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Army veteran Scott Weakley has lived his life following many of the rules about health.

A former marathon runner, Weakley, 46, avoided smoking and drinking. So in 2005, after returning from serving in Iraq, Afghanistan, Pakistan and Kuwait the former Army major was baffled why he was not able to do simple things.

"When I got back I could barely run a quarter of a mile," Weakley says. "I could barely go up two or three flights of stairs, I could barely play with my son in the front yard, baseball, maybe 10, 15 minutes."

CT scans showed nothing, so he feared it was all in his mind. "Outwardly, I looked very healthy, so I was like, 'Is this psychological, or am I just making this up?'"

A lung biopsy found Weakley had a rare disease known as constrictive bronchiolitis, a condition he says he did not have before deployment.What's more, Weakley learned he was just one of a growing number of U.S. veterans who had served in Iraq and Afghanistan that had received the same diagnosis --or had asthma or some other respiratory illness.

Doctors at National Jewish Health in Denver, Colo. recently received a $500,000 grant to obtain lung samples to try to figure out what is going on.

"Unfortunately we don't have enough information to know precisely what's causing the increase in respiratory symptoms in people who are deployed," says Dr. Cecile Rose, who has been studying the issue. "What we do know is people who are deployed in southwest Asia, that is Iraq and Afghanistan are exposed to high levels of dust and particulate matter, they're exposed to the combustion products that emanate from the burn pits were things are burned," says Rose.

A study authorized by the Veterans Administration's Medical Center conducted by researchers in New York examined data from 7,000 troops who served between 2004 and 2010. They found those who were located in Iraq and Afghanistan were seven times more likely to suffer from a respiratory illness than those serving elsewhere.

"One of the concerning parts of the science," Rose says, "that's starting to come out now is that the conventional breathing tests and CT scans of the chest that we're doing on people who have shortness of breath for no reason, are normal. The tests are normal."

That was precisely what happened to Weakley.  He now has only 30 percent of normal lung function, and as a result must take a fistful of medications to get through the day.

He says he is not bitter in any way toward the Army and is grateful for the opportunity to serve his country.

Rose says they have a good working relationship with physicians in all branches of the U.S. military. "They're definitely interested in working with us."

Her hope is that the Department of Defense will eventually run breathing tests on troops before deployment to have for a comparison upon return.

Rose says doctors see parallels between this new phenomenon and the respiratory illnesses developed by first responders exposed to high levels of particulate dust at the World Trade Center on 9/11.

What was so helpful with the World Trade Center responders was that there were a population of firefighters who had lung function tests before the World Trade Center, before their exposures on the pile. And in looking at pre-9/11 and post-9/11 lung function there was clearly a difference," says Rose.

Weakley says for him, the difference is clear every time he goes to play with his kids.