Aromatic Volatile Organic Compounds in Croatian Domestic Environments: Initial Findings

Abstract

This study aimed to investigate the concentrations of BTEX (benzene, toluene, ethylbenzene, ortho-xylene (o-), and meta and para-xylene (m-,p-)) in Croatian households between December 2023 and January 2025. The results showed that BTEX concentrations were higher indoors than outdoors, suggesting a considerable contribution from indoor sources. Significant statistical differences were found between indoor and outdoor levels of ethylbenzene, m-,p-xylene, and o-xylene, especially during cold periods when indoor activities increase and ventilation decreases. Spearman’s correlation analysis showed weak correlations between benzene and other BTEX compounds, implying multiple distinct sources such as cooking, smoking, and outdoor air infiltration.

1. Introduction

Volatile organic compounds (VOCs) are important air pollutants with a noticeable effect on indoor air quality (IAQ) and human health. The Environmental Protection Agency states that VOCs evaporate rapidly at room temperature and have boiling temperatures from 50 to 250 °C. Because of their volatility and comparatively low molecular weight, they are found in the atmosphere as gases [1]. VOCs are a major concern for IAQ, as they are associated with long-term health risks, and people spend over 90% of their time indoors [2]. Previous studies have shown that indoor VOC concentrations are influenced by both indoor sources, such as the use of consumer product and tobacco smoke, and outdoor sources, as well as building factors, such as an attached garage or building materials [2]. Research also shows that houses with mechanical ventilation had higher ventilation rates and lower VOC concentrations compared to homes with natural ventilation [1,2]. Yang et al. [3] and Madureira et al. [4] found that mechanical ventilation systems are associated with lower VOC levels, emphasizing the importance of air exchange. Studies have further shown that some VOCs are responsible for health issues such as headaches, asthma, eye and skin irritation, and exhaustion in homes with insufficient ventilation [5,6]. Additionally, VOCs have the capacity to induce oxidative stress in the body and damage macromolecules, including DNA. Oxidative stress can subsequently harm DNA, lipids, proteins, and other biological constituents. Chronic DNA damage can compromise the genome’s integrity, increasing the risk of chromosomal abnormalities and mutations. This instability may also play a role in the emergence of some noncommunicable disorders, such as cancer [7].

Monoaromatic hydrocarbons known as BTEX (benzene, toluene, ethylbenzene, ortho-xylene (o-), and meta and para-xylene (m-,p-)) are categorized as VOCs. In countries with a temperate climate, such as Croatia, and marked temperature differences between the seasons, warming can affect BTEX concentrations both indoors and outdoors. Emissions from vehicle exhausts further increase concentrations in the air [8]. Consequently, indoor air concentrations can increase during the winter months due to a combination of higher outdoor concentrations, reduced ventilation, and indoor pollution from incomplete combustion processes, such as home heating, showing a similar pattern to previously analyzed polycyclic aromatic hydrocarbons (PAHs) [9]. As a result of the high number of cars and large companies situated in urban areas, VOC concentrations in the urban atmosphere have remained extremely high in some areas over the years [10,11]. Urban areas, where natural gas is generally used, had the lowest average concentrations of PAHs, while rural areas, where mainly heating oil is used, had the highest average values [6]. Lai et al. [12] collected indoor and outdoor two-day samples into adsorption tubes and analyzed them by gas chromatography coupled with mass spectrometry. Samples were collected during all seasons from 1996 to 2000 in six European cities (Athens, Basel, Helsinki, Milan, Oxford, and Prague). Benzene levels were higher in the Mediterranean region than in the northern part of Europe. The results show that Mediterranean cities had higher temperatures, which caused more benzene evaporation. In these cities, natural ventilation was used, whereas mechanical ventilation was more common in northern Europe [13].

Due to BTEX toxicity, the air concentrations of some of these compounds, especially benzene, are limited by legislation (Directive 2024/2881/EC). However, there are no regulations for the amount of these VOCs indoors. Building materials such as paints and varnishes, consumer products such as nail polish, cleaning agents, and human activities such as smoking are some indoor sources of these chemicals [14]. The main outdoor sources include traffic, petrol stations, and nearby petrochemical or oil refineries [15,16].

The World Health Organization (WHO) has created IAQ guidelines based on health [17]. Using a list developed as part of the INDEX project [18], to critically evaluate the development and implementation of indoor air quality limits, the European Commission (EC) initiative initially ranked various substances in order of priority. Based on INDEX’s assessment of the health significance of the substances, they were categorized into three groups. Toluene and o-xylene are considered low-priority pollutants, while benzene is one of the five priority pollutants due to its toxicity.

