Particle Protection. N95 masks protect against 95% of airborne particles. N99 masks protect against 99% of airborne particles. The efficiency doesn’t refer to the particle size, it refers to the amount of leakage. So the N95 mask has slightly more leakage than the N99 mask. The testing is done with 200 mg of sodium chloride, as described in this CDC publication. The 3M N100 respirator has 99.97% leakage and provides the highest level of protection, virtually leak-proof.
The best protection from airborne illnesses such as those caused by bacteria, influenza virus and other viruses is to avoid contact. The disease is also spread by contact with contaminated objects or body fluids.