In the four years since the 1978 Symposium on the Application of Short-Term Bioassays in the Fractionation and Analysis of Complex Environmental Mixtures the use of short-term bioassays to evaluate potential health hazards of complex environmental mixtures has substantially increased. Increased research activity has been particularly noticeable in mobile source emissions, where initial observations on the mutagenic activity of diesel particulate extracts reported at the 1978 symposium stimulated the development of major research programs in government and industry. In the absence of appropriate reference materials, the U.S. Environmental Protection Agency initiated comparative genotoxicity studies to determine the relative mutagenic and carcinogenic activity and, ultimately, the potential human health risk due to exposure to various complex emission products. Among the materials investigated were those of known health risk, such as coke oven and roofing tar emissions and cigarette smoke condensates, and those of unknown hazard, such as exhaust from diesel-and gasoline-powered vehicles. Studies on diesel emission products proved useful in short term bioassay development, as the diesel exhaust extracts were genetically active with low cellular toxicity and could be obtained in relatively large quantities. Availability of such samples aided chemical characterization, and it was eventually determined that the nitro-polynuclear aromatic hydrocarbons were among the mutagenic components of diesel exhaust particulate.

With this proceedings of the fourth symposium on complex mixtures, we continue to revise and extend our knowledge of genetic methods for the evaluation of chemical mixtures in the environment. The early chapters of this volume are devoted to new bioassay techniques that are directly applicable to the monitoring of environments contaminated with genotoxic chemicals. Microbiological methods have been further refined to meet the special needs of atmospheric monitoring so that very small samples may now be efficiently tested. New in situ methods utilizing green plants actually avoid many of the usual difficulties of sample collection and preparation and offer special advantages in monitoring wastewater, sludges, and hazardous wastes. Insects also are being employed very effectively in the evaluation of gaseous air pollutants in controlled laboratory investigations. Increased emphasis has been placed on a comprehensive assessment of the potential of complex mixtures t9 cause various kinds of genetic damage. New assays for chromosome structural and numerical aberrations in mammalian cells in vitro have been developed and are being applied in laboratory studies. Efforts to link tests for gene mutation and cell transformation in vitro with assays for tumorigenesis in vivo are contributing to the validation of the short-term testing approach. Studies comparing in vitro and in vivo data on a coal conversion by-product, on polycyclic aromatic hydrocarbons, and on mineral fibers are reported in separate papers. Later chapters are devoted to investigations on the fractionation and biological evaluation of specific chemical components within complex mixtures.

More than one hundred short-term bioassays are now available for detecting the toxicity, mutagenicity, and potential carcinogenicity of chemicals. These bioassays were developed and validated with individual compounds, and their principal application was perceived to be in evaluating the health hazard of such materials. However, man is rarely exposed to single chemicals; his exposure to hazardous chemicals is more commonly a multifactorial phenomenon. Although chemical analysis can be used to detect known hazardous compounds, it would be a staggering and expensive task to analyze large numbers of samples for all known or suspected hazardous constituents. Furthermore, the biological activity of a complex mixture cannot be reliably predicted from knowledge of its components. On the other hand, bioassays alone cannot tell us which components of complex mixtures are responsible for the biological activity detected. Thus, cost effectiveness and technical feasibility dictate stepwise and perhaps iterative application -of both chemical and biological methods in evaluating the health effects of complex environmental mixtures. Through the coupling of reliable biological detection systems with methods of chemical fractionation and analysis, it is frequently possible to isolate the individual chemical species that show biological activity. Initially, complex mixtures may be separated and bioassayed in carefully defined chemical fractions. The results of such short-term screening bioassays then may be used td guide the course of further fractionation and to determine the need for more stringent and comprehensive biological testing.