ON THE MORNING of Sept. 12, 2001, the management of the Blue Plains water treatment plant at the southern tip of Washington made a decision. In the past, the plant had treated sewage with gaseous chlorine, a chemical that, if leaked, could have enveloped the entire city in a poisonous cloud. Although the plant had safety procedures, its managers figured it could not withstand a terrorist attack, and they substituted another chemical. The change cost money, both to the plant and, marginally, to customers in the Washington area, who now pay as much as 50 cents more annually for sewage treatment. It may also have created some new, albeit lesser, safety hazards. Still, the costs, given the risks, seem bearable. So shouldn't the same kind of decision be made elsewhere?

In the coming weeks Congress may well find itself pondering this question. The Environmental Protection Agency reckons that 123 of the United States' chemical plants are very close to major urban centers -- so close that a large explosion could put more than a million people at risk. Nor is this a secret: Reportedly, Mohamed Atta, the Sept. 11 ringleader, scouted a chemical plant in Tennessee.

Some leading members of the chemical industry -- the companies that are members of the American Chemistry Council -- have voluntarily conducted a systematic review of security measures. Last year, Sen. Jon S. Corzine (D-N.J.) introduced legislation designed to make such reviews mandatory. He failed, however, thanks largely to the lobbying of the American Chemistry Council and others.

Prodded by the continued efforts of Mr. Corzine and others, it seems the administration is now planning to introduce its own chemical safety bill, which may include a mandatory inspection regime. In principle, this is to be applauded. But when considering it, Congress should pay close attention to the fine print, and to one issue in particular: that of "hazard reduction." In effect, there are two ways to make a plant safer: with fences, guards and crash barriers or through hazard reduction, which alters the kinds of chemicals that the plant stores and produces, as the Blue Plains plant did. The chemical industry is balking at any suggestion that outsiders should meddle in the intimate workings of its plants, arguing that the industry already conforms to EPA and other regulations concerning toxic chemicals. But in the case of plants close to major urban areas, some meddling is warranted: None of the current regulations deals specifically with terrorist threats.

Plants must be required not only to conduct and submit a hazard assessment to the federal government but to develop and implement a response plan that includes technological change -- hazard reduction -- as well as better fences, if necessary. At the same time, regulatory decisions should take into account both the relative risks and the relative costs, which means that the regulatory process cannot apply in the same way to every plant, even those that use similar procedures. For that reason, the main responsibility for inspections should go to the Department of Homeland Security and not the EPA, which has a different regulatory tradition. No doubt it will be difficult to hammer out a working compromise on this issue, which isn't to say that it shouldn't be done. Too much is at stake.