There are two main categories: the air-purifying respirator, which forces contaminated air through a filtering element, and the air-supplied respirator, in which an alternate supply of fresh air is delivered. Within each category, different techniques are employed to reduce or eliminate noxious airborne contents.

Early development of respirators

The history of protective respiratory equipment can be traced back as far as the 16th century, when Leonardo da Vinci suggested that a finely-woven cloth dipped in water could protect sailors from a toxic weapon made of powder that he had designed. [1] Alexander von Humboldt introduced a primitive respirator in 1799 when he was working in Prussia as a mining engineer.

Practically all the early respirators consisted of a bag placed completely over the head, fastened around the throat with windows through which the wearer could see. Some were rubber, some were made of rubberized fabric, and still others of impregnated fabric, but in most cases a tank of compressed air or a reservoir of air under slight pressure was carried by the wearer to supply the necessary breathing air. In some devices certain means were provided for the adsorption of carbon dioxide in exhaled air and the rebreathing of the same air many times; in other cases valves were provided for exhalation of used air.

The first US patent for an air purifying respirator was granted to Lewis P. Haslett in 1848 for his 'Haslett's Lung Protector,' which filtered dust from the air using one-way clapper valves and a filter made of moistened wool or a similar porous substance. Following Haslett, a long string of patents were issued for air purifying devices, including patents for the use of cotton fibers as a filtering medium, for charcoal and lime absorption of poisonous vapors, and for improvements on the eyepiece and eyepiece assembly. Hutson Hurd patented a cup-shaped mask in 1879 that became widespread in industrial use, and Hurd's H.S. Cover Company was still in business in the 1970s.

Inventors were also developing air purifying devices across the Atlantic. John Stenhouse, a Scottish chemist, was investigating the power of charcoal, in its various forms, to capture and hold large volumes of gas. He put his science to work in building one of the first respirators able to remove toxic gases from the air, paving the way for activated charcoal to become the most widely used filter for respirators. British physicist John Tyndall took Stenhouse's mask, added a filter of cotton wool saturated with lime, glycerin, and charcoal, and invented a 'fireman's respirator,' a hood that filtered smoke and gas from air, in 1871; Tyndall exhibited this respirator at a meeting of the Royal Society in London in 1874. Also in 1874, Samuel Barton patented a device that 'permitted respiration in places where the atmosphere is charged with noxious gases, or vapors, smoke, or other impurities.' German Bernhard Loeb patented several inventions to 'purify foul or vitiated air,' and counted among his customers the Brooklyn Fire Department.

World War I: Chemical Warfare

At 5 p.m. on April 22, 1915, the German army released 150 tons of chlorine gas on French troops, the 87th Territorial and 45th Algerian Divisions. The Canadian 1st Division, to the right of the French, suspected immediately that something was up, such as a German attack. When the Canadians spotted the retreating Algerians, with their ashen purple faces, gasps for air, and an overbearing stench of chlorine, they knew something extraordinary and horrifying had occurred. The modern age of chemical warfare had begun, and with it an immediate need for new, reliable, and portable respirators.

Initially responding with moral outrage, the British quickly realized they could not allow German superiority in chemical warfare. In addition to sending to France any substance that might have the slightest ill effects on the enemy, the War Office authorized two companies dedicated to chemical warfare, and civilian scientists went to work. John Scott Haldane, a chemist at Oxford, designed a respirator from fabric used to manufacture veils for women. Volunteers assembled millions of cotton pads to cover the nose and mouth and shipped them to France, not knowing that they were quite dangerous; when dry they did nothing to stop or filter chlorine, and when wet nothing could pass through them, not even air, making it impossible to use. British officers on the front also tried to find solutions; Lt. Col. L.J. Barley designed a respirator and had 80,000 manufactured in local villages; the British War Office sent bottles of hyposulphite solution which acted as a neutralizing agent. Major Cluny McPherson, of the Newfoundland Medical Corps, invented a helmet made of a flannel bag with celluloid impregnated with hyposulphite, bicarbonate of soda, and glycerine. (2)

By the time the United States entered the war in 1917, the Germans and the Allies used five kinds of poisonous gases between them. Still, the U.S. Army was unprepared for chemical warfare and initially had to borrow equipment from the French and British.

