WHEN A POWER PLANT burns coal, the smoke and fumes constitute a serious threat to the health of the people who live downwind. The federal government is now trying to decide how to enforce, further reductions in homestack pollution. Simply set a tighter emission standard? Maybe. But the basic federal law is a peculiar one. The federal regulators suspect that they are dealing with a genuine paradox. The smokestack may turn out to be one place where slightly less restrictive standards actually result in cleaner air.

Coal-burning plants emit sulfur dioxide and particles of ash. Statisticians have shown that death rates in cities throughout the country are correlated with the presence of those pollutants in the atmosphere. They appear to shorten the life expectancies of the entire populations exposed to them. There is no threshold of that pollution below which people are safe. The case for pushing it to low levels is extremely strong, despite the cost. The issue is not whether to get the air pollution down, but rather how to do it most reliably.

For some years federal law has set a limit on the amount of sulfur that a power plant could emit, in relation to the energy generated. When Congress rewrote teh Clean Air Act last year, it was clear that new technology-stack scrubbers-permitted significantly lower ceilings. Congress could, and probably should, have simply written a lower number into the law: But there was double objection. Scrubbers are expensive. Coal from the eastern fields generally has a high sulfur content; western coal is cleaner. The eastern coal states feard that the utilities would avoid scrubbing by turning to low-sulfur western coal. Their senators and representatives insisted on a formula that would require new plants to install scrubbers, regardless of fuel quality. In one of the season's oddest alliances, some of the environmental organizations supported them. Stack emissions are very difficult to monitor. The environmentalists feared that some plants might promise to use low-sulfur coal but, since it is expensive, cheat and violate the standards.

The result is a law that requires plants to scrub a percentage of the pollutants out of the stack gas-with the percentage to be set by the Environmental Protection Agency. That percentages is the center of the present struggle. The EPA has tentatively proposed that 85 percent of the sulfur must be removed, a proportion that is just about the limit of the present scrubbers' capacity. But you will observe the anomaly: The requirement is 85 percent, regardless of whether there is a lot of sulfur in the fuel or only a little.

This rule has a dismaying implication. Since it would requre utilities to run their scrubbers more or less continuously anyway, it constitues an incentive for them to use the dirtiest, the cheapest coal they can find. The Energy Department wants a sliding scale, permitting utilities to do less scrubbing when they burn a higher grade of coal. It has its own reasons. It is depending on the low-sulfur western coal fields for the vast expansion of coal production on which the Carter energy plan depends, and it does not want the clean-air rules tilted against the West. But it also warns the the scrubbers are not as reliable as the EPA assumes.

Which will protect the public health better-continuous scrubbing or a sliding scale to encourage the use of clean coal? Although EPA adminsistrator Douglas Costel leans toward the first alternative, he has put it forward with a degree of doubt that he does not hesitate to express openly. He is now asking for public comment.

A lot of that comment will come from the utilities. They prefer a sliding scale, which, among other things, saves money. But if they want to persuade Mr. Costel, and the country, they might best talk about guarantees. If the rules do not require continuous scrubbing how can the government ensure that sulfur emissions will be held to the lowest possible level? Since there is no device that can monitor sulfur emissions automatically, how can the companies guarantee compliance? That, and not much else, is the consideration on which the EPA's final decision has to turn.