Czech Model Z Gas Mask
The typical gas mask filter uses a combination of three different techniques in order to treat the air you are breathing. These techniques usually include particle filtration, chemical absorption or adsorption and chemical reactions to neutralize chemical agents. The most common filtration method used is by activated charcoal. The majority of gas masks work by using a combination of activated charcoal to filter out organic gasses and particles, super small filters used to trap smaller particles, and various chemicals imbedded in pads to neutralize non-organic gasses and to absorb and block the gas or gases while purifying the air.
NATO 40 mm Filter Cannister Top Seal
NATO 40 mm Filter Cannister Bottom Seal
Filters need to be replaced on a regular basis because when they are exposed to humidity or have been unused for long periods of time they lose their ability to absorb or block chemicals and particulates. This in effect makes them non-functional and of little value for NBC protection purposes.
Speech Diaphragm and Filter Connection
Many military surplus gas masks are available in the $20 price range but most come with expired filters that provide little more protection than the average dust mask. Shelf life of gas mask filters can range from to 25 years depending upon how they are packaged. Those gas mask filters which have been vacuum-packed will have the best shelf life. Once they are exposed to the air their useful shelf life is generally a matter of a few days before they become ineffective. You will need to purchase new filters which will cost you on average twice ($40) what you paid for the mil-surp gas mask.
Fog Prevention Vents
Depending upon the level of contamination, gas masks will afford protection for as little as 30 or 45 minutes to as much as eight hours when concentrations are minimal. Gas masks are basically designed to give you time to evacuate from the area in which hazardous agents or chemicals are present. In order to work in a hazardous environment, you would need an SCBA (Self Contained Breathing Apparatus) similar to that used by First Responders or firefighters. This type of equipment is extremely expensive (costing thousands of dollars) and is well outside the budget of most people. New modern gas mask versions such as the M40 would make a good choice for some people wanting a more extensive level of protection but this is also an expensive proposition with costs in the $400 to $600 range.
Czech Model Z Carry Pouch
It is also important to remember than many chemical agents are readily absorbed through the skin. A chemical protection suit would be advisable for use in combination with your gas mask. You would do well also to get some inexpensive chemical detection badges and radiation dosimeters (these are fairly expensive) as well so that you will actually know a threat exists or if the danger has passed.
Additional information about gas masks and how they work can be found here:
Staying above the water line!