First it was PCBs. Then asbestos took center stage. Today, dioxin is the ''pollutant-of-the-month,'' observes a congressional aide. Dioxin is the latest substance to develop, almost overnight, an image as a villainous hazardous waste.
It is undoubtedly a very dangerous chemical. But many experts say that public and media reaction to the substance's sudden prominence was overblown and that very little is known about dioxin's affect on humans.
In fact, say these sources, the whole area of hazardous waste regulation is fraught with scientific uncertainty, and complicated by what Environmental Protection Agency (EPA) Administrator William D. Ruckelshaus calls public ''emotionalism.''
''We're not on real firm science in this area,'' says Virginia Gibbons, EPA director of legislation. ''We struggle with this whole question - is this chemical hazardous? What does 'hazardous' mean, anyway?''
Yet many people think the world of hazardous substances is a world of known facts. In a recent speech, Mr. Ruckelshaus spoke of the public's ''thirst for certitude'' in this area, saying that the voters assume EPA knows what all bad pollutants are, what damage they cause, and how to control them.
This isn't true. As a recent Congressional Office of Technology Assessment report concludes, scientists can't yet prove with any certainty that there is a link between industrial hazardous wastes and human health problems. ''Even with the amount of exposure documented for residents at Love Canal (in western New York state), the relationship between the exposure and health problems is not readily apparent at this time,'' says the report.
That doesn't mean the chemical stews in waste sites across the country aren't dangerous. It means, for the most part, that scientists haven't yet figured out what health problems particular chemicals cause, what quantities are dangerous to people, and how toxic substances behave when released into the environment.
Dioxin, for instance, has proven fatal to animals. But scientists aren't sure if humans would suffer the same consequences, and they don't know how, or if, dioxin changes once it's mixed with soil.
In the midst of this uncertainty, Congress has charged the EPA with compiling a list of all hazardous wastes, deciding their relative dangers, and setting priorities for cleaning the dump sites around the country.
The uncertainty over a chemical's effects complicates the process. Before Times Beach, Mo., hit the evening news, for example, most forms of dioxin weren't on EPA's official list of regulated hazardous wastes.
''They just hadn't gotten around to it,'' says Blake Early, a Washington representative of the Sierra Club.
EPA officials admit that until Times Beach, the study of dioxin had not been a high priority at the agency. But rulemaking proceedings have begun to place most dioxins on the hazardous waste list which the Resource Conservation and Recovery Act of 1976 (RCRA) requires the agency to keep. And, these officials point out, dioxin was already partially controlled under a separate law that governs dangerous chemicals.
Currently, the RCRA hazardous substance list includes approximately 80 general groups of waste, says Mike Cook, deputy director of the EPA office of solid waste. In addition, several hundred everyday products, such as paint, are considered ''hazardous'' once they are thrown away. ''We've listed everything for which there is enough scientific information at hand,'' he says.
Since the list was first compiled in 1980, he adds, EPA has launched several major studies, particularly in the area of organic chemicals, to seek out other hazardous substances.
But some critics charge that EPA is moving with glacial slowness to find and regulate new toxic wastes. After all, they point out, the RCRA list has remained essentially unchanged for three years.
A bill to modernize and tighten RCRA, now on the House floor, would order EPA to quickly add most dioxins, PCBs (polychlorinated biphenyls), dyes and pigments , paint production wastes, and other substances to its hazardous waste list, unless the agency can prove these toxics don't need to be regulated.
''The dioxin hysteria was useful. It helped us to quickly move this bill,'' says a congressional source who asked not to be named.
Blake Early of the Sierra Club says EPA's definition of ''hazardous'' is too narrow. He complains that the agency doesn't consider the carcinogenic effects of a substance when testing it for hazardous qualities.
Indeed, the Office of Technology Assessment found that 32 state environmental offices use broader definitions of ''hazardous'' than EPA does.