More areas violated federal clean air standards last year for ozone, a principal part of smog, while fewer violated the carbon monoxide standard, the Environmental Protection Agency said today.

Sixty-eight urban and rural areas failed to meet the limit for ozone, up from 62 last year. Fifty-nine urban areas failed to meet the carbon monoxide standard, down from 65 last year.There is substantial overlap between the two lists, and all the nation's 24 largest metropolitan areas are on one or both. The largest urban area not on either list is Kansas City.

"We had a hot, dry summer in the East, which tends to push up ozone levels," said EPA spokesman Chris Rice. "It was much like the summer of 1983, except confined to the East."

The decline in carbon monoxide was expected as more and more cars made before 1981 are junked. Cars made before then had to meet less stringent standards for exhaust pollution.

In addition, the same weather that elevates ozone tends to depress carbon monoxide.

The carbon monoxide list is based on data for two years. Looking only at 1987 readings, 37 cities would have failed to comply, while 50 would have flunked in 1986.

Motor vehicle exhaust is the primary source of carbon monoxide, which can interfere with the oxygen-carrying capacity of the blood.

In most areas, motor vehicles also are the major source of a key raw material for making smog, unburned gasoline from both exhaust pipes and natural evaporation from the fuel system.

Those gasoline vapors react in sunlight with oxides of nitrogen, also present in exhausts as well as in power plant emissions, to produce ozone. The sunnier the day, the more ozone that is produced.

Ozone is the part of smog that interferes with breathing and makes the chest feel tight.

Winds can blow the raw materials for ozone away from the source, but they may still react in sunlight, which accounts for violations in several rural counties in Maine, EPA says. The chemicals that made their ozone were emitted in the Boston-Washington corridor.