Benzene, toluene and xylene (BTX) are major volatile hydrocarbons emitted during petroleum refinery operations, and all three chemicals are at least suspected carcinogens and mutagens (10, 17, 18). The major process streams in petroleum refining are associated with higher BTX content, and thus pose significant exposure risk to workers in these tasks (1). Consequently, jobs and tasks were categorized in this study according to their associated highest potential contact with refinery product streams containing BTX. Based on job descriptions workers were divided into high, moderate, and low-exposure categories. However, on closer examination, during any of the four years of monitoring, some of the workers in both the low and moderate exposure categories had very high exposures for all the BTX chemicals, even exceeding levels of some individuals in the high exposure job group.
Workers of all age groups in the refinery are at risk of exposure to BTX (p <0.05), however exposure to xylene was significantly elevated in 2012 with an increase of between 70% and 129%. This increase was clearly due to an occupational incident (chemical spill, new raw material, type of crude oil/gasoline, etc.) since all workers’ exposure to xylene (as shown by an increase in its metabolite, methylhippuric acid, in urine) increased twofold from 2011 to 2012. In 2013, there was a decrease in urinary methylhippuric acid across all job categories.
The results also suggest that both men and women generally have the same exposure risk with regard to BTX in this petroleum refinery. However, an increase in exposure to xylene was observed in 2011 where women showed exposure difference of approximately 60% higher than their male counterparts compared to all the other years where men were more exposed. The reason for this significant difference in xylene exposure could be from the fact that there were fewer women than men in the study. The results could thus be showing statistical bias and/or it could also be due to the fact that in most cases men and women have different lifestyle habits (e.g., heavy smokers and drinkers, etc.) as confounders.
Although there was no significant difference between men and women with regard to toluene and xylene exposure, in 2012 a difference in exposure to benzene was observed between men and women. In the four years of biomonitoring, women showed increased levels of exposure to benzene and toluene compared to men. The difference in benzene exposure between men and women in 2012 happened to be from an individual worker (process controller) whose benzene exposure levels increased rapidly (Table 3) compared to all other workers. The increase could have been due to either non-occupational factors (e.g., contaminated food, newly painted/renovated house, etc.) or occupational factors like chemical spills, or improper use of personal protection equipment while on duty. This individual’s increased exposure led to general increase in average exposure to benzene for women while there was a decrease in average exposure for men (2011 to 2012), which then resulted in the significant difference in exposure compared to the other years.
However, generally there was no significant difference (p ≥0.05) in toluene exposure with regard to the different occupations assessed in the study, with all job categories in the refinery having the same risk of being exposed to toluene. Also, all the workers in the different occupations, within the different exposure categories in the refinery had the same risk of exposure to benzene (p≥0.05), except in 2010 where high and moderate exposure job categories showed a difference in benzene exposure (p<0.05). This suggests that levels of benzene that workers in these two categories were exposed to were not the same; which could be either from an occupational setting or non-occupational factors.
For 2010, 2012 and 2013 all the workers from the different job roles in the refinery had the same risk of exposure to xylene, but in 2011 there was a difference in exposure between the different job categories. The difference observed was from the workers in the moderate exposure category (Table 5) and this increased exposure is evident throughout all the four years of biomonitoring, except only in 2012 where the high exposure job category workers were highly exposed (Table 3). The xylene levels of exposure in the plant seem to have been constant in the four years with an overall average decrease of 7%, while exposures to toluene and benzene have an average increase of 146% and 260%, respectively.
In all the four years of biomonitoring in this petrochemical refinery, there has been an increase in benzene exposure, as observed from the analysis of its biomarker in urine, phenol, which is not the case for xylene (methylhippuric acid in urine) and toluene exposure (o-cresol in urine). This is probably due to its higher stability in the environment with a lifespan estimate of a couple weeks compared to 2 days for toluene and several hours for xylene (19, 20, 21). The decrease in xylene exposure throughout the four years could have also been influenced by time of urine collection, which was not necessarily constant for all participants.
There are a number of factors, both occupational and non-occupational that may contribute to elevated levels of exposure to BTX or the presence of BTX metabolites in urine. Non-occupational factors that may contribute to BTX exposure are smoking cigarettes, diet (fish, nuts, alcoholic drinks, poultry and offal); paint and consumer products (glues, adhesives, lacquers, waxes, detergents and cleaning products); traffic intensity (car exhaust emissions, type of fuel, age and type of motor vehicle, speed rate). Petrol stations, electronic devices (printer/photocopiers), some furniture (chair/tables) and infrastructure at new buildings also emit BTX s (22, 23).
Worker exposure to BTX was monitored at the petroleum refinery in this study, probably due to legislation amid growing concerns over the years based on cancer incidence data gathered in epidemiological studies of petroleum refinery workers (24). Based on the results for benzene exposure, measured as urinary phenol, it was generally assumed that there was no significant risk for this group of workers throughout the four years of continuous monitoring. On the other hand, some workers were significantly exposed to high levels of toluene, measured as urinary o-cresol. The sporadic individual overexposure to benzene and toluene in this refinery could be attributed to single chemical incidents. Workers at highest risk of significant exposure to BTX were those workers at the helm of production streams and maintenance tasks.