A very nice article that explains the N95 respirator rating.

Note that it recommends infected persons NOT wear a respirator with an exhalation valve, since it can allow aerosols to exit the respirator.

https://pksafety.com/blog/respiratory-basics-n95-vs-p100/

I'll have to admit that I don't understand why 95% filtering efficiency is considered sufficient protection against the virus. Maybe it's like drinking water cleanliness. The body can fight just a few nasties (such as 5%), but not when too many nasties are injected (or inhaled). If so. I suppose the choice of N95 vs. N99 depends upon the nasty involved.

OR, is it that the ~5 micron COVID-19 aerosol droplet is big enough that the N95 respirator is much more efficient than 95% for the COVID-19 aerosol droplet??