1,3-Butadiene,
isoprene and
chloroprene have all been evaluated more than once by the IARC Monographs Programme on the Evaluation of Carcinogenic Risks to Humans, most recently in February 1998 (Volume 71). Summaries are available on-line at http://monographs.iarc.fr.
1,3-Butadiene is currently classified in Group 2A (probably carcinogenic to humans), on the basis of limited evidence for increased occupational
cancer risk in humans plus sufficient evidence of carcinogenicity at multiple organ sites in rats and especially in mice exposed by inhalation. Four epidemiologic studies are available on
cancer risk among workers exposed to
1,3-butadiene, one large study among
styrene-butadiene rubber (SBR) workers, and one large and two small studies among
1,3-butadiene production workers. The results of the study of SBR workers suggest an association between
butadiene exposure and leukaemia risk, which is consistent with the results of the large study of production workers. This latter study also suggested an increased risk of lymphoreticulosarcoma (ICD-8, 200). The major factors hampering the assessment of the available results are (i) possible misclassification of lymphoid and haematopoietic
neoplasms, (ii) limitations in the assessment of past exposure (with the exception of the study of SBR workers) and (iii) a potential confounding effect of agents other than
butadiene. Future research priorities include (i) the incorporation of newly developed
biomarkers of exposure, (ii) the possible application of intermediate
biomarkers, (iii) the replication of the study among SBR workers, possibly in Europe, and (iv) reanalysis of existing data in light of revisions of the classifications of leukaemias and
lymphomas in the International Classification of Diseases for Oncology, Third Edition (2000).
Isoprene is classified in Group 2B (possibly carcinogenic to humans), on the basis of sufficient evidence for carcinogenicity at multiple organ sites in both mice and rats, especially male mice, exposed by inhalation. No epidemiologic studies are available on
cancer risk from occupational exposure to
isoprene. Such studies could be conducted within the framework of existing or future studies of SBR workers, assuming that
isoprene exposure can be disentangled from
butadiene and
styrene exposure.
Chloroprene is classified in Group 2B on the basis of sufficient evidence for carcinogenicity at multiple organ sites in both mice and rats exposed by inhalation. Studies of
chloroprene exposed workers now include chemical workers from the United States, China and Armenia as well as shoe workers from Russia. The results of the studies from China, Armenia and Russia suggest an excess risk of
liver cancer. The risk of other
neoplasms was not consistently increased. Limitations of available studies include possible bias from cohort enumeration, follow-up, and choice of reference population. In most studies the exposure assessment was poor, the possible confounding effect of co-exposures was not addressed and the statistical power was low. The pathology of the cases of
liver cancer should be reviewed. Future research priorities include a replication of available studies in well-defined populations and the development of
biomarkers of exposure.