1. Pulmonary and Critical Care Medicine Section, Medical Service, Veterans Affairs Boston Healthcare System, Boston, Massachusetts, USA
2. Channing Laboratory, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA
3. Exposure, Epidemiology, and Risk Program, Department of Environmental Health
4. Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, USA
5. School of Economics, University of Maine, Orono, Maine, USA
6. Environmental and Occupational Medicine and Epidemiology Program, Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts, USA
7. Environmental Health Sciences Division, School of Public Health, University of California, Berkeley, California, USA