1. Division of Environmental and Occupational Health Sciences, Department of Medicine, National Jewish Health, Denver, Colorado
2. Division of Pulmonary and Critical Care Medicine, Department of Medicine, University of Colorado Anschutz Medical Campus, Aurora, Colorado
3. Department of Environmental and Occupational Health, Colorado School of Public Health, Aurora, Colorado
4. Division of Pulmonary and Critical Care Medicine, Department of Medicine, Veterans Administration Eastern Colorado Health Care System, Aurora, Colorado
5. Department of Epidemiology, Colorado School of Public Health, Aurora, Colorado