""A mask rated 95 will stop 95 percent of airborne particles that are 0.3 microns in size and larger, while a mask rated 100 will filter out 99.97 percent of particles 0.3 microns and larger, making it comparable to a HEPA (high-efficiency particulate air) filter. (The average human hair is 70 micrometers in diameter.)
Activated carbon vs. chemical agents
Paper-filter masks, however, are useless against most chemicals, whether paint fumes, sarin gas or airborne organophosphates. Those threats require a mask with activated carbon, also known as activated charcoal.
Charcoal bonds with the active ingredients in nerve gas and other chemical threats, effectively filtering those toxic compounds out of the air inhaled by the mask's wearer. As long as there's space on the charcoal filter's fibers for incoming toxins, the wearer is safe.
But after a period of time — depending on the density of toxic chemicals in the air, the wearer's rate of breathing and other factors — an activated charcoal filter will stop working and will need to be replaced. Most activated charcoal masks have disposable filters that are easily replaced. [End of the World? Top Doomsday Fears]
There are other limits to the effectiveness of these masks: They don't always provide eye protection, and any mask is only as good as the seal around the wearer's face — a leak or a weak seal would render them useless.
That's why many professionals, including hazmat teams and emergency first-responders, wear full-face masks that are connected to an air supply, such as a small air canister or a scuba-style air tank.""
https://www.livescience.com/39217-ho...asks-work.html
