1. Department of Public Health & Primary Care, Institute of Public Health, University of Cambridge, Cambridge CB2 2SR, UK
2. Institute of Occupational Medicine (IOM) Consulting Ltd, Research Avenue North, Riccarton, Edinburgh EH14 4AP, UK
3. Respiratory Epidemiology and Public Health Group, Imperial College London, National Heart & Lung Institute, 1B Manresa Road, London SW3 6LR, UK
4. Department of Earth Sciences, Institute of Hazard, Risk and Resilience, Durham University, Science Labs, South Road, Durham DH1 3LE, UK