Several research needs, as noted below, were identified during preparation of this document.
1. PCBs have been shown to be carcinogenic. However, in the literature, the doses used to study carcinogenicity of these chemicals in animals are relatively much larger than those to which animals may be exposed in the environment. Furthermore, it is not clear from the studies conducted to date how the information (e.g., carcinogenicity to animals) generated using rodents can be used to predict carcinogenicity in humans. More discriminating studies are required to study these aspects.
2. Commercial PCB formulations are mixtures of PCB isomers and congeners. Several of these congeners (co-planars as well as non co-planars) are very toxic to terrestrial and aquatic animals. Research on congener-specific toxicity, bioaccumulation, and environmental levels is needed for establishing congener-specific guidelines or criteria.
3. Toxicity equivalency (with respect to 2,3,7,8-TCDD) for various PCB coplanar congeners has been established using AHH and EROD enzyme induction in hepatoma cell cultures of terrestrial animals. The toxicity equivalency factors, based on the enzyme induction in terrestrial animal cells, need to be checked for their application in aquatic environments.
4. Several organochlorines, including PCBs, furans and dioxins, have shown a good correlation between in vitro induction of the AHH and EROD enzyme activity in rat hepatoma cells and several receptor-mediated in vivo responses (e.g., body weight loss, thymic atrophy, etc.). The big question, however, still remains to be answered; i.e., should a PCB concentration in water, however small, be considered toxic if it results in the induction of enzyme activity? The relationship between induction of the enzyme (e.g., EROD) activity and long-term toxicity of PCBs to aquatic animals exposed to the same concentration in water needs to be addressed.
5. Recently, an approach based on structure-activity relationships or toxicity equivalency to 2,3,7,8-TCDD has been used to assess the contamination potential of various organochlorines (e.g., PCBs, furans, and dioxins). It is generally assumed that the toxicities (expressed as toxic equivalent of 2,3,7,8-TCDD) of various organochlorines are independent of one another and that the toxicity of all congeners for a given group of organochlorines is additive. Recent investigations have shown that PCB mixtures such as Aroclor 1254 are a dioxin antagonist in rat hepatoma cells. If it is true, then summation of toxic equivalents cannot be a viable approach for estimating the toxic significance of these chemicals in the environment. Interaction between organochlorines and their isomers and/or congeners needs more research.
6. More research is needed on PCB effects (toxicity and accumulation) on humans from primary-contact recreational waters, although this is likely to be of low priority in British Columbia.