Liver function, kidney function, glycohemoglobin, neurotransmitters, and heart markers and factors affecting blood biochemistry among workers at gas service stations, Thailand

Introduction

Background

Fuel compounds such as benzene, toluene, ethylbenzene, and xylene (BTEX), given inadequate self-protection and chemical control, may contaminate the air in work areas, particularly at connection points during fuel transfers, such as the transfer of fuel from trucks to storage tanks at stations (1) and the dispensing of fuel to engines (2). This is especially true for gas stations located in commercial areas, where benzene and toluene levels were previously found to be higher than in residential and suburban areas (3). Additionally, a positive correlation was discovered between BTEX levels and per capita gross national income (4). As Rayong Province in Thailand is an economically prosperous province with the highest gross national income per capita in the country (5) and the province contains a variety of airborne organic solvents such as benzene levels exceeding the annual standard (6), this airborne contamination may result in BTEX exposure among area workers.

Workers who live near fuel sources, particularly those who work at gas stations, are at a high risk of BTEX exposure (3). According to one study, refueling attendants, cashiers, and convenience store employees were exposed to benzene at a higher rate than the control group (7). In vivo biomarkers can be used to assess exposure. The American Conference of Governmental Industrial Hygienists (ACGIH) recommends evaluating urinary BTEX in the following forms: trans,trans-muconic acid (t,t-MA) (8), hippuric acid (HA) (9), mandelic acid (MA), and methyl hippuric acid (MHA) (8). Previously, these biological markers were used to evaluate refuelers in India who were exposed to t,t-MA, HA, MA, and MHA at concentrations of 0.59, 6.55, 1.91, and 1.20 mg/g Cr, respectively (10), which may have caused them to experience health toxicity as a result of chemical exposure.

BTEX toxicity can manifest itself in both acute and chronic forms. It can have a toxic effect on the nervous system during the acute stage. Exposure to more than 500 parts per million of benzene can cause headaches and dizziness. It can also have a toxic effect on the blood system (11), liver and kidney functions (12), and heart functions during the chronic stage (13). Remarkably, benzene is a human carcinogen (14). Additionally, researchers have begun to suspect that BTEX may be toxic to other blood biochemicals, including the metabolic system (15), neurotransmitter enzymes (16), and heart functions (17). A correlation was discovered between pollutants and metabolic syndromes. An increased t,t-MA quartile was associated with a 2.77-times increased risk of diabetes and a 1.42-times increased risk of obesity (15). One study was conducted on rats to determine heart function markers. There was a statistically significant correlation between serum troponin levels in rats exposed to diesel vapors (17). Another study examined the effects of gasoline and color products on humans. At a later age, the exposed group was more likely to develop cardiovascular diseases (13). However, previous assessments of worker exposure to BTEX lacked personal assessment.

Individuals who work at gas stations and are exposed to BTEX may experience health effects associated with significant target organ toxicity, such as hematologic effects (11,18) and an increased risk of cancer (11,19). However, research on other blood biochemistry markers such as neurotransmitter enzymes, glycohemoglobin, and heart function markers is still in its infancy. There are studies on the effects of BTEX on laboratory animals (17,20,21) or the general population living in chemical-contaminated areas (12), but no studies exist concerning workers at gas service stations who are at risk of being exposed to BTEX directly from their sources. The purpose of this study, the first of its kind, was to elucidate factors affecting liver function, kidney function, glycohemoglobin levels, neurotransmitters, and heart markers among gas service station workers in Thailand as baseline data for health surveillance. It may also aid in preventing adverse health effects by determining their causes.

Rationale and knowledge gap

Working at a fuel service station in Thailand involves many different types of work, both inside and outside the fuel area. To the authors’ knowledge, there has never been a study on many occupational groups such as this before. The workers are exposed to BTEX for long periods but at low concentrations. Therefore, a clear effect on blood biochemistry is desired. In addition, less studied markers of multiple system effects were examined, including HbA1C, AChE, and cardiac troponin-T (cTnT).

