In 1987 the International Agency for Research on Cancer (IARC) in Lyon, France, upgraded styrene's classification from a Group 3 "not classifiable" to a Group 2B "possibly carcinogenic to humans." This reclassification resulted from revisions to IARC's classification scheme and considerations of styrene oxide, an intermediate formed during styrene metabolism. No new data from human epidemiology or long-term animal studies prompted the reclassification.
In 1994 and 2002, IARC again considered styrene and kept it in Group 2B. This classification continues to be controversial among some scientists. It is important to note that IARC specifically states that its classifications are intended for hazard identification only, and should not be used for regulatory purposes.
Based on IARC's classification, the Danish Occupational Safety and Health Administration chose in 1994 to adopt the IARC 2B, and set a 25-ppm workplace ceiling value.
In 1994, however, Health Canada, co-administrator with Environment Canada of the Canadian Environmental Protection Act (CEPA), concluded that styrene is "non-toxic," and as such does not require regulation under CEPA. Health Canada gave this designation after examining all aspects of human and environmental health, including a full assessment of styrene's carcinogenic potential. As part of its internal evaluation, Health Canada's classification structure required that styrene be classified as a Class III possible human carcinogen, largely following the IARC conclusion.
However, the CEPA Assessment Report shows that the evaluators were well aware of the complexity and shortcomings of the database on styrene. They further noted that environmental exposure levels for styrene are so minimal that they do not pose any threat to the Canadian general public or the environment. Canada's classification scheme does not drive its regulatory programs to the same extent as the EPA's does in the United States; when real-life exposures were factored in, the final Canadian decision was that styrene did not warrant regulatory consideration.
The United Kingdom's (UK's) Health and Safety Executive (HSE) has drafted a comprehensive risk assessment for styrene. The UK headed the development of this Screening Information Data Set (SIDS) as part of a major chemical evaluation and reporting program under the Organization for Economic Cooperation and Development (OECD). The project was done in connection with the European Union (EU) directive on the evaluation of high-volume existing chemical substances.
The most recent version of the UK's draft review concludes that styrene's health effects data do not suggest that it should be treated as a carcinogen. Representatives of the European styrene industry have subsequently met with the UK HSE to provide reports on more recent styrene research, as the HSE worked to finalize its conclusions. The styrene risk assessment has undergone several review stages by EU member states, and is expected to be finalized in 2004 or 2005.
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