1. Human Environments Analysis Laboratory, Department of Geography, University of Western Ontario, 1151 Richmond Street, London N6A 3K7; Canada
2. Children's Health Research Institute, 800 Commissioners Road East, London N6C 2V5; Canada
3. Department of Epidemiology & Biostatistics, & Department of Paediatrics, University of Western Ontario, 1151 Richmond Street, London N6A 3K7; Canada
4. Human Environments Analysis Laboratory, Department of Geography, School of Health Studies, Department of Epidemiology & Biostatistics, & Department of Paediatrics, University of Western Ontario, 1151 Richmond Street, London N6A 3K7; Canada