Objective

This study was performed to elucidate factors affecting liver function, kidney function, glycohemoglobin, neurotransmitters, and heart markers among gas service station workers in Thailand. We present this article in accordance with the STROBE reporting checklist (available at https://jphe.amegroups.com/article/view/10.21037/jphe-24-26/rc).

Methods

Population and sample

The sample size for this descriptive study was used by the G*Power program (22) to compare the difference between two means to determine the effect size of medium =0.5, α error prob =0.05, and power (1-β error prob) =0.95. The sample comprised two groups of 100 people each: 100 employees who worked inside fuel dispenser areas, such as refuelers and cashiers, and 100 employees who worked outside fuel dispenser areas, such as food shop, coffee shop, and convenience store employees. They were randomly selected using cluster sampling until the required number of workers was reached, owing to the similar work conditions at each station. The criteria for inclusion included working at stations for at least three months, agreeing to participate voluntarily, and not having diabetes as a congenital disease. Exclusion criteria included the presence of illness on the day of sampling and absence from work on the day of data collection.

Research tools and quality

• Interview forms were divided into two sections: general information and work history. General information included the following: (I) gender, age, weight, height, smoking habits, and alcohol consumption; and (II) work history, which included the nature of work, length of work, frequency of work, overtime work, personal hygiene, and personal protective equipment (PPE)—wearing behavior. Three experts, two occupational medicine physicians, and one university professor of occupational health and safety evaluated the interview forms using the item-objective congruence (IOC) index. For all questions that met the assessment and evaluation criteria, the IOC score was greater than 0.5.
• Urine collection devices and analyzers included plastic cups, 50 mL polyethylene jars, temperature-controlled foam boxes, and ice packs, while high-performance liquid chromatography (HPLC) was used to analyze the urinary BTEX metabolites according to the method of Onchoi et al. (23).
• Blood collection devices and analyzers included a tourniquet, dry cotton swabs, alcohol, needles, 10 mg ethylene diamine tetraacetate (EDTA) blood tubes, Transpore tapes, and infectious waste bags, while the laboratory biochemistry analyzers used were classified by such parameters as serum glutamic oxaloacetic transaminase (SGOT), serum glutamate pyruvate transaminase (SGPT), blood urea nitrogen (BUN), creatinine (Cr), and acetylcholinesterase enzyme (AChE) via the enzymatic automated analyzer. Hemoglobin A1C (HbA1C) was analyzed by HPLC, and cTnT was analyzed by electrochemiluminescence immunoassay (ECLIA) according to the standards of the Department of Medical Sciences of Thailand (24). Blood and urine analytical laboratories were certified according to the International Organization for Standardization (ISO) 15189: 2012 No. 4247/63.

Data collection and interpretation

• Interview form collection: interview forms were collected by the researcher and research assistants. On the day the samples were collected, the interview locations at gas service stations prepared by the station owners were used. The interview time was approximately 10–15 minutes per person.
• Urine sample collection: the researcher gave the sampling devices to the workers during their shifts and clarified that the urine samples should be collected at the end of their shifts. They were instructed to collect midstream urine in a plastic cup and then pour it into a 50 mL polyethylene jar. The urine samples were then collected in a foam box that was kept at a temperature below 4 ℃. The results of the analysis were reported for the BTEX metabolites t,t-MA, HA, MA, and MHA according to the methods described in these studies (23,25). The data obtained was quantitative. The interpretation of the results was divided into two groups, using the median to divide the data: the group exposed to substances less than or equal to the median and the group exposed to substances more than the standard.
• Blood sample collection: the blood collection locations were the same as the interview locations. Blood samples were collected by a medical technician or professional nurse after work (3 mm per person) and placed in clotted blood tubes. The following biochemistry were analyzed: SGOT, SGPT, BUN, Cr, HbA1C, AChE, and cTnT, according to the method specified by the Department of Medical Sciences (23). The interpretation of the results was divided according to the method specified by the Department of Medical Sciences of Thailand. The analysis was grouped into two levels: normal (within criterion) and abnormal (under/over criterion).

