LONG HOT DAYS bring Washington's air pollution to its annual peak. The present season is not a bad time to think about the costs and benefits of improving air quality and the next steps that the law will require -- steps that involve everybody's cars. The recent series of articles by Phil McCombs in this newspaper provides a useful and balanced briefing on the coming round of public decisions.

The most dangerous forms of air pollution are those produced by burning coal to generate power, and controlling pollution from power plants has demonstrably lowered death rates. The costs, while substantial, are not high in relation to those results. Pepco's investment in pollution abatement, over the past decade, has worked out to about $35 a year for each customer. That figure will rise in the years ahead, but for the health of the people who live downwind from the stacks, the benefits are clearly worth the money.

Automobiles produce a different range of pollutants, which, in their effects on human health, are neither so direct nor so easily demonstrable as those of coal smoke. Ozone is the most common air pollutant in Washington, and it comes mainly from cars. But, at its present levels, it does not seem to have severe or permanent effects on people's health.

The Council of Governments has been reporting "unhealthful" levels of air pollution in this area during the past week. Under the federal definition of the term, "unhealthful" means "mild aggravation" of symptoms in people who suffer from respiratory illness or other special susceptibilities, and "irritation" among healthy people. How much should society spend to reduce these kinds of irritation further?

Allergic people don't like being made ill by car exhausts, if only mildly, and healthy people don't like eye and nose irritations, if only temporary. There's always the disquieting possibility, as well, that automobile fumes may eventually turn out to have more serious effects over the long term. That's why the public consensus in favor of reducing automobile pollution has proved remarkably durable over the past decade, despite the cost that it adds to the price of a new car. That consensus is going to be put to a more difficult test over the next several years when -- beginning next January in Virginia, and a year later in Washington and Maryland -- you will have to put your car periodically through an emissions inspection.

That will be another of life's minor exasperations. Will it be worth the trouble? Emissions inspections are now mandatory in two states. New Jersey and Rhode Island, and are getting under way in cities in seven others. The requirement doesn't seem to have incited rebellion there.

The outcome of this regulatory experiment will mainly depend on people's attitudes about the smog that cars cause. Sensible public policy will recognize the large elements of uncertainty in the scientific evidence, and will acknowledge the need to keep revising environmental standards, tightening some and loosening others, as the data accumulate. These new regulations and inspections can be justified as an attempt to diminish a frequent nuisance, if not a major threat. But sensible public policy will also require reconsideration of this inspection rule from time to time, to see whether those benefits stay in balance with the costs.