The Washington area's controversial smog-cleanup efforts are now within range of eliminating hazardous air pollution by the federal deadline of 1987, a new study concludes. But environmental officials say the antipollution drive may be thwarted if dirtier auto exhaust is allowed by Congress.

The findings by the Metropolitan Washington Council of Governments, the region's air-quality planning agency, point to long-term decreases in photochemical smog and carbon monoxide. Nevertheless, the report also underscores uncertainties surrounding several bitterly contested antipollution measures that have become targets of criticism from several industries, members of Congress and the Reagan administration.

Since the 1960s, governments, businesses and consumers in the Washington area have spent millions of dollars to curb air pollution, amid repeated court contests, regulatory shifts and legislative battles. Through cleanups of utility smokestacks, reductions in leaded gasoline and numerous other measures, the region has reached federally mandated targets for sulfur dioxide, soot, nitrogen dioxide and lead.

The most intractable pollutants have been smog and carbon monoxide, two substances closely linked to auto, truck and bus exhausts. Carbon monoxide, 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, hydrocarbons and nitrogen oxides. 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.

If Congress passes proposed legislation to relax emission controls for car manufacturers, "it would make things much more difficult in the Washington area," Austan S. Librach, COG's environmental programs director, said in a recent interview. Librach warned that the District of Columbia, Maryland and Virginia might then have to consider setting their own "special standards" for pollution equipment on autos sold in the region, as California now does.

The COG report estimates that such stricter standards might add $615 to the average price of a car sold here, increasing total costs to Washington area motorists by $89 million a year. An industry spokesman also said the price differential might amount to several hundred dollars. Bill Sessa, a California Air Resources Board spokesman, contended, however, that such predicted price rises may be "manipulated by the auto companies."

The antipollution drive, moreover, is expected to rely heavily on highly controversial, annual exhaust testing of Washington area drivers' cars--an inspection system that has already stirred angry opposition from the Virginia and Maryland legislatures. The Environmental Protection Agency recently threatened sanctions against Maryland, including cutoff of $600,000 in federal air-quality grants, because of a six-month delay in the start of emissions tests.

Another key antipollution measure is a hotly debated plan to require special gasoline-pump nozzles and other new equipment to trap hydrocarbon vapors at filling stations here. The retail gasoline industry vehemently opposes the proposal, citing increased costs and equipment malfunctions. An initial attempt by D.C. officials to mandate the nozzles set off a furor during the late 1970s, though new moves are now under way. Meanwhile, Maryland and Virginia have balked at the plan, pointing to EPA's failure to issue long-sought technological guidance.

Despite the continuing controversies, the new COG study marks a sharp turnabout since last year, when air-quality officials had openly warned that the Washington area would probably fail to reduce pollution enough to meet the 1987 federal deadline. Librach attributed the brighter forecast now to discoveries of several substantial arithmetic mistakes in earlier estimates and to more accurate analyses of some major pollution sources.

"It was simply a mathematical error--but it was a significant one," he remarked. "The picture is definitely different."

In recent years, smog episodes have declined markedly, according to COG statistics. The number of days on which health advisories were issued dropped from 18 in 1977 to one last year and none so far this year. None of the more severe pollution warnings known as alerts has been issued since 1975. (Health advisories are triggered by violations of the federal ozone standard of 0.12 parts per million at two or more monitoring stations during stagnant weather. Alerts signal two or more ozone readings exceeding 0.2 parts per million.)

Officials attribute the smog reductions to numerous factors, including improved auto-pollution control devices, expansion of the Metro subway system, shifts from petroleum- to water-based paints, and installation of new equipment to trap hydrocarbon vapors from solvents at auto repair shops, dry cleaners and other businesses.

To comply with federal ozone limits by the 1987 deadline, the Washington region must cut hydrocarbon emissions to 183 tons a day--a steep drop from the estimated 598 tons emitted 10 years ago and the 339 tons measured in 1980, according to the COG report. The study concludes that this goal may be met by a narrow one-ton margin if federal auto-exhaust curbs are not relaxed, Maryland and Virginia impose the controversial nozzle requirements at gas stations, and annual auto-exhaust testing is carried out throughout the region.

Under the Clean Air Act, the Washington area would face sanctions, including possible cutoff of federal highway and sewer funds, if pollution targets were not met. The new COG study was drafted to comply with a federal deadline this month for showing what further steps must be taken before 1987.

The Clean Air Act has been under attack since last year, however, partly because of the auto industry's economic slump and Reagan administration efforts to ease environmental regulations. It is unclear whether a revised bill will clear Congress this year, although House and Senate committees are considering substantial rollbacks of auto-exhaust standards for nitrogen oxides and carbon monoxide, postponement of the 1987 deadline to 1990 or 1993, less stringent rules for annual auto-emissions inspections and other shifts.

One measure aimed at relaxing pollution standards is stalled in the House Energy and Commerce Committee, where it is being pushed by Chairman John D. Dingell, a Democrat from the auto-producing Detroit area. The bill has industry and Reagan administration backing. Another measure that would reduce some pollution standards--and which is considered more palatable by environmentalists--is under consideration by the Senate Environment and Public Works Committee.

Against this backdrop, the uncertainties posed for regional air-quality officials are evident in several developments, among them the recent dispute over Maryland's delayed car-emissions tests. The Maryland General Assembly earlier this year pushed back the startup date to July 1, 1983--six months later than EPA's deadline.

Last month, EPA Assistant Administrator Kathleen M. Bennett responded by warning Gov. Harry Hughes in a letter that Maryland faces sanctions, including the air-quality grants cutoff and a ban on construction of plants that emit hydrocarbons or carbon monoxide. At the same time, however, she made clear her distaste for the sanctions she was threatening to impose.

"We must enforce the provisions of the Clean Air Act until it is changed," she wrote, noting her "opposition" to mandatory institution of auto-emissions tests as well as to the congressionally established sanctions.

Maryland officials complain, in turn, that they have gotten conflicting signals about the costly testing program from Reagan administration officials and they argue that sanctions are not warranted because the state is making "reasonable progress" toward starting the tests and curbing pollution. Ejner J. Johnson, Hughes' chief of staff, said Maryland is prepared to contest the issue in court if EPA carries out its threat.

In Northern Virginia, annual emissions tests started last December, when then-governor John N. Dalton vetoed a bill that would have delayed them. Mandatory exhaust tests are scheduled to start in the District next January.

COG's proposals for special hydrocarbon-trapping nozzles, hoses and other equipment at gas stations are also mired in conflicts among state, city, federal and industry officials.

After protracted controversy and study, the District is now moving to require the special devices at gas pumps by Oct. 1. Maryland is studying the issue. Virginia officials say they do not plan to mandate the vapor-trapping equipment until EPA issues technological guidance. An EPA official said recently that the agency may soon consider whether to revive its long-shelved vapor-recovery study.

"We're really opposed to the use of the vapor recovery nozzles," responded Vic Rasheed, executive director of the Service Station Dealers of America, an industry group. "It's counterproductive in every way. . . . I think it's going to be a long time before we see it in Maryland and Virginia."