1 Introduction

Over the past few decades indoor air quality has become a major concern among researchers studying the effects of exposure to different pollutants in occupational and non-occupational environments on human health & wellbeing, which is not surprising as people tend to spend the majority of their time in indoor environments rather than outdoors [1,2,3].

The workforce of various industries are exposed to a variety of chemical pollutants including gases, vapors, solid and liquid particulate matter on a daily basis [3,4,5,6,7,8,9]. The extent of resulting adverse health effects ensuing from exposure to such pollutants is dependent on the specific type of the chemical, exposure duration, exposure route and chemical concentration. Among different pollutants, Volatile Organic Compounds (VOC) are one of the most important pollutants found in the air of indoor environments and the associated adverse health effects following exposure to these compounds have been investigated in myriad studies. Several studies have reported adverse health effects such as nausea, throat irritation, fatigue and dizziness despite exposure to low concentrations of VOCs [1, 8]. Used as an anti-knock agent in gasoline, VOCs are also used as solvents in many different occupations and industries including vehicle repair and maintenance, paint shops and paint and varnish workshops [5,6,7,8,9,10,11,12]. Amongst different compounds in the VOC category, benzene, toluene, ethylbenzene and xylenes (usually referred to as BTEX) have been the focus of a number of studies not only due to high volatility, well-studied high health risks and classification as hazardous air pollutants (HAP), but also their role in secondary pollutant generation. Owing to high volatility, inhalation is considered the main exposure pathway of said compounds, although dermal exposure is also possible. Being classified as human carcinogen (group 1) and potential human carcinogen (group 2b) by the International Agency for Research on Cancer (IARC), the health effects of chronic and acute exposure to benzene and ethylbenzene are well-known. Multiple studies have linked increased rate of birth defects, anemia and leukemia cases to high occupational benzene exposure while eye and throat irritation, dizziness and effects on kidney are associated with exposure to ethylbenzene. Although not considered carcinogenic compounds, various adverse health effects such as effects on respiratory system, short-term memory impairment and central nervous system depression have been reported as a result of exposure to toluene and xylenes. Jo and Song [13] reported highly elevated aromatic VOC levels among individuals with potential occupational exposure to VOCs such as traffic police, parking garage attendants, service station attendants, roadside storekeepers and underground storekeepers. Fandi, Jalaludin [14] submitted similar results, reporting exacerbated risk of adverse health effects due to prolonged exposure to benzene and ethylbenzene among traffic police officers. Investigating BTEX concentrations in ambient air of refueling stations and worker’s occupational exposure, Cruz, Alve [9] described very high risk of cancer due to BTEX exposure. Likewise, Kitwattanavong, Prueksasit [15] detailed high risk of cancer among petrol station workers. Abd Hamid, Jumah [16] also suggested high risk of cancer amongst refueling station workers. Nsonwu-Anyanwu, Nsonwu [6] outlined elevated levels of liver enzymes, oxidative stress, oxidative DNA damage and lower antioxidants among vehicle manufacturing industry workers compared to the control group. Despite measuring BTEX levels lower than the established limits, Badjagbo, Loranger [11] reported potential occupational cancer risk. Castaño, Ramírez [5] revealed medium risk resulting from exposure to toluene, xylene and ethylbenzene among vehicle manufacturing industry painters. Golbabaei, Dehghani [17] observed a significant correlation between HQ and cognitive function test scores in vehicle manufacturing industry painters.

Literature review uncovered that nearly every study on occupational BTEX exposure in the vehicle manufacturing industry focused almost exclusively on paint shops, thus the need to assess the amount of exposure to said compounds in other units as well as ensuing health risks was felt. [5, 17,18,19], hence the present study was conducted aiming to assess occupational BTEX exposure health risks not only for paint shop unit workers, but for other usually neglected units such as gluing, foam injection, repair shop and molding as well.

2 Material and methods

This cross-sectional study was conducted in 5 different units of a vehicle manufacturing industry including foam injection unit (FIU), gluing unit (GU), repair shop unit (RSU), molding unit (MU) as well as 5 paint shop units (PSU). Since inhalation is considered a major exposure pathway for BTEX, measuring workers’ exposure was conducted according to NIOSH 1501 method. Four technicians were selected randomly from each unit (forty technicians in total) and personal sampling was carried out utilizing calibrated MSA personal sampling pumps set at 0.2L flow rate equipped with coconut charcoal sorbent glass tubes for forty minutes. It’s worth noting that sampling equipment was calibrated using a rotameter in advance of sampling sessions. Sampling was conducted from the breathing zone of two technicians from each unit in two consecutive days in March under identical normal working conditions. As minimizing the risk of contamination & desorption is imperative, samples were capped, stored at 4° C and transferred to the laboratory promptly at the end of each sampling session. The charcoal from front and back sections of sampling tubes were extracted into separate vials and prepared for analysis by introducing 1 ml of carbon disulfide. Analysis was performed via gas chromatography equipped with a flame ionization detector (GC-FID).