Statistics for data analysis

The data analysis was divided into two sections. The first section, (I) descriptive statistics, comprised the following: (i) a calculation of the percentage of qualitative variables, such as gender, smoking habits, alcohol consumption, PPE-wearing behavior, and proportion of blood biochemistry abnormalities; (ii) the mean and standard deviation calculations for quantitative variables with a normal distribution, such as age, body mass index, and work frequency; and (iii) the median (min–max) calculations for quantitative variables with an abnormal distribution, such as BTEX metabolites. The second section, (II) inferential statistics, included (i) an independent sample t-test used to compare the means of blood biochemistry between groups located inside and outside fuel dispenser areas and (ii) logistic regression analysis used to determine the crude odds ratio (OR) and 95% confidence interval (CI) for various factors affecting blood biochemistry.

Ethical consideration

The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). This study was conducted with the approval from the Burapha University Institutional Review Board for Protection of Human Subjects in Research (BUU-IRB) (certificate no. 019/2020). Before beginning the survey, informed consent was obtained from all the study participants.

Results

Personal information

Approximately two-thirds (68.5%) of the workers were female, and the majority were between the ages of 20 and 29 years (43.5%). A little less than half were overweight or obese (44.0%), and approximately half (52.5%) were single, while approximately one-third were secondary school graduates (31.0%) and earned between 10,000 and 15,000 baht per month (31.0%).

Health behavior

The majority of workers (63.5%) were nonsmokers, though some were secondhand smokers (13.0%). They consumed alcohol at a rate of 45.5%, with the majority (41.8%) drank infrequently (less than one glass per week), but some (31.9%) drank four or more glasses per week. Approximately half slept for about 7–8 hours per day (51.0%), though some slept for about 5–6 hours per day (19.5%).

Work history

These workers had 2.44±4.06 years of work experience, most of which involved working 8 hours per day. However, some worked for more than 8 hours per day (36.5%). About half (55.0%) worked 6 days per week, while one-fourth (25.5%) worked every day, and they worked overtime for more than 5 hours per week (13.5%).

The refuelers mostly refueled diesel (36.0%), followed by gasoline (10.0%). According to the interviews concerning the use of PPE, most did not use PPE—N95 masks, protective clothing, and protective glasses at 98.0%, 97.0%, and 95.0%, respectively. For PPE-wearing behavior, they wore cloth masks and long sleeves or armbands at 66.0% and 62.0%, respectively.

BTEX exposure

On the urinary BTEX metabolite data, the workers had medians (min–max) of 393.62 (59.71–1,482.46) µg/g Cr for t,t-MA, 0.32 (0.06–0.77) g/g Cr for HA, 0.06 (0.03–0.11) g/g Cr for MA, and 0.40 (0.20–0.88) g/g Cr for MHA. When urinary BTEX metabolite exposure was compared to the standards, the t,t-MA content was found to be greater than the ACGIH threshold (>500 µg/g Cr) at 29.5%, while the other substances were found to be within the standards (Table 1).

Blood biochemistry marker

The majority of workers had normal health status as measured by biochemistry markers, but some had abnormalities, such as glycohemoglobin measured by HbA1C exceeding the criterion at 30.5%, followed by liver functions measured by SGPT and SGOT exceeding the criterion at 9.0% and 8.0%, respectively. When the means of biochemistry marker parameters were compared between groups inside and outside fuel dispenser areas, the inside group had a statistically significant higher mean of Cr and HbA1C (P<0.001 and P=0.046, respectively), albeit a statistically significant lower mean of neurotransmitters (P=0.043) (Table 2).

Factors affecting blood biochemistry markers

According to the findings of this study, married workers had a higher risk of abnormal Cr and HbA1C than single workers (OR =3.724; 95% CI: 1.325, 10.470 and OR =2.168; 95% CI: 1.161, 4.049, respectively), while smokers and those exposed to secondhand smoke had a higher risk of abnormal AChE than nonsmokers (OR =2.600; 95% CI: 1.096, 6.168).

In terms of work factors, employees who worked more than 8 hours per day had a higher risk of abnormal SGPT than those who worked less than 8 hours per day (OR =2.789; 95% CI: 1.003, 7.761), and employees who were exposed to more than the median amount of urinary HA had a higher risk of abnormal SGPT and AChE (OR =3.295, 95% CI: 1.080, 10.050; and OR =3.326, 95% CI: 1.256, 8.805), respectively (Table 3).