VOC concentrations in Zagreb, Croatia have so far only been measured in outdoor air, where the average daily concentrations for benzene, toluene, and xylene isomers were 2.5 μg m⁻³, 4.7 μg m⁻³, and <1 μg m⁻³, respectively [19]. Currently, there are two air quality monitoring stations with continuous measurements of benzene that show mass concentrations below the limit values set by Croatian and European legislation (5 µg m⁻³ for an annual average) [20]. On the other hand, indoor air measurements in Croatia in the last thirty years have been performed only for benzene in schools [21].

This study aims (i) to determine for the first time indoor and outdoor BTEX concentrations in Croatian households, (ii) to investigate the relationships between indoor and outdoor air, and (iii) to analyze seasonal variations of BTEX concentrations during warm and cold periods. The study was part of the Zagreb pilot of the EDIAQI project (Evidence-Driven Indoor Air Quality Improvement) [22].

2. Materials and Methods

2.1. Sampling and Analysis of BTEX

Parallel sampling of outdoor and indoor air was carried out in thirty-two households between December 2023 and January 2025. The households were distributed across Zagreb, Croatia, and their locations are shown on Figure 1. With 767,131 inhabitants, Zagreb is the largest city in Croatia and the capital of the country (Census 2021). On the north side are the Medvednica mountains, and the river Sava flows through the city.

Sampling was carried out once in each household. Air sampling was carried out both outdoors and indoors using multibed tubes (Markes International Ltd., Bridgend, UK) filled with a porous polymer, graphitized black carbon, and a carbonized molecular sieve. Before sampling, the tubes were conditioned with a continuous flow of high-purity helium carrier gas and stored in a refrigerator at a temperature below 5 °C. For indoor sampling, the living room was used, at a height of about 1 m above the floor and 1–2 m away from building walls or other obstacles. In Ilgen et al. [23], it was found that outdoor concentrations in urban areas can vary as a function of the height above street level, so the authors decided to sample directly in front of the windows of the monitoring households. This motivated us to use terraces or balconies for the outside air sampling in this study. Sample units were positioned by providing good air circulation around the sampler. BTEX samples were collected for 50 min to 6 h using a mini-pump with a flow rate of 30 mL min⁻¹ to 200 mL min⁻¹, depending on the characteristics of the households. The flow rates of the pump were calibrated before and after sampling using an Ultra-flow Calibrator (SKC Inc., Eighty Four, PA, USA).

BTEX was analyzed by thermal desorption coupled with a gas chromatography and mass spectrometry detector (TD-GC/MS) (Agilent Technologies, Santa Clara, CA, USA). A thermal desorption device (TD) (Markes International, Bridgend, UK) was used as a tool for the GC/MS sampling introduction. The TD is designed to heat the separated components and transfer them to the GC/MS. First, the TD unit was subjected to a thorough leak test. The tubes were conditioned before sampling, and chromatograms of blank samples were recorded. The tubes were then sealed with long-storage brass hexagonal caps with Teflon^® seals secured on both ends, wrapped in aluminum foil, and stored at 4 °C. Upon arrival at the selected households, the long-storage brass analytical caps were removed, and sampling for VOCs was conducted. Then, upon returning to the laboratory, the samples were analyzed within 24 h. Figure S1 in the Supplementary Materials shows the chromatograms of BTEX in the blank (a) and sample (b). The sample underwent purification to eliminate air and moisture, followed by continuous heating to separate the volatile components. These components were cooled and concentrated in a low-temperature cold trap, then reheated and transferred via a heated line to the GC column.

Calibration curves were obtained by spiking conditioned tubes with a reference standard mixture (CRM 4877, Supelco, Sigma-Aldrich, St. Louis, MO, USA). The spiked tubes were heated to 320 °C in the desorber, and a helium flow rate of 50 mL min⁻¹ was applied. The DB-624 UI capillary GC column (6% cyanopropyl/phenyl, 94% polydimethylsiloxane, 60 m, 0.32 mm internal diameter, 1.80 µm film thickness, Agilent Technologies, Santa Clara, CA, USA) was used for BTEX separation. The mass spectrometer was operated in positive electron impact ionization (EI) mode. The working conditions for the transfer line and ion source temperatures were set at 230 °C, and the quadrupole temperature was maintained at 150 °C. A mixed VOC standard of bromochloromethane, 1-bromo-4-fluorobenzene, and chlorobenzene-d5 and 1,4-difluorobenzene (Restek Corporation, Bellefonte, PA, USA) in the gas phase was used as the internal standard. Quantification limits (QLs) were determined to be ten times the standard deviation of ten blank samples, while detection limits (DLs) were set at three times that value. DLs ranged from 0.032 µg m^-3 for ethylbenzene to 0.072 µg m⁻³ for m-,p-xylene. QLs ranged from 0.1 µg m⁻³ for o-xylene to 0.244 µg m⁻³ for m-,p-xylenes.

