1. Environment and Health Research Unit, South African Medical Research Council, Johannesburg, South Africa, Environmental Health Department, Faculty of Health Sciences, University of Johannesburg, Johannesburg, South Africa
2. Department of Geography, Geoinformatics and Meteorology, University of Pretoria, Pretoria, South Africa
3. Department of Geography, Geoinformatics and Meteorology, University of Pretoria, Pretoria, South Africa, Environment and Health Research Unit, South African Medical Research Council, Pretoria, South Africa
4. Department of Family Medicine, University of Pretoria, Pretoria, South Africa