Smokers who quit will breathe easier.

Middle-aged smokers who kick the cigarette habit can stave off a deadly lung disease, according to a study published this week.

Chronic obstructive pulmonary disease, a condition that includes emphysema and chronic bronchitis, occurs almost exclusively in smokers. It is the fourth leading cause of death in the United States, killing about 90,000 Americans each year.

Nicholas R. Anthonisen of the University of Manitoba in Winnipeg and his colleagues at 10 clinical centers in the United States and Canada began the Lung Health Study by recruiting 5,887 smokers, age 35 to 60, with mild airway obstruction but no noticeable breathing trouble. The recruits were all at high risk of developing full-blown chronic obstructive pulmonary disease, which is characterized by lung tissue damage and airway narrowing.

The team randomly assigned volunteers to one of three groups. One group underwent an aggressive smoking cessation program and received a prescription broncholdilator that, when inhaled, opens the lung's airways. Another group participated in the antismoking program but got a placebo inhaler with an inactive solution. The final group received the usual care for people at risk of this disease -- advice to stop smoking.

About 22 percent of the people in the antismoking programs successfully quit their habit, compared to 6 percent of people in the usual care group.

During the study, the researchers monitored lung function, or the ability to move air in and out of the lungs. As time goes on, people with chronic obstructive pulmonary disease may experience breathlessness when walking even a short distance. Eventually, they may become bedridden and require the administration of oxygen.

At the end of 5 years, the researchers found significantly less loss of lung function in the smoking cessation groups than in the usual care group. On average, volunteers assigned to the antismoking programs experienced a 7.4 percent drop in lung function, compared to a 10 percent decline in the usual care group.

When the researchers homed in on a subgroup -- those who quit smoking early in the program and stayed smokefree for 5 years -- a bigger benefit emerged. These exsmokers experienced a 2.5 percent decline in lung function during the study, a rate similar to the age-related drop in lung function seen in people who have never smoked. People who smoked throughout the study had an 11.4 percent drop.

Previous studies had hinted that regular use of a bronchodilator might mitigate this lung disease. The Lung Health Study's data showed that people using this inhaler experienced a slight improvement in their lung function during the first year of the study. That edge disappeared as soon as the bronchodilator treatment was stopped. "The bronchodilator did not change the underlying course of the disease," Anthonisen says. The team describes its findings in the Nov. 16 JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION.

"The study is a landmark in chronic obstructive pulmonary disease research," says Nicholas J. Gross of the Loyola University of Chicago School of Medicine in Maywood, Ill. Gross wrote an editorial in the same issue of the journal.

Researchers know that chronic obstructive pulmonary disease includes a long period, from 20 to 40 years, in which the smoker experiences no definitive symptoms except smoker's cough. Nonetheless, where there is smoke, there is irreversible damage to the lungs.

This study shows that smokers who quit during that at-risk period can reap substantial benefits. "If you stop smoking, you virtually eliminate your chance of getting this disease," Anthonisen says.