The impact of occupational exposure to BTX on lung function was evaluated by quantitative assessment of exposure to BTX as health-relevant VOCs. Our main finding was that working as a gas station attendant is consistently associated with changes in lung function with a pattern that is distinct from what was observed in office workers. In the gas station workers exposure to benzene during work hours (c.f. delta benzene) was observed to be a much more prominent risk factor, whereas in controls toluene and xylenes appear to be driving lung function changes associated with pre-shift exposure that could indicate a potential contribution from exposures outside work hours.
The gas station attendant and the control populations were comparable by age, ethnicity, anthropometric measurements including BMI and smoking status. The gas station attendants being labourers who receive a lower income compared to the non-executive office workers who were recruited as controls. The subgroups of workers for which BTX exposure data were available, were comparable (for the same parameters) to the much larger group of workers for which spirometry was successfully completed.
With the collection of pre- and post-shift samples of end-exhaled air we attempted to find out to what extent BTX exposures are work-related. The rationale for this strategy is the fast kinetics of inhalation and elimination, suggesting that an increase from pre-shift to post-shift suggests exposure occurring during work hours. That this is a reasonable assumption is indicated by the strong correlations observed in gasoline station workers. The pattern of strong correlations within the BTX mixture indicates that exposure occurred during work hours, whereas the pre-shift results were clearly correlated less strongly. The high correlations in post-shift and delta results are explained by the common source of all BTX components from primarily fuel vapors and to some extent also from road vehicles where two-stroke engines of motorcycles are an important source due to incomplete combustion (IARC, 2021). The much weaker correlation with delta benzene may be explained by 5–10 times higher vapor pressure of benzene compared to toluene and xylenes. This property may affect environmental fate as well as kinetics of uptake and elimination and explain this deviation. In gas station attendants a contribution from smoking is not likely as smoking is strictly forbidden during work hours.
In our earlier report, BTX was characterised for 29 gas station attendants and 16 controls (Scheepers et al., 2019). In that study, pre- and post-shift end-exhaled air was evaluated. BTX concentrations were significantly higher among gas station attendants compared to office workers (p < 0.005). In the subgroup with spirometry values in the current analysis (20 gas station attendants and 11 controls), showed an increase in VOC levels from pre- to post-shift to be statistically significant (p < 0.03), signifying an elevated exposure to VOCs in this group of gas station attendants.
BTX can be considered both a source of indoor air pollution and outdoor air pollution 14. Toluene and xylene are commonly found in household items such as glues, paints and rubber; thus, exposure to these chemical compounds is inevitable. Benzene is primarily observed in combustion sources such as tobacco smoke and also solid fuel burning as important indoor sources. Both toluene and xylene exposure has been reported to decrease spirometry values even in residential (non-occupational) exposures. The site of toxicity is indicated to be both small and large airways with the mechanism being mediated through oxidative stress induced damage 15. That these minute exposures correlate with a reduction in lung function parameters irrespective of lack of direct and prolonged exposure to VOCs raises concern.
Gasoline station attendants and lung function
The correlation matrix indicates a strong correlation of all measured and calculated lung function parameters in controls. In gas station workers this consistent pattern of interrelationships appears to be disturbed, potentially because of different responses of lung function parameters to BTX exposures and other work-related factors. Obstructive and restrictive ventilatory patterns were similar in gas station attendants and controls who were smokers. The reduction in PEFR seen is likely to be attributable to unsatisfactory technique and effort in some attempts, and unlikely to be of any significance. The other lung function parameters were not reduced in either group, and were not significantly different between gas stations workers and controls. Measurement of airway resistance in future studies could reveal more subtle changes in lung function.
Taking together the interrelationships between BTX exposure and lung function parameters the correlation matrix in the group of gas station attendants is different from the controls. The associations observed in both gas stations attendants and controls suggest that effects on spirometry parameters are work-related. This is indicated by a consistent moderate negative correlation of % predicted FVC with post-shift and delta BTX as an indicator of the development of exposure over a shift. Associations with changes in other lung function parameters are much weaker. The more consistent negative associations with different expressions of lung function decrease with delta benzene exposure (taken as a proxy for exposure occurring during work hours) are remarkable and not observed in controls.
We used the BTX levels in end-exhaled air to correlate with the spirometry findings. Because of the high vapour pressure and poor water solubility, the retention and absorption of BTX are limited. When air concentrations are high, most of these substances have fast kinetics of uptake and elimination via the airways. This results in airways being exposed by BTX entering the lungs and a second time when these substances are eliminated via the airways as primary route of excretion. The residue leads to a build-up of BTX in the body until a steady-state is reached. When the worker is exposed to fresh air, the absorbed parent BTX (if not metabolised in the meantime) leaves the body unchanged via the lungs. This means that both during and after work, the exhaled air contains BTX, and the upper airways will be exposed during both uptake and elimination. This makes the exhaled air analysis a useful surrogate measure for airway dose (Murayama et al., 2006).
