T Greenhalgh1, Zhe Peng2, Jose L Jimenez2, W Bahnfleth3, S J Dancer4,5, L Bourouiba6. 1. Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford OX2 6GG, UK. 2. Department of Chemistry and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA. 3. Department of Architectural Engineering, Pennsylvania State University, University Park, PA 16802, USA. 4. Department of Microbiology, NHS Lanarkshire, Glasgow, UK. 5. School of Applied Sciences, Edinburgh Napier University, Edinburgh, UK. 6. Fluid Dynamics of Disease Transmission Laboratory, Massachusetts Institute of Technology, Cambridge, MA, USA.