The Environmental Protection Agency said average air pollution levels have decreased nationwide during the past 10 years, but ozone and carbon monoxide remain stubborn urban problems.

In its annual report on long-term air pollution trends, the EPA said Wednesday that between 1978 and 1987, significant reductions have been achieved in the six primary air pollutants regulated by the government - ozone, carbon monoxide, soot, sulfur dioxide, nitrogen dioxide and lead.The most dramatic success story involves lead levels, which have fallen 88 percent during the past decade. The reduction is largely attributed to the phase-out of leaded gasoline. High levels of lead can permanently damage the nervous system, especially in children.

However, the agency said that while improvements have been made, many cities still do not meet federal air quality standards and that ozone smog, in particular, appears to be worsening.

Last year, the EPA cited 66 urban areas for exceeding federal ozone limits and preliminary 1988 monitoring data show another 30 cities may be added to the list of ozone violators.

The agency listed 59 urban areas in 1988 that violated the carbon monoxide standard, but the figure was six fewer than in 1987.

Don Clay, acting assistant EPA administrator for air and radiation, said ozone was an especially difficult pollutant because it is not emitted directly from cars and factories. It is produced by the chemical reaction of volatile organic compounds and nitrogen oxide emissions when they are exposed to sunlight.

"Obviously, we are disappointed in some areas, such as ozone, but overall the data clearly show things have gotten better over the long term," Clay said.

The report said 102 million Americans lived in areas which exceeded at least one federal air quality standard in 1987. Some 88 million Americans were exposed to excessive ozone, more people than for the other five primary pollutants combined.

Ozone is beneficial in the upper atmosphere, where it screens out harmful levels of ultra-violet radiation from the sun, but low-lying ozone impairs breathing and irritates eyes.