The health risk assessment of workers’ exposure to BTEX was carried out using U.S. Environmental Protection Agency guidelines and EPA Risk Assessment Information System through the following steps:

  1. 1.

    Problem identification

  2. 2.

    Designing conceptual site models

  3. 3.

    Selecting Chemicals of Potential Concern (COPCs)

  4. 4.

    Toxicity assessment

  5. 5.

    Risk calculation

The conceptual site model was created in Stem-and-Leaf format depicting the pollution sources, exposure mediums, exposure route and exposed population (Fig. 1).

Fig. 1
figure 1

Conceptual site model, stem and leaf forma

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2.1 Chemicals of potential concern

Four compounds were selected as chemicals of potential concern by reviewing previous studies and site visits: Benzene, toluene, ethylbenzene and xylene(s).

2.2 Toxicity assessment

Toxicity assessment was carried out via the respective SDS for each compound and past studies.

2.3 Risk calculation

Cancer risk resulting from chronic exposure to carcinogenic air pollutants via inhalation is calculated using the formulas below, Eq. 1 is used to calculate Chronic Daily Intake (CDI, µg/m3 & mg/m3) of carcinogenic substances via inhalation & Eq. 2 is used to calculate inhalation risk (Inhalation Unit Risk (IUR)):

$$\mathrm{CDI iw}-{\text{air}}-\mathrm{ca }\left(\upmu \frac{{\text{g}}}{{\text{m}}3}\right)=\frac{{C}_{air}\left(\frac{\mu g}{{m}^{3}}\right)\times E{F}_{iw}(\frac{250Days}{Year})\times E{D}_{iw}(25 Years)\times E{T}_{iw}(\frac{8 Hours}{Day})\times (\frac{1 Day}{24 Hours})}{A{T}_{iw}(\frac{365 Days}{Year}\times LT(70 Years))}$$
(1)
$$Inhalation\; Risk=DI\times IUR$$
(2)

According to EPA guidelines, the upper limit of acceptable lifetime cancer risk is at about 1 in 10,000 (or 10–4), in other words, if the risk to that individual is no higher than approximately 1 in 10 thousand, it is considered acceptable. It’s worth mentioning that due to high toxicity, classification as a human carcinogen and in order to protect most people possible, the upper limit of acceptable lifetime cancer risk for benzene is 1 in 1 million (10–6) [20]. Inhalation unit risk (IUR) and Reference Concentration (RfC) values are depicted in Table 1. The IUR is defined as the upper-bound excess lifetime carcinogenic risk estimated to result from continuous exposure to an agent at a concentration of 1 µg/m3 in air. Inhalation unit risk toxicity values are expressed in units of (µg/m3)−1. Furthermore, Reference Concentration (RfC) is an estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime [20, 21].

Table 1 Inhalation unit risk and reference concentration for Benzene, Ethylbenzene, Toluene & Xylenes [20]
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The inhalation hazard resulting from exposure to non-carcinogenic compounds is calculated using the formulas below, Eq. 3 is used to calculate CDI of non-carcinogenic substances via inhalation & Eq. 4 is used to calculate Inhalation Hazard Quotient (HQ):

$$\mathrm{CDI iw}-{\text{air}}-\mathrm{nc }\left(\frac{{\text{mg}}}{{\text{m}}3}\right)=\frac{{C}_{air}\left(\frac{\mu g}{{m}^{3}}\right)\times E{F}_{iw}(\frac{250Days}{Year})\times E{D}_{iw}(25 Years)\times E{T}_{iw}(\frac{8 Hours}{Day})\times (\frac{1 Day}{24 Hours})}{A{T}_{iw}(\frac{365 Days}{Year}\times E{D}_{iw}\left(25 Years\right))\times (\frac{1000\mu g}{1mg})}$$
(3)
$${\text{Inhalation HQ }} = {\text{ DI}}/{\text{RfC}}$$
(4)

Values equal to 1 or lower imply exposure levels equal to or lower than the level at which no adverse effects are expected, or to put it another way, a hazard quotient of 1 or lower means adverse non-cancer effects are unlikely and negligible, while HQs greater than 1 implicate an increase in the potential adverse effects, howbeit unclear to what extent [20]

Other variables used in above equations which were not discussed before are mentioned in Table 2.

