Ozone, a quirky form of ordinary oxygen, is once again helping to spread a summertime blanket of throat-burning smog over Washington and other polluted cities, renewing worries for joggers, health researchers, gasoline dealers, government officials, environmentalists and youngsters at camp.

"There seems to be evidence to indicate very strongly that the whole Northeast is one big sewer, if you will," said Maryland air management director George P. Ferreri, adding that some of Washington's smog seems to waft over Baltimore. "Baltimore is downwind from Washington."

This year marks the deadline for cities to comply with smog limits set under the federal Clean Air Act, and the Washington area is considered certain to fail the test for ozone, a key pollutant used to gauge smog levels. So are dozens of other major urban areas.

"It doesn't look like there's any way the region could meet the standard" by the Dec. 31 deadline, said Robert C. Kaufmann, an environmental planner for the Metropolitan Washington Council of Governments, which has coordinated local cleanup efforts. A new plan to attack ozone will have to be devised, he added.

Competitive cyclists, fast-paced runners and people lugging loads upstairs are among those most likely to incur ozone's ill effects, scientists say. Exercise has been found to heighten smog symptoms. Children at camp are a focus of concern because they spend many hours at outdoor sports, researchers say.

Despite a multimillion-dollar pollution campaign that has led to increased costs and inconvenience for car owners, the outlook for getting rid of smog appears, at best, hazy. Additional curbs on car exhaust and gasoline fumes are among steps being weighed by local, state, congressional and federal officials.

"Ozone is a very tough pollutant to control," said S. William Becker, who heads a national group representing state and local air pollution officials. "The states find themselves up against a wall on this one."

Ozone is one of two federally regulated air pollutants that have proven more than a match for environmental strategists here. Carbon monoxide, a poisonous gas also linked to car exhaust, is the other. It is unclear whether the Washington area will meet this year's carbon monoxide deadline, officials say.

Both of these unwanted vapors have hung over the region despite major steps to eliminate other airborne hazards. Sizable reductions have been recorded in such pollutants as sulfur dioxide, soot and lead as a result of federal and local controls on utility smokestacks, car exhaust and other emissions.

Ozone, a Jekyll-and-Hyde substance that displays its conflicting natures in disparate parts of the earth's atmosphere, differs from ordinary oxygen only by one atom. A molecule of oxygen, a biological necessity, consists of two oxygen atoms. A molecule of ozone contains three. As a result, ozone is unstable and highly reactive.

High in the stratosphere, about 12 miles above the earth's surface, ozone is beneficial. There it absorbs ultraviolet radiation, preventing these cancer-causing rays from harming human beings and other forms of life. Scientists are worried about possible losses of stratospheric ozone.

At ground level, however, the gas is considered a hazard. In the absence of ultraviolet radiation, ozone, a potent oxidizing agent, reacts rapidly with animal and plant tissue. Some scientists say it may cause premature aging of the lungs and other respiratory damage.

Ozone is difficult to control because it forms in complex ways from other gases. When sunlight strikes a molecule of nitrogen dioxide, a pollutant linked to car exhaust, an oxygen atom may be knocked off. This unconnected atom may then combine with an ordinary two-atom molecule of oxygen to form ozone.

Additional ozone may build up through a series of reactions involving volatile carbon and hydrogen compounds common in car exhaust, gasoline and other petroleum-based products. Along with ozone, these reactions may yield other smog ingredients, such as peroxyacetyl nitrate (PAN).

Because of sunlight's prominent role in the reactions, ozone is prevalent on hot, cloudless summer days, when it often mounts to afternoon peaks. Ozone lasts longest in stagnant weather because wind tends to blow pollution away. Amid air currents, Washington's ozone may drift over Baltimore or Boston.

In recent years, scientific studies have shed new light on ozone. The research, conducted in part on human volunteers exercising on treadmills in special ozone chambers, has dispelled some past claims, raised new worries and left many uncertainties.