2.2. Health Risk Assessment

The United States Environmental Protection Agency (US EPA) categorized xylene, toluene, ethylbenzene, and benzene as harmful air pollutants. The International Agency for Research on Cancer (IARC) has classified benzene into Group 1 (carcinogenic to humans), while ethylbenzene is classified into Group 2B (possible carcinogens), and toluene and xylene are classified into Group 3 (non-carcinogenic) [24,25]. Although humans can be exposed to BTEX through ingestion, inhalation, or skin contact, inhalation is the most common route of exposure.

BETX exposure levels can be estimated using the methods described by the US EPA Equation (1):

$$\mathrm{E}=\mathrm{C}\times {\displaystyle \frac{\mathrm{ET}}{24}}\times {\displaystyle \frac{\mathrm{EF}\times \mathrm{ED}}{\mathrm{AT}}}$$

(1)

In this case, E stands for exposure level, C for the concentration of a specific BTEX compound (μg m⁻³), ET for exposure time (h day⁻¹), EF for exposure frequency (days year⁻¹), ED for exposure duration (years), and AT for the average time over years.

A lifetime cancer risk (LCR) can be estimated by multiplying exposure levels by inhaling unit risk (IUR), according to Equation (2):

$$\mathrm{LCR}=\mathrm{E}\times \mathrm{IUR}$$

(2)

IUR values were provided only for benzene and ethylbenzene, at 2.9 × 10⁻⁵ and 2.5 × 10⁻⁶, respectively [26,27,28]. LCR values higher than 1.0 × 10⁻⁴ are considered a definite risk, while values between 1.0 × 10⁻⁵ and 1.0 × 10⁻⁶ are considered a probable risk.

2.3. Statistical Analysis

The STATISTICA 14 software package (TIBCO Statistica, San Ramon, CA, USA) was used for statistical analysis. The results of the measured BTEX mass concentrations were summarized using the median and standard deviation. The statistical analyses were performed separately for indoor and outdoor areas. The normality distribution of BTEX concentrations was assessed by Shapiro–Wilk test. As the data were not normally distributed, paired indoor–outdoor and warm–cold measurements were compared using by Mann–Whitney U test. Spearman regression analysis was performed using the statistical software program R (version 2024.12.1, Posit Software, PBC, Boston, MA, USA) to analyze intercorrelations between indoor and outdoor BTEX levels.

3. Results and Discussion

3.1. Outdoor and Indoor Concentrations of BTEX

The median and mean values of BTEX mass concentrations measured in the 32 Zagreb households, as well as other basic statistical parameters, are shown in

Table 1. Statistical parameters for BTEX in indoor and outdoor air (µg m⁻³).

		Median	Mean	Geometric Mean	Minimum	Maximum	Standard Deviation	25th Percentile	75th Percentile
Indoor	Benzene	2.95	4.35	2.00	0.02	24.77	5.243	1.12	5.15
	Toluene	12.20	22.62	15.34	3.52	88.07	21.844	7.49	27.55
	Ethylbenzene	0.92	1.74	0.92	0.04	15.47	2.836	0.56	1.59
	m-,p-Xylenes	1.29	2.24	1.32	0.11	21.27	3.752	0.71	2.19
	o-Xylene	0.92	1.27	0.72	0.04	10.83	1.895	0.37	1.51
Outdoor	Benzene	1.84	4.80	1.30	0.01	42.84	8.275	0.49	5.68
	Toluene	8.50	19.63	10.37	1.51	64.00	20.016	3.31	42.04
	Ethylbenzene	0.33	2.85	0.54	0.05	48.70	8.846	0.21	1.12
	m-,p-Xylenes	0.53	1.32	0.61	0.07	11.40	2.338	0.32	1.09
	o-Xylene	0.33	1.97	0.47	0.04	36.49	6.448	0.22	0.78

. The median benzene concentration in indoor and outdoor air was 2.95 ± 5.243 µg m⁻³ and 1.84 ± 8.275 µg m⁻³, respectively. Benzene concentrations in outdoor air were in the range of values measured at air quality monitoring stations [20]. The average outdoor concentration (4.80 µg m⁻³) was lower than the limit of 5 µg m⁻³ set by the EU. However, the values cannot be compared directly due to the short monitoring periods used in this study, while the EU limit value refers to an annual average. There is no safe level of indoor benzene exposure, according to the WHO and South African guidelines. However, China and France have established a threshold of ≤0.03 mg m⁻³ for 1 h of exposure and 10 µg m⁻³ for 1 year of exposure, respectively [29]. The primary indoor sources of benzene include tobacco smoke, organic solvents, adhesives, and gasoline. Reported indoor benzene concentrations vary considerably depending on the type and use of the indoor environment, with measured values ranging from 1 to 109 µg m⁻³ across residential buildings, offices, and schools [30].

