1. Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada
2. Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA
3. School of Environmental Health, University of British Columbia, British Columbia, Canada
4. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
5. Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada
6. Population Studies Division, Health Canada, Ottawa, Ontario, Canada