1. Pulmonary, Critical Care, Sleep & Allergy Division, Department of Internal Medicine, University of Nebraska Medical Center, Omaha, NE 68198-5910, USA
2. Research Service, Department of Veterans Affairs Omaha-Western Iowa Health Care System, 4101 Woolworth Avenue, Omaha, NE 68105, USA
3. Department of Environmental, Agricultural, and Occupational Health, College of Public Health, University of Nebraska Medical Center, Omaha, NE 68198-7850, USA