Table 2 Description of variables used in carcinogenic and non-carcinogenic chronic daily intake equations
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2.4 Quality assurance and control

The front & back sections of sampling tubes were analyzed separately in order to check for possible breakthroughs in the back section, which was not found in any of the samples. The GC-FID device was calibrated before sample analysis and six blank samples were used to acquire 0.6 µg Limit of Detection (LOD).

2.5 Site schematics

Site schematics for some of the units are depicted in Fig. 2.

Fig. 2
figure 2

a Paint shop unit (Unit No 4), b Repair shop unit (Unit No 7), c Foam injection unit (Units No 8&9), d Gluing unit (Unit No 10), e Paint shop unit (Unit No 2), f Paint shop unit (Unit No 5). Approximate sampling stations are marked with a star

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3 Results & discussion

3.1 Sampling results

Sampling results and TLV-TWA for BTEX are presented in Table 3.

Table 3 BTEX sampling results (Average and standard deviation)
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Threshold Limit Values (TLVs) are concentration levels of airborne chemical substances at which it is believed nearly all workers may be exposed daily per shift over a working lifetime without adverse health effects [22].

3.2 Health risk assessment

The results of health risk assessment carried out using EPA RAIS for carcinogenic risks & HQ for non-carcinogenic risks are summarized in Table 4.

Table 4 Health risk assessment results
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The results depicted high concentration of benzene in gluing (Unit No 10) and paint shop units (Unit No 2), 3641.7 µg/m3 (equivalent to 1.142 PPM) and 2768 µg/m3 (equivalent to 0.86 PPM) respectively, which is higher than the TLV-TWA concentration of 0.5 PPM. High benzene concentrations were also detected in foam injection (9) and repair shop units (Unit No 7): 1417µg/m3 (equivalent to 0.444 PPM) and 1433 µg/m3 (equivalent to 0.449 PPM) respectively, although these measurements were lower than TLV-TWA. The highest concentration of toluene which was lower than the established TLV-TWA was measured in foam injection (Unit No 8) followed by repair shop (Unit No 7) and foam injection units (Unit No 9), 33,925.18 µg/m3 (equivalent to 9.006 PPM), 25,052.81 µg/m3 (equivalent to 6.651 PPM) and 24,028.55 (equivalent to 6.379 PPM) respectively. Ethylbenzene and xylene(s) concentrations were found to also be below TLV-TWA, with the highest ethylbenzene concentration measured in repair shop (Unit No 7), 4936.15µg/m3 (equivalent to 1.137 PPM) and the highest xylene(s) concentration in paint shop (Unit No 2), 37,528 µg/m3 (equivalent to 8.647 PPM).

As presented in Fig. 3, the cancer risk in 8 out of 10 units was calculated in 10–4 to 10–2 range which is higher than the EPA’s upper limit. The highest total cancer risk was observed in gluing (Unit No 10) closely followed by paint shop (Unit No 2), 2.32E-03 and 2.28E-3 respectively. Albeit the concentration of each BTEX compounds was measured below the established TLV-TWA, the calculated total cancer risk values in repair shop (Unit No 7) and foam injection (Unit No 9) units were found to be 1.92E-03 and 9.05E-03 respectively, which is also higher than the EPA upper cancer risk limits.

Fig. 3
figure 3

Total inhalation risk for each unit

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Total HQ in 8 out of 10 units was calculated higher than 1, as depicted in Fig. 4. The highest HQs were registered in paint shop (Unit No 2), repair shop (Unit No 7) and gluing units (Unit No 10), 108, 67.6 and 28.3 respectively. Simply put, the exposure to BTEX compounds in paint shop (Unit No 2), repair shop (Unit No 7) and gluing (Unit No 10) units were calculated to be 108, 67.6 and 28.3 times higher than the exposure level at which no adverse health effect is observed (total RfC) respectively.

Fig. 4
figure 4

Total HQ for each unit

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4 Discussion

High benzene concentration measured in paint shop (Unit No 2) of 2768 µg/m3 (equivalent to 0.86 PPM) is in line with the results of Golbabaei’s study on the effect of BTEX exposure on paint shop workers’ cognitive functions [17]. It’s worth mentioning that quantifiable effects in workers’ neurobehavioral functions were observed in spite of exposure levels below TLV-TWA. Measuring the atmospheric concentration for benzene yielded values 34.9 & 9.5 times the Iranian Occupational Exposure Limit (OEL), 19.9 ± 4.8 & 5.4 ± 2.3 PPM respectively, in an study in two automobile paint plants conducted by Aliasghar [18], higher than measurements in current study. High benzene concentration—although barely below TLV-TWA—was measured in repair shop unit (Unit No 7): 1433 µg/m3 (equivalent to 0.449 PPM), which is higher than previous studies by Badjagbo [11] and Abd Hamid [16] in similar units. It is speculated that lack of Local Exhaust Ventilation (LEV) system & windows which limited natural ventilation to repair shop’s doors in current study could play a role in higher measurements in this unit. In the study by Badjagbo, benzene measurements in general mechanic garages were higher than other pollutants while toluene, ethylbenzene & xylenes were prevalent in paint garages. Abd Hamid identified gasoline vapors as the main source of BTEX concentration, reporting that higher measurements were recorded in motorcycle repair workshops compared to automobile repair shops.