Office work and lung function
If the calculated changes from pre-shift to post-shift within each individual worker (delta) can be used as a proxy for work-related exposures it is clear that our data do not suggest BTX exposure to be associated with clinically relevant changes in lung function. A remarkable finding is the consistent and strong association of both toluene and m/p-xylene across all expressions of lung function, moderate for pre-shift and even more pronounced for post-shift. The lack of a contribution of benzene in this pattern confirms that fuels and traffic exhaust are less likely involved. Exposures to consumer products that contain toluene and xylenes may also explain these exposures and their effect on lung function, e.g., use of paint, glue and decreasing/cleaning agents with a white spirit profile of light-weight aliphatic and aromatic hydrocarbons (white-spirit type) are source of toluene and xylenes but not benzene. As we have not collected any information on the use of such products in the office environment, we cannot be sure if this exposure can be explained from work or not. As indicated by the strong negative correlation between pre-shift and post-shift values we can only suggest that is most probably derived from recent exposure, i.e., the same day.
We hypothesise that the control group was to some extent exposed to BTX outside work hours. This exposure was associated with reduced spirometry values, but to a lesser extent than the group of gas station attendants. This is reflected in somewhat lower correlations of end-exhaled air BTX levels and spirometry values. Although the BTX levels were significantly lower among the controls, even these low exposure levels were negatively correlated with the % predicted FVC. We would expect some of these correlations to become statistically significant in a larger study sample.
In the current study, significantly lower % predicted FVC was found among gas station attendants who were employed for 5 years or more compared to those workers with employment durations of less than 5 years. The other parameters including FEV1/FVC ratio, % predicted FEF25-75% were lower among those employed for more than five years although not significant. Chawla and Lavania (2008) reported a significant reduction in FVC, FEV1, PEF and FEF25-75 with increasing years of work, with FEF 25–75 being the most affected. A similar pattern has been repeatedly observed in other studies.16–19
Context of existing knowledge
In a recently published systematic review on spirometry parameters among gas station attendants, 20 studies observed a statistically significant reduction in FVC in the exposed group compared to the controls. An additional seven studies reported lower values, although not to a statistically significant effect. The meta-analysis by the same group reported the same trend in spirometry parameters 18. A restrictive pattern in lung function among gas station attendants has been reported in previous studies 5,19. Other studies showed an inconsistent effect on spirometry 16,17,20−22 with all spirometry parameters (FVC, FEV1, FEV1/FVC ratio, PEFR) reduced in gas station attendants compared to controls. Residents living close to fuel stations had a significantly lower FVC, and the exposure duration correlated with observed reductions in FVC values 20. Similarly, in the current study, we found that duration of employment was a predictor of the FVC whereby a longer duration of employment as a gas station attendant was associated with FVC decrements but no other lung function parameters. Similar findings were reported by Rahimi et al. where persons working as petrol station attendants for more than 12 months recorded lower spirometry parameters compared to employees with less than a 12-month exposure 18. In our study, we recruited only persons exposed to fuel handling for a minimum of 12 months.
Although we did not see a statistically significant difference, a higher percentage of smokers had an abnormal spirometry pattern in both control and study groups. In a study on gas station attendants in Malta, smokers had a statistically significantly lower FVC and FEV1 than non-smokers 23. In our study exposure to toluene and the duration of employment were found to be negative predictors of % predicted FVC, whereas exposure to benzene was a negative predictor of % predicted FEF25-75% when controlled for smoking and all expressions of BTX exposure.
Strength and limitations
This is the first study of the Sri Lankan population where the BTX exposure levels have been correlated with spirometry values of the gas station attendants. Equally important were the exposure measurements among controls. The exposure correlation matrix of the controls is quite different from that of gas station attendants.
Important limitations in the current study are related to the small sample size due to the low response and missing values. There was random selection according to the allocation of duty on the days of exposure evaluation and the availability of acceptable spirometry recordings. Members of the research team did not select participants for either investigation based on physical or other characteristics. Only males are employed as gas station attendants in Sri Lanka, hence we did not include females also not as controls. Due to the limited availability of study participants, matching gas station attendants with controls by smoking status was not feasible. We used partial correlation and included smoking as a dependent variable in regression analysis to develop a prediction model to overcome this limitation, albeit not entirely. We recommend that our findings be reconfirmed in larger study populations to confirm associations between exposure and health outcome and allow multivariate regression analysis to better adjust for confounders. In that future work female workers should also be included if possible.
In conclusion, we did not observe changes to the lung function of any clinical relevance as a result of exposure to BTX in gas-station attendants or office workers. However, we found that obstructive responses in spirometry were most pronounced for the level of work-related benzene exposure and also associated with duration of employment. In controls a distinct pattern of associations with exposure was observed, indicating a moderate to strong association of the exposure level of toluene and m/p-xylenes. We recommend reducing long working hours and installing self-service fuel stations to minimise exposure and related adverse health effects for gas station attendants.