The gas was long gone, it wasn't being filtered. However, I don't see that it proves pollution is the cause. What about infection? Air filtration can reduce

The gas was long gone, it wasn't being filtered. However, I don't see that it proves pollution is the cause. What about infection? Air filtration can reduce the spread of pathogens. Schools throw together a large pool of people, the bug of the day will go around. Less if there are good filters.