In an experiment four years ago, federal researchers asked a group of Washington residents to carry air monitoring gadgets with them for a day to help determine how many people are exposed to excessive amounts of carbon monoxide, a chronic pollutant.

The answer came as a surprise. More than 120,000 people, about 10 percent of the nonsmoking adults in the Washington area, were shown to breathe more carbon monoxide than is considered safe. The results were viewed as troubling partly because air pollution indexes had recorded few violations at the time.

"These findings raise the question of whether regulatory agencies can continue to rely exclusively on a system of outdoor monitoring stations to protect the public from elevated exposures to pollutants whose sources are often to be found indoors or in vehicles," said a recent report on the study.

Carbon monoxide has remained a persistent pollutant in the Washington area despite federal and local efforts to reduce emissions, chiefly by regulating car exhaust. Officials say it is uncertain whether the Washington area will comply with carbon monoxide standards by Dec. 31, the federal deadline.

But as the federal study appeared to demonstrate, carbon monoxide may continue to pose health risks for some residents here even if current standards are met. The study, sponsored by the U.S. Environmental Protection Agency, indicated that the standards may not provide adequate safeguards, scientists say.

Carbon monoxide, a colorless, odorless gas, is formed by incomplete combustion in car engines and by cigarette smoke, gas stoves and other appliances. It is frequently detected at congested intersections amid stagnant winter weather, when car engines are cold and run inefficiently.

At high concentrations, carbon monoxide can cause death. At lower levels like those reported in the Washington area, it is considered harmful to people with heart ailments such as angina, a condition marked by recurrent chest pain. Some research indicates it may be a health risk for pregnant women.

Carbon monoxide is dangerous, scientists say, because it can deprive body tissues of necessary oxygen. It forms a stable compound with hemoglobin, reducing the amount of oxygen carried by the bloodstream.

The federal study was designed to measure the amount of carbon monoxide inhaled by 712 nonsmokers here. Similar tests were conducted in Denver, where carbon monoxide is a major pollutant. People who smoke were excluded, officials said, because cigarette smoke is known to contain high levels of carbon monoxide.

Researchers said the results could be interpreted to show that about 10 percent of the 1.22 million nonsmoking adults in the Washington area are exposed to excessive amounts of carbon monoxide.

People were found likely to encounter relatively high carbon monoxide levels if they have long highway trips to work, spend time near people who smoke, own gas stoves or work at polluted sites.

Among those identified by federal officials as often subjected to carbon monoxide are garage attendants, police officers, firefighters, auto mechanics, crane operators and bus, truck and taxi drivers. Officials said carbon monoxide also may pose risks in office buildings with inadequately ventilated garages.