The comparative potency approach is used to esti-mate the potency of mixtures without having to identify or quantify the individual components (WHO, 1998). This approach differs from others in that it attempts to consider all compounds in a mixture such as air pollu-tion (i.e., PAHs and others). The potency in the bioas-says and the risk to humans are expressed in terms of risk per mass of the organic content. To use this approach, the carcinogenicity of the major source mixtures that contribute to a given ambient environment must be established. It would be costly and impractical to attempt to generate carcinogenicity data on many such PAH mixtures. The levels of each source mixture must then be estimated for a given ambient environment (e.g., industrial city), because contributing sources differ in their carcinogenicity. This can also be difficult to perform in practice. It is also noted that some source mixtures can vary considerably in composition. For example, Nesnow et al. (1982) showed that B[a]P concentrations in four diesel emissions varied by over a 600-fold range. Other authors have proposed that coal tar pitch (CTP) might serve as a suitable standard for polluted air, although information on the concentration of high potency PAHs is incomplete (e.g., no data on DBA was reported) (Foureman and Smith, 1999). Therefore, the suitability of this particular mixture as a surrogate for air pollution is uncertain.
We suggest that this approach lacks the flexibility to be adapted to risk assessment of high potency PAHs.