The concentration of toluene ranged from 3.52 µg m⁻³ to 88.07 µg m⁻³ in indoor air, while the outdoor values ranged from 1.51 µg m⁻³ to 64 µg m⁻³. The outdoor and indoor concentrations varied considerably between individual houses and showed high standard deviations, which is probably due to the locations of the households and different sampling times. The WHO guideline value for toluene (260 µg m⁻³ ) was not exceeded in any of the households. However, it should also be noted that the sampling periods were short, while the WHO guideline value applies to chronic exposure. The China Standards for Indoor Air Quality level of ≤0.20 mg m⁻³ for 1 h of exposure was not exceeded [29]. The common indoor sources of toluene include gasoline, cigarettes, solvents, adhesives, paint, and pest control, with concentrations ranging from 5 to 358 µg m⁻³ in various indoor environments such as schools, homes, offices, and hospitals [30].

However, some households (H6, H2, and H10) had a very high benzene concentrations (17.18 µg m⁻³, 11.26 µg m⁻³, and 24.77 µg m⁻³, respectively). The concentrations of toluene were also higher in these households, at 20.56 µg m⁻³, 22.71 µg m⁻³, and 45.50 µg m⁻³, respectively. A weakness of this study may lie in the fact that it only utilized one quick VOC measurement per residence, which may not be sufficient to account for variations in indoor air concentrations. Indoor VOC levels are known to vary throughout the day and across seasons due to factors such as cooking, cleaning, human activity, ventilation techniques, and variations in outdoor air infiltration. Consequently, a single sample can reflect an unusually high or low point, potentially misrepresenting long-term exposure. Cooking significantly influences indoor VOC levels, with higher cooking temperatures linked to increased emissions of VOCs about 5 minutes after cooking and dispersing more rapidly than particulate matter [31]. Compared to homes using natural gas or electricity, homes that rely on gas, particularly LPG, display higher amounts of harmful VOCs [32]. Studies show that indoor VOC levels can vary based on the day and time of week. Typically, they decrease from morning to evening and drop significantly on weekends due to less human activity and reduced traffic-related emissions [33,34,35,36].

Toluene was the most frequently occurring chemical in all households, followed by benzene. Both indoor and outdoor concentrations of the individual BTEX compounds were in the following descending order: toluene > benzene > m-,p-xylenes > ethylbenzene = o-xylene. Vera et al. [30] reported that indoor sources of o-xylene and m-,p-xylenes included cigarettes, paint thinner, and spray lubricants, with indoor concentrations in various environments ranging from 0.2 to 20.5 µg m⁻³ for o-xylene and from 2 to 88.2 µg m⁻³ for m-,p-xylenes.

Benzene and o-xylene were detected in every household, while ethylbenzene and m-,p-xylene were detected with a frequency of 97%. Toluene was detected in all (100%) households, illustrating its widespread presence in the indoor environment. This was likely due to its extensive use in numerous household products like paints, adhesives, and cleaning products. However, detector saturation prevented accurate quantification of toluene concentrations in five homes. This saturation indicated that the toluene levels exceeded the upper limits of detection for the measurement tool used, which hindered precise numeric reporting. According to these findings, indoor toluene concentrations may be significantly higher in some residential areas, potentially caused by recent remodeling, the use of solvent-based materials, or inadequate ventilation. Data from these households were excluded from the statistical analysis for toluene.

Table 1 shows that BTEX concentrations were higher indoors than outdoors, and a statistical test was performed to determine whether the difference was statistically significant. The normality of BTEX distribution was tested by Shapiro–Wilk test. Mann–Whitney U test was used to analyze the differences between indoor and outdoor concentrations, as the distribution of the variables was abnormal (p < 0.05). The results showed that the differences in toluene and benzene levels indoor and outdoor were not statistically significant. On the other hand, there was a significant difference between indoor and outdoor levels of ethylbenzene (p < 0.05), m-,p-xylene (p < 0.01), and o-xylene (p < 0.05).

Table 2. Indoor and outdoor concentrations of BTEX in European cities (µg m⁻³).

Compound	Indoor	Outdoor	Time Sampling, Type, Year	City, Country	Reference
Benzene a Toluene a Ethylbenzene a o-Xylene a m-,p-Xylene a	2.20 4.25 0.62 0.68 1.36	1.05 2.27 0.57 0.27 0.80	24 h, active, 2001–2002	Antwerp, Belgium	[37]
Benzene b Ethylbenzene b o-Xylene b m-Xylene b p-Xylene b	2.7 2.1 2.2 3.8 1.8		24 h, active, 2014–2019	Luxembourg	[38]
Benzene c Toluene c Ethylbenzene c o-Xylene c m-,p-Xylene c	1.48 20.46 0.70 0.79 2.92	1.13 4.46 nd nd 1.20	1 week, passive, November/December 1994	Hamburg, Germany	[8]
Benzene c Toluene c Ethylbenzene c o-Xylene c m-,p-Xylene c	2.17 37.29 1.67 1.20 4.17	1.62 4.98 nd nd 1.76	1 week, passive, November/December 1994	Erfurt, Germany
Benzene b Toluene b m-,p-Xylene b	3.0 15.1 3.8		4 weeks, passive, October 1997–February 1999	1000 homes throughout the United Kingdom	[39]
Benzene b	8.2 2.2 1.7 10.1 2.8 6.5		2 days (non-working hours), active, 1996–2000	Athens, Greece Basel, Switzerland Helsinki, Finland Milan, Italy Oxford, UK Prague, Czech Republic	[12]
Benzene c Toluene c Ethylbenzene c o-Xylene c m-,p-Xylene c	1.7 6.3 2.7 0.4 4.7	1.3 2.3 0.4 0.06 1.2	2 weeks, passive, May–June 2011	Puertollano, Spain	[40]
Benzene a Toluene a Ethylbenzene a o-Xylene a m-,p-Xylene a	1.6 9.4 2.4 3.1 7.6		10 days, passive, October–November 2017	Aveiro, Portugal	[41]

