1. Occupational Medicine Division and Centre for Environmental and Occupational Health Research, School of Public Health and Family Medicine, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa
2. Occupational Medicine Division and Centre for Environmental & Occupational Health Research, School of Public Health, University of Cape Town, Cape Town, South Africa
3. Department of Environmental and Occupational Studies, Faculty of Applied Sciences, Cape Peninsula University of Technology (CPUT), Cape Town, South Africa
4. Department of Epidemiology, Johns Hopkins School of Public Health, Baltimore, USA