from what i understand from you initial post, you are concerned about terrorist attacks. In that case, there will be no warning of a virus or bacteria being released. It's highly unlikely to know you'r breathing them during the event, because they don't have a smell and are generally not visible (unless the aerosol deliverly system malfuctions). This mean, you either have to wear it all the time or accept the risk.

Secondly a mask only give you limited protection. Even when a N-100 maks is used properly and on time during a bio-attack, you need to know how to decontaminate yourself, when you have reached a "cold" zone (out of the danger area and it's direct surroundings). Unproper care of your protective equipment, can render the protective properties useless.

A N-100 mask is effective aganist dust release by a structure, due to blast effects. But when a (large) fire has started inside a confined space, you may have to ask you self wheather the protection against dust is worth the time to find and put on your mask. Dust generally won't kill you quickly, smoke and lack of o2 will.