The way to tell how bad the air is on a polluted summer day -- besides counting how many times you cough -- is by a number called the Air Quality Index announced over radio and television

The number ranges from 0 to 500, and the higher it is the worse the air. It is calculated by officials of the Metropolitan Washington Council of Governments, an association of local governments.

But there are problems with the index that make it an imperfect way to tell what the air is really like.

For one thing, the number represents the highest readings at any two 54 monitors in the Washington area even though the air may not be consistenly bad everywhere. Wind and other factors may keep air cleaner in some places.

Also, the Evironmental Protection Agency in 1979 relaxed the standard for ozone -- the main component of the automoble-caused smog that Washington suffers from in the summer -- from .08 to .12 parts per million. That meant the index changed, too, and conditions that triggered polution alerts in the past no longer do.

Under the new standard, there were no pollution alerts last summer, but there would have been two under the old standard.

Local officials are critical of the change. Ruth Spector, chairwoman of the Montgomery County Council, said many people "that you wouldn't imagine -- ashmatics, elderly, athletes -- "rely on the index She said these people must now recalibrate their thinking about the relationship between the index and the level of physical activity that they can undertake because now the index "doesn't tell you what you're really used to."

There was an outcry by environmentalists when the standard was relaxed, but then-EPA Administrator Douglas Costle said it was done only after "careful reevaluation of medical and scientific evidence."

He said the new standard would still protect the nation's 5 to 10 million asthmatics, but added that, "Even healthy individuals engaged in vigorous outdoor activity . . . may well experience . . . chest tightness, coughing, wheezing and headaches . . . as a result of ozone concentrations ranging from .15 to .30."

Index numbers between 0 and 100 indicate good or moderate air quality. Between 100 and 200 means unhealthy air, and if the weather conditions will persist, then COG issues a pollution health advisory, meaning people with heart or respiratory ailments should reduce exertion and stay indoors.

Between 200 and 300 means very unhealthy air COG issues a pollution alert, again if stagnant weather conditions are expected, and this adds the elderly to the endangered list.

Between 300 and 400 means hazaradous air, COG issues a pollution warning, and everybody should stay indoors.

Above 400 and it's an emergency: get out of town.

There hasn't been a warning in the Washington area since 1974. During the last two summers there were no alerts for the first time in a decade, although there was one three-day health advisory last July. The previous eight-year average was 16 alert days a year: [TABLE OMITTED]

Of these 31 alerts, all were for ozone except one carbon monoxide alert for two days in January 1973.

The longest ozone alert was for 13 days in late August and early September 1973. The highest ozone level was that .26 reading is Suitland -- more than twice the new EPA standard -- recorded during an eight-day alert in midsummer 1975.