements.
• Trichloramine concentrations exceeded the recommended limit of 500 µg/m³, reaching up to 1167 µg/m³ during swimming competitions. This suggests that standard ventilation operation may not be sufficient during high-intensity use. Ventilation systems with variable airflow rates, controlled by current CO₂ or NCl₃ levels, could help maintain air quality under changing load conditions. During competition it is worth using only fresh air instead of recirculated air that contains trichloramine.
• Elevated concentrations of volatile chlorine compounds, combined with high temperature and humidity, create an aggressive environment that accelerates the corrosion of stainless steel and other construction materials. The results underline the importance of proper material selection, corrosion protection, and regular inspection of structural elements exposed to such conditions.
• The findings provide practical input for engineers and facility operators involved in the design and management of indoor pools. Considering competition periods as critical operational scenarios is essential for selecting materials, planning ventilation strategies, and ensuring the durability and safety of pool structures.