NEW YORK - Some soldiers have returned from Iraq and Afghanistan with an untreatable lung disease that interferes with their ability to do physical exercise, possibly caused by inhaling toxic material, doctors report.

The illness is rare in otherwise healthy young people, Dr. Robert Miller of the Vanderbilt University Medical Center in Nashville, Tennessee, and colleagues say in Thursday's New England Journal of Medicine.

Their analysis can't show how common the condition is in the troops nor positively identify its cause. But 28 of the 38 diagnosed soldiers in the analysis had been exposed to a sulfur-mine fire near Mosul, Iraq, in 2003. That suggests they inhaled a significant dose of sulfur dioxide, a known cause of the lung disease, called constrictive bronchiolitis, Miller said in a telephone interview.

Dust storms and the burning of waste in pits may also have played a role, he said. Identifying the cause would help with prevention, he said.

The soldiers, referred to Vanderbilt from Blanchfield Army Hospital at Fort Campbell, Kentucky, were evaluated between 2004 and 2009. The diagnoses were made after lung biopsies. At least half the soldiers have left the service with a disability rating. The researchers said they have counted nine more cases since 2009.

In an email, R. Craig Postlewaite of the Office of the Assistant Secretary of Defense and two other Army medical experts said they had "some concerns" about the lung biopsy procedures and the diagnoses. They said they're working with several authors of the report and others to do further research.