It is natural that with the threat of biological or chemical terrorism comes a lot of talk about gas masks. Before making the decision to buy and use a gas mask it is important to understand the technology behind the different types of available equipment. It is also important to understand that a gas mask alone will not give adequate protection from biological or chemical contamination. Even the use of a protective suit will only give some protection from airborne contaminants. The only way to be fully protected is to use a suit that entirely covers the user and seals air tight, which means the use of SCBA gear is required. Unfortunately, this type of gear is very expensive and is not practical for civilian use (see discussion below). You should not expect to be able to stay in an affected area for any period of time without this type of gear.
The least effective type of gas mask is known as a half-mask air-purifying respirator. These cover the nose and mouth allowing the user to breathe through the filtration system of the mask. However, many chemical and biological agents use the eyes as an entry point, causing contamination.
A more effective type of mask is known as a full-face air-purifying respirator. These Gas Masks provide a clear face mask or clear eye pieces that protect the eyes, as well as the nose and mouth. The issue with these air-purifying respirators is that they may leak from either a poor fit or from a crack or hole on the mask.
Solving the leak problem is the supplied-air respirator. These use the same sort of filter attached to a battery-operated canister with a fan forcing air through it. The advantage is the positive pressure created by the system ensuring that any leak in the mask releases purified air rather than allowing contaminated air from the environment to enter. This is often the only option for infants and children because their small faces make masks difficult to fit reliably. Anyone considering using this type of gas mask should consider that the constant flow of air through the filter means that the filter needs more frequent replacement. Also consider that if the batteries wear out, the system will no longer operate.
The most effective system is known as an SCBA, or Self-Contained Breathing Apparatus. In an SCBA system, the air tank contains high-pressure purified air providing constant positive pressure to the face mask. While providing the best protection, an SCBA system is expensive and impractical for civilian use. The tanks are heavy and bulky and only contain, at best, 60 minutes of air. They may make sense for diving or firefighting, but for civilians or soldiers on the battlefield, an SCBA system is nearly impossible to manage.
Because of the problems with SCBA systems, the respirator you are most likely to use will have a filter that purifies the air you breathe. Air filters can use one (or more) of three techniques of removing poisonous chemicals and deadly bacteria from the air.
Particle filtration is the simplest of the three. Holding a cloth or handkerchief over your mouth to keep from breathing dust is an example of an improvised particulate filter. In a gas mask designed to guard against a biological threat, a very fine particulate filter is useful. An anthrax bacteria or spore might have a minimum size of 1 micron. Most biological particulate filters remove particle sizes as small as 0.3 microns.
A chemical threat requires a different approach due to the fact that chemical mists or vapors are largely immune to particulate filtration. The most common approach with any organic chemical is activated charcoal. Activated charcoal is charcoal that has been treated with oxygen to open up pores between the carbon atoms. These so-called active, or activated, charcoals are widely used to absorb odorous or colored substances from gases or liquids. When certain chemicals, such as paint fumes or nerve toxins like Sarin, pass next to the carbon surface, they attach to the surface and are trapped. Activated charcoal is good at trapping these organic chemicals, but many other chemicals are not attracted to carbon at all and pass right through. This means that an activated-charcoal filter will remove certain impurities, while ignoring others. It also means that once all of the activated charcoals bonding sites are filled, the filter stops working and must be replaced.
The third technique involves destruction by chemical reaction. This technique was adopted in some of the earliest protective equipment. In industrial respirators, you can choose from a variety of filters, depending on the chemical that you need to eliminate. The different filters are color coded by NIOSH standards for things like acids and ammonia. It may be difficult to decide which filtration to use, since in an attack, the chemical used is unknown beforehand.