Unfortunately, we (as a society) have so little understanding of how chemical and biological weapons work that it's no wonder that gas masks are flying off the shelves. But that's what happens when goverment runs the schools.
Folks, almost none of the gasmasks you can buy would help you against anything but CS/tear gas. This is because the filters they sell to civilians aren't rated for chem or bio. Plus, few understand how quickly filters wear out during actual use (in addition to their shelf life, which is often past expiration when we get them), and almost no one has adequate spares, which are expensive. Then, as several have pointed out, we have no way to know when to put the masks on, and when to take them off. Finally, most of the agents we're worried about can be absorbed via skin contact.
Even assuming you had several thousand dollars worth of the correct gear per person, you also need decontamination equipment and facilities. Who has those? AFAIK, only the military does.
You should be buying water and storable food, and not wasting money on useless items like gas masks.
BTJMO...
-Troy