Last winter while waiting for a bus I watched my breath flow past about 4 car length before it disappeared. There was a very gentle breeze, a slow walking speed. As a mechanical engineer air flow was one of the mandatory courses. So when I enter a room or buildings, I check to see if the air flow is from the ceiling to the floor. If it is not I ware a mask.  This is not rocket science, but good since. In many studies, I wonder why other disciplines review the proposed study? For example what is the effect of metabolic rate considered in epidermic studies?  2 other parameters are temperature and humidity that seem to be glossed over. A mechanical engineer comment.  Alan Robb