1. a Air Health Effects Division , Health Canada , Ottawa , Ontario , Canada
2. b Department of Medicine and Department of Epidemiology and Biostatistics, McGill University and Division of Clinical Epidemiology , McGill University Health Centre , Montréal , Quebec , Canada
3. c Division of Biostatistics, Department of Preventive Medicine , University of Southern California , Los Angeles , CA , USA
4. d Meteorological Service of Canada , Toronto , Ontario Canada
5. e Division of Biostatistics, Department of Preventive Medicine , University of Southern California , Los Angeles , CA , USA