^a average value; ^b geometric mean; ^c median; nd—not detected.

shows BTEX concentrations indoor and outdoor in various European cities. The indoor values refer to households. Benzene concentrations found in Croatian households were similar to indoor concentrations found in houses in the United Kingdom, where samples were collected by passive sampling over 4 weeks.

These levels were also slightly higher than those in Luxembourg and Antwerp, Belgium, over 24 h [37,38,39]. The mean concentrations for 10 days measured during October and November in households in Portugal were lower than in this study [41]. The geometric mean toluene concentrations measured over 4 weeks by passive sampling in 1000 houses throughout the UK in the aforementioned study were slightly higher than those reported in this study [39], while the mean indoor toluene concentration in Portugal was slightly lower [41]. In contrast, toluene levels in Erfurt and Hamburg, Germany, during a 1 week period, were significantly higher [8] (Table 2) than in this study. Only in Belgium was the indoor concentration of toluene lower, measuring 4.25 µg m⁻³ [37]. The ethylbenzene level measured in this study was similar to the concentrations found in Belgium [37] and Hamburg, Germany [8], but lower than in Luxembourg [38], Erfurt [8], and Portugal [41]. m-,p-Xylene levels were similar to those found in the study by Stranger et al. [37] and much lower than in studies by Alvarez-Vaca et al. [38], Raw et al. [39], Schneider et al. [8], and Alves et al. [41]. The concentration of o-xylene was comparable to levels found in Belgium and Hamburg [8,37], and slightly lower than in Erfurt [8] and Luxembourg [38]. Outdoor concentrations of benzene and o-xylene were similar to concentrations in Erfurt and Antwerp, respectively [8,37]. Toluene in the ambient air in Croatia was much higher (8.5 ± 20.016 µg m⁻³) than in other European cities (Table 2).

Although the results of this study were compared with similar research from other regions, it is essential to acknowledge significant differences in sampling duration, the season during which sampling was conducted, the types of indoor environments (households—different room types, offices, schools), and the analytical methods used. Comparisons should be made carefully, because these variations can have a big impact on the concentrations measured and the structure of the observed characteristics. Despite these drawbacks, our findings are consistent with VOC levels seen in comparable dwellings in previous research, suggesting that our data nevertheless offer a useful, if brief, overview of indoor air quality.

3.2. The Relationship Between Indoor and Outdoor Concentrations

Outdoor BTEX concentrations are a significant contributor to indoor air quality, so BTEX levels were measured on terraces or balconies to better assess the infiltration of pollutants into indoor environments, particularly during ventilation (e.g., open windows). Ilgen et al. [23] reported that outdoor BTEX concentrations in rural areas at three measuring stations were similar, while in urban areas, concentrations exceeded those in rural areas by 9 times for benzene, 12 times for toluene, and 15 times for isomeric xylene. To better understand how outdoor air impacts indoor air, they decided to measure outdoor air in front of the windows of households. The ratio of indoor (I) to outdoor (O) BTEX concentrations shows how indoor and outdoor sources are related and is an efficient tool for comparing measured concentrations. If the I/O ratio is low, a significant contribution from outdoor pollution can be assumed, while I/O ratios higher than 1 indicate domination of indoor BTEX sources. In a typical area, motor vehicle emissions were found to be the main source of BTEXs [42]. Figure 2 displays the indoor/outdoor BTEX ratios (box-whisker plot) determined in this study. The median ratios of indoor to immediate outside were 1.3, 1.4, 2.0, 1.9, and 1.6 for benzene, toluene, ethylbenzene, m-,p-xylene, and o-xylene, respectively. The I/O ratios were higher than 1, indicating that there may be other indoor sources besides outdoor air infiltration, such as emissions from cooking, the type of household heating, or the use of consumer products. According to studies by Edwards et al. [43], Lagoudi et al. [42], and Yu and Kim [44], building materials and cleaning agents are the main sources of VOCs indoors. Although the microenvironment inside a house may not have significant emission rates of gaseous pollutants, the proximity of sources and the obstruction of dispersion would result in indoor levels being higher than those immediately outside. This is particularly important when it comes to identifying the cause of outliers. Although 75% of the I/O values for all compounds were below 10, some outliers were observed, particularly for benzene. Two households had very high I/O values (190.4 and 831.4), which can be explained by the smoking habits of the household inhabitants and the frequent use of some types of thinners.