Discussion

Blood biochemistry markers such as liver, kidney, and heart function, glycohemoglobin levels, and neurotransmitter enzymes were evaluated in this study. Previously, certain parameters such as diabetes (15) and heart function (17) were questioned as a result of organic solvent exposure. Consequently, studies on animals (17,21) and humans have been attempted (12). However, this study distinguished itself from previous research in that it examined workers close to chemical sources and is one of the first to examine BTEX exposure and the factors affecting blood biochemistry in workers. The following parameters were used to analyze the results.

HbA1C is a result of hemoglobin glycosylation, and the values indicated are markers of blood sugar accumulation over several preceding months (26). According to the findings of this study, the workers had an HbA1C level greater than 30.5% above the criterion, which is a more abnormal proportion than those of other biochemistry parameters. The workers were more likely to have diabetes than the general population. Diabetes prevalence was 8.9% in the general population of Thailand aged 15 years and over, according to a survey (27). As can be seen, they had triple the prevalence of diabetes compared to the general population. Previously, there was concern about air pollution, particularly the volatile organic compounds (VOCs) that can cause abnormal metabolism (12). This study found no correlation between BTEX and HbA1C, which contradicts previous research (12,15). For instance, a Canadian study by Cakmak et al. (12) discovered an association between benzene, ethylbenzene, and o-xylene as well as increased insulin resistance. Additionally, m,p-xylenes were linked to an increase in glycated hemoglobin, whereas blood VOCs influenced glucose metabolism.

Married workers were 2.168 times more likely to have abnormal HbA1C levels than single workers. This was consistent with the results of Kaewsuk’s (28) study, which found a statistically significant association between marital status and a low level of blood sugar uncontrollability in diabetics (P=0.001). It was implied that eating similar diets was influenced by family. One study discovered that family eating habits were a 20.8% predictor of diabetes (29). Compared to single workers, married workers who live with their families may have fewer dining options or prefer to eat alone. The food items that Thai people eat in their households nowadays tend to have more sugar than those in the past. According to a food survey, Thai people now consume three times as much sugar per person than they did two decades ago, at approximately 36.4 kilograms per person per year (30).

SGPT and SGOT were used to evaluate liver function. According to the findings of this study, 9.0% and 8.0% of workers, respectively, exhibited abnormal liver function. When the correlation was analyzed, working for more than 8 hours per day and being exposed to more than the median amount of urinary HA was associated with an increased risk of abnormal SGPT (OR =2.789, 95% CI: 1.003, 7.761; and OR =3.295, 95% CI: 1.080, 10.050, respectively). This result was consistent with a meta-analysis of 22 high-quality studies examining the liver functions of gas station employees. One could conclude that refuelers had a 2.06-fold increase in abnormal SGPT compared to a control group (95% CI: 1.42, 2.69). Moreover, this was consistent with several other studies conducted at gas stations (18,31,32). SGPT is an enzyme that catalyzes the transfer of amino acid groups to cytoplasmic alanine. It is found in a variety of tissues, including the liver. As a result, SGPT is an excellent biomarker for hepatocellular injury (33). Despite prolonged exposure to low levels of BTEX (34), chemicals found in fuels can also cause hepatocellular injury. Another study discovered that a combination of solvents, including toluene, affected liver functions (33). As a result, this is a risk factor that increases exposure and has a greater effect on the liver if working hours are increased. Additionally, liver function tests should be performed regularly.

Kidney functions were assessed by BUN with a 5.5% abnormality and Cr with a 11.5% abnormality. According to the BTEX analysis, there was no correlation between BTEX and kidney function. This evidence contradicted several previous studies (34-36) that indicated that substances in fuel membrane tissues interfere with the activity of sodium and potassium (Na⁺/K⁺/ATPase) involved in the transport of intracellular and extracellular concentrations, thereby increasing kidney function upon exposure (34). This is possible because previous research on heavy metals in oil found that they can cause severe kidney damage. However, fuels in Thailand are now free of heavy metals, and the kidney is a disease-resistant organ. The sample group in this study had a combined work experience of 2.44±4.06 years, and this work experience may not have had an effect on kidney function. As a consequence, long and complex follow-up studies are required.

