1. Occupational Medicine Division and Centre for Environmental & Occupational Health Research, School of Public Health and Family Medicine, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa
2. College of Public Health and Human Sciences, 114B Milam Hall, Oregon State University, Corvallis, Oregon USA
3. Department of Community Health and Epidemiology, Faculty of Medicine, Dalhousie University 5790 University Avenue, Nova Scotia, Canada