According to WHO [45], rural areas had higher I/O ratios (2.52) than urban areas, suggesting that traffic density is not the main source of indoor benzene emissions. In Asian countries with high traffic emissions, I/O ratios close to 1 (0.96–1.10) were reported [45]. In rural areas with indoor sources of benzene, such as kerosene and fuel appliances, the I/O ratio may exceed 3.

Spearman correlation analysis was performed using the statistical program R to examine the intercorrelations between BTEX compounds separately for indoor and outdoor settings (Figure 3). Red signifies a positive correlation, while blue indicates a negative correlation. The intensity of the colors is proportional to the correlation coefficient. As for indoor BTEX concentrations, a positive correlation was found between all xylenes and ethylbenzene and between benzene and all of the other compounds, while toluene correlated negatively, but not significantly, with ethylbenzene and xylenes. The most significant positive correlation was found between ethylbenzene and o-xylene, with a correlation coefficient (r) of 0.95, and between ethylbenzene and m-,p-xylenes (r = 0.78), which is in agreement with the results of Esplugues et al. [16]. The m-,p-xylenes correlated very well with o-xylene, with a correlation coefficient of 0.84, suggesting that these compounds have similar common sources. Similar results for correlations among BTEX compounds were found in a study by Esplugues et al. [16] in Spanish households. For outdoor BTEX concentrations, the correlations were positive, except between toluene, ethylbenzene, and toluene and o-xylene, where the correlation was negative but not significant (r < −0.2). A significant correlation was found between ethylbenzene and o-xylene (r = 0.93), between ethylbenzene and m-,p-xylenes (r = 0.58), and between m-,p-xylenes and o-xylene, with a correlation coefficient of 0.63.

Spearman regression analysis was also performed between indoor and outdoor BTEX pairs (Figure 3). Linear regression revealed a statistically significant (p < 0.05) positive correlation between indoor and outdoor pairs for ethylbenzene and xylenes (correlation coefficients between 0.42 and 0.45) and a weak positive correlation between indoor and outdoor benzene concentrations, while no correlation was found for toluene. In addition, some other indoor–outdoor relationships were found, such as the positive correlation between indoor ethylbenzene and outdoor o-xylene, with a correlation coefficient of 0.53, between indoor o-xylene and outdoor ethylbenzene (correlation coefficient 0.38), between m-,p-xylenes indoors and o-xylene outdoors and ethylbenzene outdoors (correlation coefficients 0.38 and 0.30, respectively), and between benzene indoors and ethylbenzene outdoors and benzene indoors and o-xylene outdoors (correlation coefficients 0.37 and 0.36, respectively). This finding indicates that there are different indoor and outdoor sources of toluene. Benzene exposure in indoor air increased due to outdoor air infiltration, and additional indoor sources such as smoking, heating and cooking appliances, solvent-based paints, renovation work, and new particleboard materials [46]. Similar correlation coefficients for ethylbenzene and o-xylene between air pairs were found study by Fuselli et al. [47].

The contribution percentage of each compound to the total BTEX level is shown in Figure 4. Toluene accounted for around 67% indoors and 60% outdoors. The higher contribution of toluene in indoor spaces may have been due to its presence in thinners, varnishes, and adhesives [45]. The other compounds contributed between 4% and 6% indoors and between 4% and 10% outdoors, except benzene, which accounts for 15% indoors and 17% outdoors. Toluene is the dominant compound indoors and outdoors, followed by benzene, which is more dominant in outdoor air, because it is the main marker of fuel combustion [48]. m-,p-xylenes were the third dominant compound indoors, at concentrations twice higher than outdoors, as they are found in varnishes, coatings, air fresheners, household cleaners, disinfectants, and cosmetic solvents [45]. In outdoor air, m-,p-xylenes are relatively rapidly degraded by photochemical reactions with hydroxyl radicals, typically having lifetimes of several hours [49]. The ethylbenzene present outdoors is usually a consequence of traffic, while indoor sources of this compound are significantly more limited [50]. The percentage contributions of o-xylene were about twice as high in outdoor air compared to indoor air, which is explained by their dominant emission from traffic, as well as their lower presence in typical indoor products (paints, varnishes) [51].

