WHEN THE HOUSE of Representatives rewrote the Clean Air Act last month, it generously gave a lot of the law away to the automobile industry. It also gave a lot to utilities with plans for big coalfired generators. The Senate was less generous, and its version of the bill is a much more rigorous defense of basic public-health requirements. This interlude before the conference committee meets is not a bad moment to take a look at the Senate's case for the tighter rules.

Over the past couple of decades there's been a dismaying increase in death rates for several types to disease, particularly heart disease and cancer. Part of it is caused by environmental pollution, the evidence shows. It's a slippery and difficult argument to make with total precision, because some of the links in the chain of cause-and-effect aren't yet known. In the case of the common air-borne pollutants like smog, no scientist can say that this particular substance caused that particular death. The case is largely one of statistical patterns, but it is compelling. People exposed to certain pollutants in the air demonstrably run higher risks of these diseases.

Since 1971 the law has set a series of primary standards for air quality, to protect health. THey were established in relation to the immediate effects-the point at which, for example, smog makes your eyes water. There was originally an assumption that, below that level, there were no harmful effects, The assumption, it increasingly appears, is wrong. Constant exposure to smog has a cumulative effect on people. That effect is sometimes chronic and serious disease.

Industry chafes at restraints on siting plants when they are in areas where the extra expense when the air is already cleaner than the law requires? The answer came from Sen. Edmund Muskie in last week's debate: "Testimony on the health question over the last seven years has made the point over and over again that there is no such thing as a threshold for health effects . . .Long-term, low-level exposure to pollutants produces health effects which are not guarded against by national primary standards."

Most American cities, of course, suffer pollution considerably greater than those standards. But the automobile companies say that they haven't the technology to meet the requirements of the Senate bill. True, but they have the capacity to develop it. Certainly they won't make any more progress than the law requires. As the power companies, the bill tightens the rules but doesn't prevent the construction of new plants. The issue is the price of certain kinds of economic growth. The House and Senate conferees might keep in mind that it's not a matter of watering eyes but of mortality rates. Because the most serious effects of this pollution on people's lives are often slow but irreversible, the effects of Congress's decisions will less visible in the 1970s than in the 1990s.