Is styrene toxic to humans?

Styrene (CAS 100-42-5) is a widely used aromatic monomer, primarily used in polymer production. Existing toxicological studies indicate that styrene is toxic to humans, with toxicity depending on the route of exposure, duration, and concentration.

Acute Toxicity and Irritation

Styrene exhibits low to moderate acute toxicity in humans. Its main effects are skin and respiratory irritation and central nervous system (CNS) depression. Liquid styrene is irritating to the skin; direct contact can cause erythema, while prolonged exposure may lead to blisters and dermatitis. Human responses to different concentrations of styrene are as follows:

•  400 to 600 ppm of styrene has been reported as the lowest toxic concentration range for humans.

•  Subjects exposed to 600 ppm of styrene reported eye and nasal irritation.

• Subjects exposed to 800 ppm of styrene for up to 4 hours immediately experienced nasopharyngeal irritation accompanied by increased nasal discharge.

Central Nervous System Effects

High concentrations of styrene exposure can lead to central nervous system depression. Observed symptoms included drowsiness, lethargy, muscle weakness, and unsteady gait. One study showed that subjects exposed to 376 ppm styrene for 1 hour experienced decreased neurological test results in addition to eye and nose irritation, nausea, and headache.

Genogenic Toxicity

Occupational exposure studies have provided some evidence of styrene's genotoxicity. Workers with occupational styrene exposure had a higher incidence of chromosomal aberrations in peripheral blood lymphocytes compared to unexposed workers. Specific findings include:

• No increase in chromosome breaks or sister chromatid exchanges (SCEs) was observed in workers exposed to styrene concentrations below 50 ppm.

• However, a significant increase in the number of chromosome clefts was observed in exposed workers.

• Other studies have shown a significant increase in the number of micronuclei in styrene-exposed workers.

Carcinogenicity

Currently, the evidence regarding the carcinogenicity of styrene in laboratory animals is inconclusive, and there is a lack of convincing evidence to definitively confirm its carcinogenicity. Data regarding the carcinogenicity of its main metabolite, styrene oxides, are also controversial. However, one study found an increased incidence of mammary adenocarcinoma and elevated levels of leukemia-lymphosarcoma in female rats exposed to 600 ppm styrene.

Pharmacokinetic studies of styrene in humans show the following absorption and metabolism:

•  Absorption: Approximately 60% to 70% of inhaled styrene is absorbed by the body.

• Metabolism and Excretion: Over 85% of inhaled styrene is excreted in urine as mandelic acid and phenylglyoxylic acid.

•  Distribution: Adipose tissue may be a reservoir for styrene in the body. Studies have shown that styrene can still be detected in fat samples from workers up to 3 days after exposure.

In summary, styrene has significant irritant, central nervous system toxicity, and potential genotoxicity in humans, but evidence of its carcinogenicity is limited. Occupationally exposed populations need to take effective protective measures to control the risk.

References

[1]Bond, J.A. and Bolt, H.M., 1989. Review of the toxicology of styrene. CRC critical reviews in toxicology, 19(3), pp.227-249.

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