3.3. Seasonal Variations in Indoor and Outdoor BTEX Concentrations

Of 32 air samples, 10 were taken during a warm period (May–July 2024) and 22 during cold periods (December 2023, January–April 2024, January 2025). The average temperature and relative humidity during the cold periods were 12.6 °C and 57%, respectively. In the warm period, the temperature was higher, reaching 21.3 °C, and the relative humidity was 67.3%. Based on this, seasonal variations in indoor and outdoor concentrations were analyzed. The median values for cold and warm periods, the 75^th and 25^th percentiles, and non-outlier minima and maxima are shown in Figure 5. The highest outdoor and indoor benzene concentrations occurred during cold periods, although the median indoor concentrations were similar. Outdoor concentrations of toluene were generally lowest during the cold periods, while indoor concentrations were higher during the same period. Ethylbenzene was present at low concentrations both outdoors and indoors during the warm period, but much higher concentrations were found both outdoors and indoors during the cold periods, which also showed much greater variability between sites. Concentrations of m-,p-xylenes and o-xylene were also elevated indoors and outdoors during the cold period. Statistical analysis revealed significant seasonal differences for indoor concentrations of ethylbenzene (p = 0.03) and o-xylene (p = 0.02), but no seasonal differences for outdoor concentrations.

During both periods, the indoor concentrations of ethylbenzene and xylenes were higher than the outdoor values. In the warm period, there were no significant differences between indoor and outdoor concentrations for any BTEX compounds, while in the cold periods, differences were found for m-,p-xylenes (p = 0.01) and o-xylene (p = 0.04).

The ratio between indoor and outdoor concentrations was calculated separately for both periods. Figure 6 displays the BTEX ratios between indoor and outdoor (box-whisker plot) during two periods (warm and cold). In the warm period, the median values of the I/O ratio were 2.65, 0.88, 1.80, 1.76, and 1.28 for benzene, toluene, ethylbenzene, m-,p-xylenes, and o-xylene, respectively. With the exception of toluene, all values were greater than 1, suggesting that toluene from outdoor sources can have a substantial impact on indoor air quality. The I/O values for benzene in the cold period were marginally lower than in the warm period, measuring 1.29. In contrast, the other BTEX compounds had higher median I/O levels in the cold period, at 1.64, 2.21, 2.10, and 1.63 for toluene, ethylbenzene, m-,p-xylenes, and o-xylene, respectively. The I/O ratio also showed higher variability during the cold periods for all compounds except for benzene. A similar finding that the I/O ratio is generally greater than 1, especially in the colder months when indoor sources are more common and ventilation occurs mainly during the day, was found in a study by Ferrero et al. [46].

Spearman regression analysis was also performed for the BTEX concentrations indoors and outdoors during both cold (C) and warm (W) periods (Figure 7). During the cold period, linear regression analysis of indoor concentrations revealed a statistically significant (p < 0.05) positive correlation between Ethylbenzene_C and m-,p-Xylenes_C, with a correlation coefficient of 0.72. A positive correlation was found for their outdoor concentrations, although it was not as significant, at r = 0.47. The strongest correlation was observed between indoor Ethylbenzene_C and o-Xylenes_C (r = 0.94), followed by m-,p-Xylenes_C and o-Xylene_C, with a coefficient of 0.82. For outdoor measurements, the correlation between Ethylbenzene_C and o-Xylene_C was very strong (r = 0.92). In contrast, the correlation between m-,p-Xylenes_C and o-Xylene_C was moderate but significant. A weak correlation (0.2 < r < 0.4) was found between Benzene_C and other BTEX compounds during both periods. During the warm period, Toluene_W showed a moderate positive correlation with Benzene_W, with a correlation coefficient of 0.57 indoors and 0.37 outdoors. The correlation of Toluene_W with other compounds was negative outdoors (correlation coefficients between −0.30 and −0.38), while there were no significant correlations indoors. A strong, significant correlation was found indoors between Ethylbenzene_W and m-,p-Xylenes_W (r = 0.95), and with o-Xylene_W (r = 0.97). The outdoor correlation coefficients were 0.93 and 0.88 for Ethylbenzene_W and m-,p-Xylenes_W, and Ethylbenzene_W and o-Xylene_W, respectively. In addition, strong correlations were found between Xylenes_W (m-,p- and o-), both indoors and outdoors, suggesting that these compounds originate from similar dominant sources, which may be traffic-related during the warm period. Benzene concentrations showed only very weak correlations (r < 0.2) with other BTEX concentrations in both environments. This result indicates that benzene originates from different indoor and outdoor sources. The benzene concentration in indoor air increased due to the infiltration of outdoor air and other indoor sources, such as smoking, cooking, and the use of heating appliances.

