**Author details**

#### James I. Phillips

*National Institute for Occupational Health, National Health Laboratory Service, South Africa Deparatment of Biomedical Technology, Faculty of Health Sciences, University of Johannesburg, South Africa* 

#### David Rees, Jill Murray and John C.A. Davies

*National Institute for Occupational Health, National Health Laboratory Service, South Africa School of Public Health, Faculty of Health Sciences, University of the Witwatersrand, South Africa* 