AChE or neurotransmitter enzyme functions were assessed because BTEX is a neurotoxic agent (37) and one of the enzymes required for acetylcholine (ACh) hydrolysis (38,39). According to the results of this study, AChE was abnormal at 14.5%, and interestingly, smoking or exposure to secondhand smoke increased the risk of abnormal AChE (OR =2.600; 95% CI: 1.096, 6.168). According to reports, the chemicals in cigarettes, particularly nicotine, affect AChE and are associated with the central nervous system as well as an increased risk of silent cerebral infarction (40). Additionally, there have been reports of cigarettes contaminated with organic solvents (41). Thus, a recommendation should be made to reduce or abstain from smoking to minimize its effects on the nervous system.

Remarkably, workers exposed to more than the median amount of urinary HA had a higher risk of abnormal AChE than workers exposed to less than the median amount of urinary HA (OR =3.326; 95% CI: 1.256, 8.805). This study assessed the effects on the nervous system differently than several previous studies (37,42-43) that examined neuro-symptoms. Specifically, this study assessed AChE, but the results were consistent. A previous study conducted in Egypt indicated that toluene exposure inhibited AChE, an enzyme responsible for the hydrolysis of ACh at the neuronal synapse. As a result, neurotoxicity caused by toluene is caused by a decrease in AChE activity and an increase in extracellular ACh levels (20). One can conclude that workers exposed to fuels at their sources will exhibit an abnormality in the degradation of ACh into choline and acetic acid, resulting in effects on the human nervous system (39).

Heart functions were assessed with cTnT, a marker of the heart’s association with the risk of cardiovascular diseases (44). All workers in this study had normal cTnT levels. Previous research examined the cardiovascular effects of air pollutants on people and identified VOCs as a cause of myocardial infarction (45). Ogbuowelu et al. (13) investigated occupational health in a sample of 162 people, including gasoline pump attendants, mill workers, and a control group. Troponin levels varied significantly among the three groups (P<0.05). However, the gas station workers in this study all had troponin levels within the criteria, possibly because the samples were relatively young and their exposure to BTEX at gas stations was brief (2.44±4.06 years). As a possible consequence, the effects of working at gas stations on heart function were not apparent. Therefore, health surveillance of high-risk groups and additional research on those subject to exposure for an extended period is required.

Conclusions

Gas station employees were concerned about benzene exposure exceeding the standard criteria, and their health concerns included normal liver function, kidney function, and heart function as well as neurotransmitters. On the other hand, these workers had a higher proportion of HbA1C, a diabetes marker, than the general population. According to the significant factors affecting blood biochemistry, abnormal liver function was influenced by working hours and toluene exposure. Likewise, smoking and secondhand smoke, along with exposure to toluene, were associated with an increased risk of abnormal AChE. These are all preventable factors. Thus, it is recommended to abstain from smoking and avoid secondhand smoke as well as to minimize exposure to toluene by wearing personal respiratory protection equipment in addition to limiting work hours to no more than 8 hours per day. Additionally, sources at fuel dispensers should be avoided during rest periods to avoid exposure to fuel-borne contaminants.

Acknowledgments

We thank the staff of Rayong Hospital in honor of Her Royal Highness Princess Maha Chakri Sirindhorn for allowing us to collect samples as well as for their participation. We would like to thank Proofread Services Company for helping to check the accuracy of the English language.

Funding: This study was supported by Health Systems Research Institute (Contract No. 63-070).

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jphe.amegroups.com/article/view/10.21037/jphe-24-26/rc

Data Sharing Statement: Available at https://jphe.amegroups.com/article/view/10.21037/jphe-24-26/dss

Peer Review File: Available at https://jphe.amegroups.com/article/view/10.21037/jphe-24-26/prf

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://jphe.amegroups.com/article/view/10.21037/jphe-24-26/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). This study was conducted with the approval from the Burapha University Institutional Review Board for Protection of Human Subjects in Research (BUU-IRB) (certificate no. 019/2020). Before beginning the survey, informed consent was obtained from all the study participants.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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