Clinical tests appear to have shown, for example, that ozone does not cause the eye irritation often ascribed to it. Researchers say that, instead, on smoggy days, eye soreness probably stems from PAN.

Despite longstanding warnings that ozone may be hazardous to people with respiratory ailments, proof has been elusive. Tests have indicated little difference between ozone's effects on healthy and unhealthy people. Nevertheless, some doctors argue that lung patients cannot afford such risks.

Exercise has consistently been shown to intensify ozone's effects, scientists say. Studies have detected reductions in lung capacity, lessened air flow through bronchial tubes and symptoms similar to those accompanying asthma, even at moderate ozone levels like those often recorded on summer days here.

In one troubling discovery, researchers have shown that a substantial proportion of the overall population -- estimated at 5 to 20 percent -- is especially sensitive to ozone. No one knows who these so-called responders are or why their respiratory systems show more susceptibility to smog.

Such findings, along with laboratory studies of rats and monkeys subjected to heavier ozone doses, have caused concern among doctors, set off calls for expanded research and sparked debate over a possible federal move to tighten the ozone standard, which was relaxed eight years ago amid marked controversy.

Moreover, several key questions remain unanswered, scientists say. Does ozone cause permanent respiratory damage, or is it merely a temporary irritant? Do repeated exposures to low or moderate ozone levels have long-lasting consequences, or are the health risks limited to high smog doses?

"No, we don't know. But there's reason for concern," said Dr. Rebecca Bascom, medical director of the University of Maryland's environmental research facility in Baltimore, citing recent data showing signs of respiratory abnormalities even at levels considered safe under the current ozone standard.

"The presumption is that ozone will lead to some decrease in lung function," said Dr. Bernard D. Goldstein, a former federal environmental official and chairman of environmental and community medicine at the state-supported Robert Wood Johnson Medical School in Piscataway, N.J. "It's an unproven presumption." But, he added, "I'm concerned about that chronic effect."

Ozone has been found harmful to forests and crops, including potatoes grown in Maryland. Boris Chevone, a plant pathologist at Virginia Polytechnic Institute and State University at Blacksburg, pointed to yellowing needles and brownish tips as signs of damage to white pines in the Blue Ridge Mountains.

In the Washington area, ozone levels have decreased since the 1970s, when full-fledged smog alerts were last issued. But, officials say, little headway has been made in recent years. This year, air monitors have recorded several violations of the federal standard, 0.12 part of ozone per million parts of air.

Despite past threats of federal sanctions, including cuts in highway funds, the Washington area does not appear to be in immediate jeopardy, federal officials say. The U.S. Environmental Protection Agency has proposed to withhold penalties for polluted cities if they continue taking steps to reduce ozone.

Moreover, ozone levels here are markedly less severe than in such smog-ridden cities as Los Angeles and Houston, officials note.

In the Washington area's war on smog, not everything has gone as planned. Five years ago, officials predicted that ozone violations could be halted by now, largely by curtailing pollution from cars and gasoline.

But Maryland and Virginia officials balked at one key measure, a proposal to require special nozzles at gasoline stations to trap vapors. The District mandated the nozzles along with other pollution equipment, but a 1984 survey found widespread disregard of the rules. City officials say most stations now comply.

Also, many cars in the Washington area were required to undergo exhaust tests. However, federal officials soon cited flaws in the checkups, especially in Virginia, where only 3 percent of the cars tested failed. Virginia officials are weighing changing the tests. And the District has revamped its test apparatus.

At the same time, key federal measures have been stalled by controversy. Under one EPA proposal, car manufacturers would have to install devices to control gasoline fumes. Also, the agency is considering a plan to limit volatile ingredients in gasoline.

Clean-air advocates have pressed for these and other steps, including a clampdown on oil-based paint. Industry officials have raised the specter of higher gasoline and car prices. Some local officials have expressed dismay.

The Washington area needs to find a way to eliminate tons of pollution, said Fairfax County air pollution control chief Edgar M. Chase, "and I don't know where the devil we're going to get it."