Modern respirator technology

All respirators have some type of facepiece held to the wearer's head with straps, a cloth harness, or some other method. The facepiece of the respirator covers either the entire face or the bottom half of the face including the nose and mouth. Half-face respirators can only be worn in environments where the contaminants are not toxic to the eyes or facial area. For example, someone who is painting an object with spray paint could wear a half-face respirator, but someone who works with chlorine gas would have to wear a full-face respirator. Facepieces come in many different styles and sizes, to accommodate all types of face shapes, and there are many books and references available for determining which kind of hazard requires what type of respirator.

Air-purifying respirators

Air-purifying respirators are used against particulates (such as smoke or fumes), gases, and vapors. This class includes:

• negative-pressure respirators, using mechanical filters and chemical media
• positive-pressure units such as powered air-purifying respirators (PAPRs)

Half- or full-facepiece designs of this type are marketed in many varieties depending on the hazard of concern. They use a filter which acts passively on air inhaled by the wearer. Some common examples of this type of respirator are single-use escape hoods and filter masks. The latter are typically simple, light, single-piece, half-face masks and employ the first three mechanical mechanisms in the list below to remove particulates from the air stream. The most common of these is the disposable white N95 variety. The entire unit is discarded after some extended period or a single use, depending on the contaminant. Filter masks also come in replacable-cartridge, multiple-use models. Typically one or two cartridges attach securely to a mask which has built into it a corresponding number of valves for inhalation and one for exhalation.

Mechanical filter respirators

Mechanical filter respirators retain particulate matter when contaminated air is passed through the filter material. This was the method used by early inventors such as Haslett and Tyndall. Wool is still used today as a filter, along with other substances such as plastic, glass, cellulose, and combinations of two or more of these materials. Since the filters cannot be cleaned and reused and therefore have a limited lifespan, cost and disposability are key factors. Single-use, disposable as well as replaceable cartridge models are common.

Mechanical filters remove contaminants from air in the following ways:

1. by particles which are following a line of flow in the airstream coming within one radius of a fiber and adhering to it, called interception;
2. by larger particles unable to follow the curving contours of the airstream being forced to embed in one of the fibers directly, called impaction; this increases with diminishing fiber separation and higher air flow velocity
3. by an enhancing mechanism called diffusion, which is a result of the collision with gas molecules by the smallest particles, especially those below 100 nm in diameter, which are thereby impeded and delayed in their path through the filter; this effect is similar to Brownian motion and raises the probability that particles will be stopped by either of the two mechanisms above; it becomes dominant at lower air flow velocities
4. by using certain resins, waxes, and plastics as coatings on the filter material to attract particles with an electrostatic charge that holds them on the surface of the filter material;
5. by using gravity and allowing particles to settle into the filter material (this effect is typically negligible); and
6. by using the particles themselves, after the filter has been used, to act as a filter medium for other particles.

Considering only particulates carried on an air stream and a fiber mesh filter, diffusion predominates below the 0.1 μm diameter particle size. Impaction and interception predominate above 0.4 μm. In between, near the 0.3 μm most penetrating particle size (MPPS), diffusion and interception predominate.

For maximum efficiency of particle removal and to decrease resistance to airflow through the filter, particulate filters are designed to keep the velocity of air passing through the filter medium as low as possible. This is achieved by manipulating the slope and shape of the filter to provide larger surface area.

The greatest advance in mechanical filter technology has been the HEPA filter, invented during the Manhattan Project and now available to everyone. A HEPA filter can remove as much as 99.97% of all airborne particulate matter that passes through it. In the United States, manufacturers have established the categories below for particulate filtration. Most are NIOSH-approved:

Chemical cartridge respirators

Chemical cartridge respirators use a cartridge to remove gases and vapors from breathing air by adsorption, absorption, or chemisorption. A typical organic vapor respirator cartridge is a metal or plastic case containing from 25 to 40 grams of sorption media such as activated carbon. The service life of the cartridge varies based, among other variables, on the carbon weight and molecular weight of the vapor and the cartridge media, the concentration of vapor in the atmosphere, the relative humidity of the atmosphere, and the breathing rate of the respirator wearer.