The percentage contribution of the individual compounds to total BTEX concentration was calculated for cold and warm periods separately and is shown in Figure 8. For indoor conditions during both periods, toluene accounts for about 67%, followed by benzene, at 13% during the cold period and 22% during the warm period. The relative increase in the contribution of benzene during the warmer period can be attributed to enhanced ventilation in spaces, which facilitates the entry of benzene from external sources, primarily traffic [52]. The other compounds indoors had a higher proportion during the cold period than during the warm period. Reduced ventilation with cooler indoor temperatures can be the cause of higher indoor BTEX concentrations, because it promotes the buildup of indoor emissions from sources like construction materials and home cleaners [53]. Toluene’s contribution to outdoor habitats varied significantly with the season, accounting for 53% in the cold period and 84% in the warm period. Because of conditions like high temperatures, low humidity, and strong sunlight that promote atmospheric oxidation reactions, toluene’s contribution is larger during the warm period [54]. Benzene was the second most dominant compound, at 20%, during cold conditions, but its proportion decreased to around 11% during the warm period. A much higher contribution was observed for ethylbenzene during the cold season (13%) compared to the warm period, when the contribution decreased to 1%. During warmer periods, higher radiation and temperature accelerate the chemical oxidation of VOCs by OH radicals, resulting in decreased ambient concentrations [35,55].

3.4. BTEX Risk Assessment

Lifetime cancer risk was estimated for benzene and ethylbenzene in adults (70 years), assuming they spend 10 h per day indoors and 4 h outdoors over 350 days in 1 year. The exposure levels and LCRs obtained in this study are shown in

Table 3. Exposure levels (µg m⁻³) and LCRs indoor and outdoor during warm and cold periods.

		Indoor		Outdoor
		Warm	Cold	Warm	Cold
Benzene	E	1.116	1.245	0.322	0.295
	LCR	3.24 × 10−5	3.61 × 10−5	9.35 × 10−6	8.54 × 10−6
Overall	E	1.179		0.294
	LCR	3.42 × 10−5		8.53 × 10−6
Ethylbenzene	E	0.253	0.516	0.043	0.069
	LCR	6.32 × 10−7	1.29 × 10−6	1.09 × 10−7	1.72 × 10−7
Overall	E	0.368		0.053
	LCR	9.19 × 10−7		1.32 × 10−7

.

Benzene and ethylbenzene exposure levels were higher indoors than outdoors for both periods. Indoor benzene exposure levels during both periods were similar but slightly higher in the cold. The overall level was 1.179 µg m⁻³, while outdoor levels showed the opposite trend, with slightly higher exposure levels during the warm period of 0.322 µg m⁻³. The LCR obtained in this study was higher indoors than outdoors but similar during both periods. Indoor LCR values fell between 1.0 × 10⁻⁵ and 1.0 × 10⁻⁶, indicating a probable risk, while outdoor LCRs were at an acceptable risk level of 1 × 10⁻⁶.

Ethylbenzene indoor exposure levels were much lower during the warm period compared to the cold period. The values were 0.253 µg m⁻³ during warm periods and 0.516 µg m⁻³ during cold periods. The overall level was 0.368 µg m⁻³. Outdoor levels show a similar trend but are much lower. The overall outdoor exposure level for ethylbenzene was 0.053 µg m⁻³. The LCR for ethylbenzene was one magnitude lower than that for benzene, indicating that ethylbenzene does not pose a cancer risk, even outdoors or indoors, during either period. These results aligned with those reported by Villanueva et al. [40], who studied adults in a living room in Spain, as well as in a non-smoker’s home in Hong Kong [56].

4. Conclusions

This study presents the first comprehensive dataset on BTEX concentrations in residential indoor environments in Croatia. Among the compounds analyzed, toluene emerged as the most dominant species across all of the sampled households, accounting for up to 84% of the total BTEX concentration in outdoor air and approximately 67% in indoor air. The second most common compound was benzene, which is especially noteworthy considering its well-established toxicity and designation as a human carcinogen. Correlation analysis using Spearman’s rank coefficient indicated weak associations between benzene and the other BTEX compounds. This suggested the influence of multiple and possibly independent emission sources, such as combustion from cooking, tobacco smoke, and infiltration of polluted outdoor air. The low correlation further implies that benzene concentrations may not have been governed by the same sources or indoor dynamics as the other BTEX species. For ethylbenzene, m-,p-xylene, and o-xylene, statistical comparisons of concentrations indoors and outdoors showed notable variations, particularly during the heating season. These differences are probably caused by a greater number of indoor activities and less ventilation in the winter, which encourages the buildup of BTEX compounds indoors. Indoor exposure to benzene and ethylbenzene was higher than outdoor exposure, and LCR values indicated that benzene posed a likely cancer risk. Regardless of the season or surroundings, ethylbenzene levels stayed low and did not suggest a substantial risk of cancer. This study does not provide daily or seasonal patterns, although it offers a helpful summary of the prevalence of BTEX and their potential sources. To improve the accuracy of exposure assessments and better capture variability, future studies should employ longer sampling durations or perform repeated sampling, along with standardizing sampling times and analysis methods.