3. Health Aspects

3.5 Carcinogenicity / Genotoxicity

The carcinogenic risk to humans after long-term exposure to styrene concentrations below recommended Occupational Exposure Limits (ranges from 20 to 100 ppm across different countries) is very low. Most international agencies classify styrene as having a very low or no cancer potency. The International Agency on Cancer Research (IARC) has concluded that there is insufficient evidence for carcinogenicity in humans and limited evidence in experimental animals. Only one study on mice showed an increased incidence in lung tumours after a lifetime inhalation exposure, but recent evidence indicates that mice are not an appropriate model for effects of styrene. Case-control studies with occupationally exposed humans investigating styrene exposure and cancer incidence showed no statistically significant increase in tumour incidence. On this basis it can be concluded that the evidence for carcinogenic activity of styrene in humans is weak.

In vitro mutagenicity tests only yielded positive results for styrene after metabolic activation. In cytogenetic studies with animals or exposed workers positive and negative findings were obtained. Styrene is metabolised to styrene-7,8-oxide, an alkylating epoxide, which was shown to be mutagenic in vitro and carcinogenic in animals only at the site of direct exposure (after oral exposure: stomach). Studies of the bio-kinetic behaviour of styrene and styrene-7,8-oxide in humans, rats and mice have been used to quantify the possible human carcinogenic risk. These data indicate that there is no significant contribution to human cancer risk if the occupational exposure levels are not exceeded.

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