I posted this in a reply in the urban section, but it probably belongs here. Everyone should read this link concerning the effectiveness of masks and respirators (all types).

https://www.cidrap.umn.edu/news-perspect...ased-sound-data

This is not a you should or should not discussion for COVID-19. This is data on masks and mask substitutes and their efficiency in filtration.

One of the things we always talk about is having a bandanna for temporary respiratory protection in the event of an earthquake or other building collapse scenario. If asbestos is part of the equation (older buildings) it is notoriously hard to filter out of the air.

Regards,
Bill