1. Channing Laboratory, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts, USA
2. Exposure, Epidemiology and Risk Program, Department of Environmental Health and
3. Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, USA
4. Department of Resource Economics and Policy, University of Maine, Orono, Maine, USA
5. Pulmonary and Critical Care Medicine Section, Medical Service, VA Boston Healthcare System, West Roxbury, Massachusetts, USA