To the best of our knowledge, units such as gluing, molding & foam injection had not been previously studied as a source for BTEX emissions. Highest benzene exposure, inhalation HQ (IHQ) & inhalation risk (IR) was recorded in gluing (Unit No 10), 3641.7 µg/m3 (equivalent to 1.142 PPM), 27.7 & 2.32 × 10–3 respectively. Even though benzene exposure in molding (Unit No.6) & either foam injection (Units No. 8 & 9) units met the TLV-TWA recommendations, the IHQ & IR was significant. Exposure in molding (Unit No.6) was 555.6 µg/m3 (equivalent to 0.174 PPM), more than four times the reference concentration, resulting in an IHQ of 4.23 & an IR of 3.53 × 10–4. Concentration of benzene in foam injection (Units No. 8 & 9) was 986.525 µg/m3 (equivalent to 0.309 PPM) & 1417.875µg/m3 (equivalent to 0.444 PPM) respectively, leading to an IHQ of 7.51 & IR of 6.72 × 10–4 for unit no. 8 & an IHQ of 10.8 & IR of 9.02 × 10–4 for unit no.9.

Highest toluene concentrations, albeit considerably below TLV-TWA were measured in foam injection (Unit No 8) followed by repair shop (Unit No 7) and foam injection (Unit No 9) units, 33,925.18 µg/m3 (equivalent to 9.006 PPM-IHQ 1.55), 25,052.81 µg/m3 (equivalent to 6.651 PPM-IHQ 1.14) and 24,028.55 (equivalent to 6.379 PPM—IHQ 1.10) respectively. Abundance of toluene in repair shop (Unit No 7) supports Badjagbo [11] and Wilson [7] findings, although current study’s measurements of said compound are higher than their results. Wilson et al. reported periodic exposure among repair shop workers to VOCs as the direct result of using aerosol solvents, remarking on the amplified neurotoxicity of n-hexane in the presence of acetone.

Ethylbenzene and xylene(s) concentrations were also found to be below TLV-TWA in current study. Highest ethylbenzene concentration was measured in repair shop (Unit No 7), 4936.15µg/m3 (equivalent to 1.137 PPM) and the highest xylene(s) concentration was recorded in paint shop (Unit No 2), 37,528 µg/m3 (equivalent to 8.647 PPM). The IHQ for xylene(s) in paint shop (Unit No. 2) was 85.7 which is the highest IHQ for this pollutant in every sampled unit. Xylene(s) concentration was lower in comparison to Aliasghar’s study which was measured at 39.2 & 13.4 PPM in two automotive paint plants respectively [18].

As most research on BTEX exposure has been conducted in paint shop units of vehicle manufacturing companies, the aim of current study was to measure exposure & ensuing health risks in units that were usually neglected by other researchers such as foam injection, gluing, molding & repair shop.

Overall, the results in paint shop unit (Unit No. 2) were similar to previous studies while the measurements in repair shop were higher, a finding that could be attributed to the lack of proper ventilation, whether natural or mechanical in that unit. Exposure assessment in molding, foam injection & gluing units revealed a source for BTEX emissions that was not considered in past research, and with the highest benzene concentration measured in gluing unit as well as high IHQ & IR in molding & foam injection units, further investigation of mentioned units is required.

5 Conclusion

The results revealed not only the possibility of exposure to BTEX in units other than paint shops such as gluing and foam injection, but high concentrations in molding and repair shop units as well. Cancer risk in foam injection and gluing units was higher than the EPA acceptable risk range (10–4) while molding (Unit No 6) and repair shop (Unit No 7) units risk levels were registered in the 10–4 to 10–2 range despite meeting the TLV-TWA (0.174 PPM and 0.449 PPM respectively), suggesting the need for implementation of a health risk assessment in conjunction with exposure assessment in occupations that are subjected to BTEX exposure. Strictly speaking, regarding exposure to carcinogenic compounds, meeting the established exposure levels is found to be insufficient in managing cancer risk and acceptable cancer risk levels also should be considered in annual occupational exposure assessments.

6 Limitations

The study was conducted in two consecutive days in March (Winter) & no other sampling was performed in other seasons to compare exposure results, as seasonal variations might affect the amount of exposure.