Review Article
Environmental & Occupational Health

Assessing Biomarkers on Exposure, Effects and Susceptibility for Environmental and Occupational Exposure of Various Range of Benzene

Background

Benzene is a ubiquitous environmental and occupational pollutant classified by the International Agency for Research on Cancer (IARC) as a Group 1 human carcinogen. Exposure to benzene, even at relatively low concentrations, has been definitively linked to haematotoxicity and the development of blood cancers, particularly acute myeloid leukaemia (AML) and other haematopoietic malignancies. Benzene’s primary route of exposure is through inhalation, and the target organ is bone marrow, where its metabolites interfere with normal haematopoiesis and can induce DNA damage in stem and progenitor cells.

In the occupational setting, the most commonly exposed workers are those in petrochemical industries, petroleum refining, and petrol distribution, including filling station attendants. Environmental exposure occurs through vehicular exhaust emissions, making traffic police officers, taxi and bus drivers, and street vendors in high-traffic areas subject to chronic low-level exposure. In Malaysia, the Occupational Safety and Health Act 1994 and the Factories and Machinery Act 1967 provide the regulatory framework for controlling occupational benzene exposure, with permissible exposure limits aligned with international standards.

This review, published in the Malaysian Journal of Public Health Medicine, comprehensively examined the published literature on biomarkers used to assess benzene exposure across occupational and environmental settings, categorised into three groups: biomarkers of exposure, biomarkers of effect, and biomarkers of susceptibility.

Biomarkers of Exposure

Biomarkers of exposure are measurable indicators that provide evidence that an individual has been exposed to a particular substance. For benzene, the principal biomarkers of exposure are urinary metabolites that reflect the absorption and metabolism of inhaled benzene. The most widely used and validated biomarkers include urinary trans,trans-muconic acid (t,t-MA), urinary S-phenylmercapturic acid (S-PMA), and unmetabolised benzene in urine or blood.

Urinary t,t-MA has been extensively studied and shows good correlation with airborne benzene concentrations across a wide range of exposure levels. It is considered sensitive and specific even at exposures below 1 ppm, making it suitable for monitoring both occupational and environmental exposures. However, t,t-MA can also be produced from dietary sources (sorbic acid in preserved foods), which may confound results at very low exposure levels.

Urinary S-PMA is generally regarded as the most specific biomarker for benzene exposure, as it is produced exclusively through the glutathione conjugation pathway of benzene metabolism. Its high specificity makes it particularly valuable for distinguishing benzene-specific exposure from background sources. However, S-PMA is present in very low concentrations, requiring sensitive analytical methods such as liquid chromatography-tandem mass spectrometry (LC-MS/MS) for reliable quantification.

Unmetabolised benzene in urine and blood provides a direct measure of recent exposure. Blood benzene reflects current or very recent exposure (within hours), while urinary benzene provides information on slightly longer exposure windows. These direct measures are highly specific but require careful sample handling to prevent contamination and loss through volatilisation.

Biomarkers of Effect

Biomarkers of effect reflect the biological consequences of benzene exposure at the cellular and molecular level. These markers provide evidence of early biological changes that may precede the development of clinical disease, making them valuable for identifying health effects at exposure levels that may not yet produce overt symptoms.

Haematological parameters, including complete blood counts with differential, represent the most clinically accessible biomarkers of benzene effect. Benzene-induced haematotoxicity manifests as reductions in one or more blood cell lineages, including leukopenia (reduced white blood cells), anaemia (reduced red blood cells or haemoglobin), and thrombocytopenia (reduced platelets). In severe cases, pancytopenia (reduction of all cell lines) and aplastic anaemia can develop. These changes reflect benzene’s toxic effects on bone marrow stem cells and progenitor cells.

Cytogenetic biomarkers provide information on benzene-induced chromosomal damage. The micronucleus assay, which detects small extra-nuclear bodies formed from chromosomal fragments or whole chromosomes that fail to incorporate into daughter nuclei during cell division, is widely used as an indicator of genotoxic effect. Studies have consistently demonstrated elevated micronucleus frequencies in peripheral blood lymphocytes of workers exposed to benzene, even at relatively low concentrations.

Oxidative stress markers, including 8-hydroxydeoxyguanosine (8-OHdG), reflect benzene-induced oxidative DNA damage. This biomarker is particularly relevant because oxidative stress is believed to be one of the key mechanisms through which benzene and its reactive metabolites cause DNA damage and initiate carcinogenesis.

Biomarkers of Susceptibility

Individual susceptibility to benzene toxicity varies substantially among exposed persons, and genetic polymorphisms in the enzymes responsible for benzene metabolism play a significant role in determining this variation. Benzene metabolism involves a two-phase process: Phase I oxidation primarily mediated by cytochrome P450 2E1 (CYP2E1), and Phase II conjugation involving enzymes including glutathione S-transferases (GSTs), NAD(P)H:quinone oxidoreductase 1 (NQO1), and myeloperoxidase (MPO).

Polymorphisms in the CYP2E1 gene can alter the rate of benzene activation to reactive metabolites, while polymorphisms in Phase II enzymes affect the detoxification and elimination of these metabolites. The GSTT1 null genotype, for example, results in the complete absence of the GSTT1 enzyme, potentially reducing the capacity to detoxify reactive benzene metabolites and increasing susceptibility to haematotoxicity. Similarly, polymorphisms in NQO1 that reduce enzyme activity have been associated with increased risk of benzene-induced bone marrow toxicity.

Implications for Malaysia

In Malaysia, benzene exposure is relevant to several occupational and environmental contexts. Petrochemical industry workers, particularly those in oil refining and petrochemical manufacturing in industrial zones such as those in Terengganu, Pahang, and Johor, represent the most directly exposed occupational group. Petrol station attendants, who number in the tens of thousands across the country, experience daily exposure during fuel dispensing operations. Environmental exposure affects urban populations through vehicular exhaust emissions, with traffic congestion in the Klang Valley and other major urban centres contributing to sustained low-level ambient benzene concentrations.

The review’s findings support the integration of biological monitoring alongside environmental air monitoring in Malaysian occupational health surveillance programmes. The Malaysian occupational exposure limit for benzene aligns with international standards, but biological monitoring provides a more comprehensive assessment of actual absorbed dose, accounting for individual differences in respiratory rate, work practices, and metabolic capacity.

Limitations

The review acknowledged that most studies on benzene biomarkers have been conducted in non-Malaysian populations, and the applicability of reference ranges and dose-response relationships to Malaysian workers requires further validation. Genetic polymorphism frequencies differ among ethnic groups, and the multi-ethnic composition of the Malaysian workforce means that population-specific susceptibility profiles may differ from those reported in predominantly Caucasian or East Asian study populations. Additionally, many studies reviewed were cross-sectional in design, limiting the ability to establish temporal relationships between exposure and biomarker changes.

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