None, scientific evidence on ineffectiveness is available before and after Covid19. On my recent timeline you can find some studies.
Are you saying they conducted studies on the effectiveness in preventing spread of infection of coronaviruses like SarsCov2?
I doubt that there is sufficient data to conduct a proper statistical study, being an extremely rare product.
>"I doubt that there is sufficient data to conduct a proper statistical study, being an extremely rare product."
They're not rare at all. It's standard respiratory PPE.
Never seen anyone wear that stuff in 3 years.
>"Never seen anyone wear that stuff in 3 years."
Well you need to get out more. Oh wait, you can't because there's a pandemic raging...
@post
An N100 or P100 is 99.97% efficient per NIOSH.