When most people think about gas masks or respirators, what they usually envision is a tight-fitting plastic or rubber face mask with some sort of filter cartridge. The mask covers the nose and mouth. These are called half-mask air-purifying respirators. Depending on the chemical or biological agents in the environment, a half mask may not be sufficient because the eyes are very sensitive to chemicals and offer an easy entry point for bacteria. In this case, a full-face respirator is called for. It provides a clear face mask or clear eye pieces that protect the eyes as well.

Air-purifying respirators have two advantages:

They are the least-expensive option.
They are the least-complicated option.
The problem with air-purifying respirators is that any leak in the mask makes them ineffective. The leak could come from a poor fit between the mask and the user’s face, or from a crack or hole somewhere on the mask.
Two other types of respirator systems solve the leak problem. The supplied-air respirator uses the same sort of filter cartridge found in an air-purifying respirator. However, instead of placing the filter directly on the mask and requiring the user’s lungs to suck air through it, the filter attaches to a battery-operated canister. The canister uses a fan to force air through the filter, and then the purified air runs through a hose to the mask. The advantage is that the air coming into the mask has positive pressure. Any leak in the mask causes purified air from the canister to escape, rather than allowing contaminated air from the environment to enter. Obviously, positive pressure creates a much safer system, but it has two disadvantages:

If the batteries die, so do you.
The constant air flow through the filter means that the filter does not last as long.
However, for infants and children this may be the only option because their small faces make masks difficult to fit reliably.
The best system is called an SCBA (self-contained breathing apparatus) system. If you have ever seen a firefighter wearing a full-face mask with an air tank on his or her back, then you have seen an SCBA system. The air tank contains high-pressure purified air and is exactly like the tank used by a SCUBA diver. The tank provides constant positive pressure to the face mask. An SCBA provides the best protection, but has the following problems:
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A primitive respirator to be used by miners was introduced by Alexander von Humboldt already in 1799, when he worked as a mining engineer in Prussia.
The gas mask was patented on June 12, 1849, by American Lewis Haslett in Louisville, Kentucky. It was an “Inhaler or Lung Protector,” issued for an air purifying respirator. Haslett’s device filtered dust from the air.
One such design began as a “Safety Hood and Smoke Protector” invented by African American inventor, Garrett A. Morgan in 1912, and patented in 1914. It was a simple device, consisting of a cotton hood with two hoses which hung down to the floor, allowing the wearer to breathe the safer air found there. In addition, moist sponges were inserted at the end of the hoses in order to better filter the air. Morgan won acclaim for his device when in 1916 he, his brother, and two other volunteers used his device to rescue numerous men from the gas and smoke-filled tunnels beneath Lake Erie in the Cleveland Waterworks
The first use of poison gas on the Western Front was on 22 April 1915, by the Germans at Ypres, against Canadian and French colonial troops. The initial response was to equip troops with cotton mouth pads for protection. Soon afterwards the British added a long cloth which was used to tie chemical-soaked mouth pads into place, and which was called the Black Veil Respirator. Dr. Cluny MacPherson of The Royal Newfoundland Regiment brought the idea of a mask made of chemical absorbing fabric and which fitted over the entire head to England, and this was developed into the British Hypo Helmet of June 1915. This primitive type of mask went through several stages of development before being superseded in 1916 by the canister gas mask of 1916. This had a mask connected to a tin can containing the absorbent materials by a hose
In 1915, American chemist and inventor credited with the invention of the gas mask  James Bert Garner read a newspaper article describing a gas attack on British forces which he hypothesized had employed chlorine gas. Remembering experiments he had performed while teaching at the University of Chicago, he set about creating the first gas mask which he tested on two of his associates in a gas filled chamber. Following the successful completion of the test, he provided the results to the British government.
Garner’s mask was the first to be used on the Western front during World War I.
The British Royal Society of Chemistry claims that British scientist Edward Harrison developed the first practical gas mask for mass production, a claim supported by a thank-you note written by Winston Churchill.
In America thousands of gas masks were produced for American as well as Allied troops. Mine Safety Appliances was a chief producer. This mask was later used widely in industry.
Gas masks development since has mirrored the development of chemical agents in warfare, filling the need to protect against ever more deadly threats, biological weapons, and radioactive dust in the nuclear era. However, where agents that cause harm through contact or penetration of the skin occurs, such as blister agent or nerve agent, a gas mask alone is not sufficient protection, and full protective clothing must be worn in addition, to protect from contact with the atmosphere. For reasons of civil defense and personal protection, individuals often purchase gas masks in the belief that they protect against the harmful effects of an attack with nuclear, biological, or chemical (NBC) agents; which is only partially true, as gas masks protect only against respiratory absorption. Whilst most military gas masks are designed to be capable of protection against spectrum of NBC agents, they can be coupled with filter canisters that are proof against those agents (heavier) or just against riot control agents and smoke (lighter, and often used for training purposes); likewise there are lightweight masks solely for use in riot control agents and not for NBC situations. (more…)