IT IS TEMPTING to view it as the ultimate pollution story: the Food and Drug Administration, whose mission in life is to protect the public from taxic substances, has been found to be emitting several quite toxic chemicals from one of its testing laboratories on -- of all impolitic locations -- Capitol Hill.

Congressmen of course rose to the occasion. They were "shocked" and "alarmed." One demanded to know, "Why should I be subjected to thousands of pounds of toxic fumes a year?" The answer is that he, along with all the rest of us, lives and breathes in a pea soup of toxic, carcinogenic and otherwise unhealthy substances. The FDA emissions, amounting to 38 pounds of chemicals per day, add only a tiny increment to what is already in the air. The fact that this amount makes almost no difference is what the congressman should be worried about.

The incident is a beautiful illustration of a paradox that plagues government regulators these days. Congress has told the regulatory agencies to balance the costs of controlling a certain risk against the benefits derived from whatever substance or activity they want to control. Their mission is not to create a completely safe or risk-free environment, but to find a middle ground, based on both scientific expertise and common sense, between an environment that is unacceptably polluted and one that is unacceptably expensive. So, naturally, what most of them have done is focus on the major sources of pollution. Limits have been set -- emission of a certain number of pounds per day, for example -- below which they will not attempt to regulate.This looks like sensible, sound decision-making until the citizen discovers an unregulated polluter just around the corner. Then, with some outrage, he will demand to know why the government hasn't been doing its job.

This pattern of events is going to be with us for a long time -- and it is going to get worse. The technology for detecting and measuring tiny amounts of chemicals is improving by quantum leaps. Only a few years ago regulations dealt with substances found in parts per million. Now measurements, and in some cases evidence of harmful effects, can be made in parts per billion, and even parts per trillion. This ability to measure infinitesimal amounts creates a mass of new information -- and also of unanswered questions. Are substances known to be dangerous actually harmful? How are choices to be made concerning what to test first and what to clean up first? How are regulatory agencies to deal with tens of thousands of small polluters who contribute to the vast sum of pollution without buying the nation in regulations, inspectors and paper work?

Those are the things the FDA's neighbors on Capitol Hill should be thinking about, instead of indulging in a lot of toxic fuming of their own.