CLEAN AIR has a price in a country that uses as much electric power as ours does. You may have noticed the heavy construction in progress at Pepco's Potomac River generating plant. That's the one in the Virginia shore of the river between downtown Alexandria and the airport. The construction there, together with the friction between Pepco and the Environmental Protection Agency, offers a useful air-pollution rules at work.

Smoke from burned coal is harmful to people who breathe it over the years. Scientists cannot say with precision how many people are affected. But there is no question that for some the effects of coal smoke are debilitating, even fatal. The tiny particles carried up the plant chimneys in the smoke can do severe damage to the lungs of people who inhale them over long periods.

The new equipment being installed at the Potomac River plant is $45 million worth of electrostatic precipitators, to take the particles out of the smoke. Running them will require something over $1 million worth of power a year. Pepco estimates that the precipitators will add about 47 cents a month to the typical residential consumer's bill. The company has 410,000 residential customers, and their bills this year have been averaging $32.66 a month. That 47 cents seems a modest cost for the precipitators - but it is hardly negligible, particularly when you consider that this project is only one of many.

Pepco is now in a continuous struggle with the Environmental Protection Agency over air-pollution standards. EPA officials term Pepco recalcitrant, the least cooperative of the half-dozen major utilities in this four-state region. Pepco replies that EPA takes little account of the soaring costs of pollution-control equipment and keeps pressing for restrictions that cannot be justified by any commensurate benefit to public health. Who's right? The health risks clearly associated with coal will incline most people to side with the EPA. But Pepco is right in saying that the costs of protection run high. The utilities are entitled to pass these costs along to their customers, who might best look on them as health-insurance premiums.

Take the case of sulfur, another dangerous polutant in coal. Pepco's Benning Road plant in Washington used to burn coal. But the EPA switched it to oil because it lies in a heavily populated neighborhood. The Potomac River plant, in a less congested area, is permitted to burn coal - but only coal with a sulfur content of less than 1 per cent. Pepco's big plant at Dickerson, in upper Montgomery County, is in much more open country, and it's currently burning coal with a sulfur content of 2 per cent. That violates the current Maryland State standard, the EPA has ordered Pepco to get in line. Pepco is resisting the order. It argues that low-sulfur coal is much more expensive and going to the state standard would nearly double fuel costs at Dickerson with little benefit to air quality.

It's pretty likely that EPA will eventually win that one. That would mean less sulfur in the air in upper Montgomery, and a few more cents on everybody's light bill. That's one reason why it's foolish for political candidates - like those in the recent Virgina State election - to talk about holding down utility bills.The country is in the process of changing its standards of public health protection. That change is altogether desirable. But it is no small undertaking, as you can see the next time you take the bike path that passes the Potomac plant.