“I think we are going to find that there is some increase in respiratory symptoms, and maybe even respiratory diagnoses,” said Col. Lisa Zacher, a doctor who is the pulmonary consultant to the Army’s surgeon general. “But I think we’ll find the majority who deploy do not have long-term chronic pulmonary diseases related to deployment.”

Mr. Durham’s breathing struggles have proved to be long-term. When he returned to Fort Campbell, Ky., in 2004, Mr. Durham was coughing up phlegm daily. Running became impossible. Yet a battery of lung tests showed nothing wrong. Before he was medically discharged as a sergeant in 2005, an Army doctor suggested that his problem might be psychological, records show.

Then last year, Mr. Durham read about a specialist at Vanderbilt University Medical Center who had treated Iraq veterans for breathing problems. The doctor did a lung biopsy on Mr. Durham and concluded that he had a debilitating and largely untreatable injury known as constrictive bronchiolitis.

Though the diagnosis might have seemed devastating, Mr. Durham felt vindicated. “I had been told there was nothing wrong with me by so many doctors,” he said. “I just wanted to know what was wrong with me.”

Dr. Robert F. Miller, who treated Mr. Durham, has conducted similar biopsies on 56 previously deployed veterans, many from Fort Campbell. He found that 40 of them had constrictive bronchiolitis, an irreversible scarring of the small airways that can make breathing during moderate exercise feel like “sucking air through a straw,” Dr. Miller said. Fifteen other biopsies led to diagnoses of other lung ailments.

Almost all of his patients had been through standard lung function tests like CT scans and spirometry that found nothing wrong. Constrictive bronchiolitis is typically found in people with lung transplants or rheumatoid arthritis, or who work with industrial chemicals, but is rare in the general population.

“My concern is that these guys come back from war, can’t do a two-mile run and then are dismissed from the Army,” Dr. Miller said. “They are told: ‘Maybe you’re out of condition.’ ”