NEARLY FOUR years ago, Congress passed the landmark law aimed at preventing toxic substances from being among the 1,000 or so new chemicals marketed each year. The law also required that the huge backlog of nearly 70,000 existing chemicals be systematically reviewed, and those found to be poisonous regulared or banned. But it was its first-strike capability -- the authority to act before any damage had been done -- that made the Toxic Substances Control Act special.

Congress had good reason to grant this pre-marketing power. Once chemicals are on the market, it becomes extremely difficult to get them off. Heavy economic and political pressures can be generated against any proposed regulation, and seemingly endless litigation is the rule, not the exception. The Environmental Protection Agency estimates, for examples, that fully one-third of the 1,500 active ingredients of currently used pesticides are toxic, and one-fourth carcinogenic. The regulatory process is so slow and difficult that only about 1 percent of all the chemicals in use have been either given a clean bill of health or regulated.

Nevertheless, the new authority to prevent dangerous chemicals from being marketed had not been used until two weeks ago, when EPA announced that it was prohibiting the manufacture of six new chemicals because of a serious potential health threat. many similar compounds are in wide use. But EPA acted on the basis of some recent disturbing research findings and because the manufacturer submitted no evidence of having performed any of the necessary safety tests.

Under the law, the manufacturer -- whose identity is treated as a business secret -- could have taken EPA to court, but instead it decided not to market the chemicals after all. The company may have decided that the value of the products was not worth the regulatory delay, but it seems at least as likely that it believed that the proposed new chemicals could not pass the safety tests. It would be interesting to know whether the company knew of the dangers all along and was just hoping that the hopelessly overburdened EPA office would not notice, or whether it was unaware of the recent research showing similar substances to be highly toxic.

Either explanation, of course, is a powerful argument for this type of before-the-fact regulation. The process is quick, requires the company -- rather than the government -- to provide the necessary evidence of safety, and involves a minimum economic penalty to the manufacturers. The process should eventually encourage chemical companies to give environmental and health considerations a higher priority in their long-range planning so that money won't be wasted developing products too dangerous to market.