1. Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada
2. Harvard−Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, United States
3. University of Colorado, Boulder, Colorado 80309, United States
4. School
of Population and Public Health, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
5. Health Effects Institute, Boston, Massachusetts 02110-1817, United States