1. Environment and Health Research Unit, South African Medical Research Council, Pretoria 0001, South Africa
2. Department of Geography, Geoinformatics and Meteorology, University of Pretoria, Pretoria 0001, South Africa
3. Environment and Health Research Unit, South African Medical Research Council, Johannesburg 2090, South Africa
4. Department of Environmental Health, University of Johannesburg, Johannesburg 2000, South Africa
5. School of Public Health, University of the Witwatersrand, Johannesburg 2000, South Africa
6. The Public Health Agency, Johannesburg 2000, South Africa