3 School of Public Health, Imperial College London, London, UK.
4 Imperial College London, School of Public Health, National Heart and Lung Institute, London, UK.
5 Faculty of Medicine and Health, Sydney University, Sydney, Australia.
6 School of Public Health and National Heart and Lung Institute, Imperial College London, White City, London, W12 7RQ, UK. xiubin.zhang@imperial.ac.uk.