The University of Michigan is starting two studies of chronic lung diseases, trying to discover why some smokers get them and others don't and whether 24-hour oxygen can help treat them.

The National Institutes of Health and National Jewish Health are funding a multiyear study of 12,000 people at Michigan and 20 other medical centers. It will look at why 75 percent of smokers don't develop chronic obstructive pulmonary disease.

The Ann Arbor school is recruiting hundreds of people ages 45 to 80 who are current or former smokers. It's looking for genetic factors in development of lung disease.

A second study will look at hundreds of people over 40 years old to test the effectiveness of round-the-clock oxygen therapy.