Hexabromocyclododecane (HBCD) is a globally produced brominated flame retardant (BFR) used primarily as an additive FR in polystyrene and textile products and has been the subject of intensified research, monitoring and regulatory interest over the past decade. HBCD is currently being evaluated under the Stockholm Convention on Persistent Organic Pollutants. HBCD is hydrophobic (i.e., has low water solubility) and thus partitions to organic phases in the aquatic environment (e.g., lipids, suspended solids). It is ubiquitous in the global environment with monitoring data generally exhibiting the expected relationship between proximity to known sources and levels; however, temporal trends are not consistent. Estimated degradation half-lives, together with data in abiotic compartments and long-range transport potential indicate HBCD may be sufficiently persistent and distributed to be of global concern. The detection of HBCD in biota in the Arctic and in source regions and available bioaccumulation data also support the case for regulatory scrutiny. Toxicity testing has detected reproductive, developmental and behavioral effects in animals where exposures are sufficient. Recent toxicological advances include a better mechanistic understanding of how HBCD can interfere with the hypothalamic-pituitary-thyroid axis, affect normal development, and impact the central nervous system; however, levels in biota in remote locations are below known effects thresholds. For many regulatory criteria, there are substantial uncertainties that reduce confidence in evaluations and thereby confound management decision-making based on currently available information.

Polar and nonpolar fractions prepared from an organic extract of inhalable air particulate material collected from an urban location in downtown Toronto, Ontario, Canada, were examined for estrogen and Ah receptor-mediated activities using in vitro gene expression assays. The presence of estrogenic activity was determined using MCF-7 human breast cancer cells transiently transfected with a Gal4-human estrogen receptor chimera and a Gal4-regulated luciferase reporter gene. 2,3,7,8-Tetracholordibenzo-p-dioxin (TCDD)-like activity was detected using Hepa 1c1c7 cells transiently transfected with a CYP1A1-regulated reporter gene (pGudLuc 1.1). Significant estrogenic and TCDD-like activity was detected in the crude extract and in the nonpolar fractions. Results from the analyses of nine environmentally prevalent polyaromatic hydrocarbons (PAH) indicated that PAH might be significant contributors to the observed activity. Surprisingly, three PAH, namely benzo[a]pyrene, chrysene, and benz[a]anthracene, were found to substantially induce in vitro estrogenic and TCDD-like activities that were mediated by the estrogen and Ah receptors, respectively. Benzo[k]fluoranthene, dibenz[a,h]anthracene, and anthracene also exhibited significant in vitro TCDD-like activity. These results demonstrate the utility of in vitro gene expression assays to identify the presence of potential endocrine disruptors within complex mixtures.