The highest readings for the city's two leading air pollutants can be found at L Street NW between 20th and 21st streets and on Piney Branch Road NW near the Takoma Elementary School, according to a recently released report on city air quality.

The report on readings taken at 12 air monitoring stations around the District showed that in 1983, the L Street site violated federal standards for carbon monoxide six times. Preliminary findings for 1984 show about five violations at that site, caused by the heavy amount of downtown traffic using L Street.

The Takoma School site violated the ozone standard 10 times in 1983 and about four times last year, largely because of the pattern of air currents across the city.

But overall, the report indicated that the District's air is getting cleaner, except for perennial trouble spots of excessive levels of carbon monoxide and ozone.

The 1983 report was delayed and the 1984 report is due out this spring, but officials had preliminary results.

"Every air pollution episode since 1973 has been based on ozone," said Jim Sweeney, acting chief of the air monitoring section of the Department of Consumer and Regulatory Affairs. "It has to do with a lot of stagnant air that builds up. If the air doesn't move out of the city, the pollution just keeps building up."

Both carbon monoxide and ozone are closely linked to auto, truck and bus exhaust. Carbon monoxide, a colorless and odorless gas whose ill effects may range from headaches and nausea to death in extreme instances, is spewed from tailpipes as a result of incomplete gasoline combustion. It builds up especially in winter at congested traffic intersections.

Photochemical smog, which consists chiefly of ozone, is formed through a chemical reaction in sunlight between two auto-exhaust byproducts. It is viewed as a possible health hazard for the elderly, infants and persons with respiratory and other ailments. Smog is most prevalent on hot summer afternoons amid stagnant air.

The District, along with the 50 states, has until Dec. 31, 1987, to meet standards for clean air imposed by Congress to cover dust particles, sulfur dioxide, nitrogen dioxide, lead, carbon monoxide and ozone. Failure to meet the standards could result in reduction of federal funds for highways and sewage projects. So far, the District has met all standards except those for carbon monoxide and ozone.

To bring the air into conformity with federal standards, District officials are looking to two major programs: the vapor recovery program, started in 1977, which requires service station owners to use gas pumps that recycle gas fumes during pumping, and the vehicle inspection program, begun in 1982, which requires all D.C. motorists to have auto exhaust systems checked as part of annual inspections.

Sweeney said the District compares well with other large cities in the nation, most of which also have problems with carbon monoxide and ozone, because Washington lacks the heavy industries that spew out dust and sulfur dioxide. The District also has good wind patterns to help blow pollution out of the area, he said.