airborne particles but is not resistant to oil. other categories filter 99% or 99.97% of particles, or have varying degrees of resistance to oil. in the european union, european standard en 143 defines the 'p' classes of particle filters that can be attached to a face mask, while european standard en 149 defines classes of "filtering half masks" or "filtering facepieces", usually called ffp masks. according to 3m, the filtering media in respirators made according to the following standards are similar to u.s. n95 or european ffp2 respirators, however, the construction of the respirators themselves, such as providing a proper seal to the face, varies considerably. (for example, us niosh-approved respirators never include earloops because they don't provide enough support to establish a reliable, airtight seal.) those standards include the chinese kn95, australian / new zealand p2, korean 1st class also referred to as kf94, and japanese ds. chemical cartridge combined gas and particulate respirator filter, type bkf (бкф), for protection against acid gases. it has a transparent body and a special sorbent that changes color upon saturation. this color change may be used for timely replacement of respirators' filters (like an end-of-service-life indicator, esli). chemical cartridge respirators use a cartridge to remove gases, volatile organic compounds (vocs), and other 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 charcoal or certain resins. 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. when filter cartridges become saturated or particulate accumulation within them begins to restrict air flow, they must be changed. if the concentration of harmful gases is immediately dangerous to life or health, in workplaces covered by the occupational safety and health act the us occupational safety and health administration specifies the use of air-supplied respirators except when intended solely for escape during emergencies. niosh also discourages their use under such conditions. filtering facepiece filtering facepiece half mask with exhalation valve (class: ffp3) filtering facepiece respirators are discarded when they become unsuitable for further use due to considerations of hygiene, excessive resistance, or physical damage. these are typically simple, light, single-piece, half-face masks and employ the first three mechanical filter mechanisms in the list above to remove particulates from the air stream. the most common of these is the white, disposable standard n95 variety; another type is the blue, surgical n95 mask. it is discarded after single use or some extended period depending on the contaminant. elastomeric new york police department officer wearing a 3m elastomeric respirator with p100-standard particulate filters in the aftermath of the 2007 new york city steam explosion main article: elastomeric respirator elastomeric respirators are reusable because the facepiece is cleaned and reused, but the filter cartridges are discarded and replaced when they become unsuitable for further use. these are replaceable-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. powered air-purifying respirators main article: powered air-purifying respirator powered air-purifying respirators (paprs) have a battery-powered blower that moves the airflow through the filters. they take contaminated air, remove a certain quantity of pollutants and return the air to the user. there are different units for different environments. the units consist of a powered fan which forces incoming air through one or more filters to the user for breathing. the fan and filters may be carried by the user or they may be remotely mounted and the user breathes the air through tubing. the filter type must be matched to the contaminants that need to be removed. some papr's are designed to remove fine particulate matter, while others are suitable for working with volatile organic compounds as those in spray paints. these must have their filter elements replaced more often than a particulate filter. atmosphere-supplying respirators these respirators do not purify the ambient air, but supply breathing gas from another source. the three types are the self contained breathing apparatus, in which a compressed air cylinder is worn by the wearer; the supplied air respirators, where a hose supplies air from a stationary source; and combination respirators that integrate both types. according to the niosh respirator selection logic, atmosphere-supplying are recommended for concentrations of hazardous particulates or gases that are greater than the immediately dangerous to life or health level; where the required assigned protection factor exceeds those of air-purifying respirators; during firefighting (self-contained breathing apparatuses only); in oxygen-deficient atmosphere; and in an unknown atmosphere. self-contained breathing apparatus main article: self-contained breathing apparatus a self-contained breathing apparatus (scba) typically has three main components: a high-pressure air cylinder (e.g., 2200 psi to 4500 psi), a pressure gauge and regulator, and an inhalation connection (mouthpiece, mouth mask or full face mask), connected together and mounted to a carrying frame or a harness with adjustable shoulder straps and belt so it can be worn on the back. there are two kinds of scba: open circuit and closed circuit. most modern scbas are open-circuit. open-circuit industrial breathing sets are filled with filtered, compressed air. the compressed air passes through a regulator, is inhaled and exhaled out of the circuit, quickly depleting the supply of air. air cylinders are made of aluminum, steel, or of a composite construction like fiberglass-wrapped aluminum. the "positive pressure" type is common, which supplies a steady stream of air to stop fumes or smoke from leaking into the mask. other scba's are of the "demand" type, which only supply air when the regulator senses the user inhaling. all fire departments and those working in toxic environments use the positive pressure scba for safety reasons. the closed-circuit type scba filters, supplements, and recirculates exhaled gas like a rebreather. it is used when a longer-duration supply of breathing gas is needed, such as in mine rescue and in long tunnels, and going through passages too narrow for a large open-circuit air cylinder. supplied air respirator main article: supplied-air respirator supplied air respirators make use of a hose to deliver air from a stationary source. it provides clean air for long periods of time and are light weight for the user, although it limits user mobility. they are normally used when there are extended work periods required in atmospheres that are not immediately dangerous to life and health (idlh). disadvantages extended use of respirators can result in higher levels of carbon dioxide than recommended in a work environment, and can lead to headaches, dermatitis and acne. regulation the choice and use of respirators in developed countries is regulated by national legislation. to ensure that employers choose respirators correctly, and perform high-quality respiratory protection programs, various guides and textbooks have been developed: textbooks and guidelines for the selection and use of respirators country language year of publication pages institution (hyperlink to document) usenglish1987305niosh () usenglish200532niosh () usenglish1999120niosh () usenglish201748pesticide educational resources collaborative (perc) () usenglish & spanish--osha () usenglish2011124osha () usenglish201596osha () usenglish201244osha () usenglish201444osha () usenglish201632osha () usenglish201438osha () usenglish201751osha () usenglish2001166nrc () usenglish1986173niosh & epa () canadafrench2013, 200260institut de recherche robert-sauve en santé et en sécurité du travail (irsst) () canadaenglish2015-institut de recherche robert-sauve en sante et en securite du travai (irsst) () canadafrench2015-institut de recherche robert-sauve en sante et en securite du travai (irsst) () francefrench201768institut national de recherche et de sécurité (inrs) () germanygerman2011174spitzenverband der gewerblichen berufsgenossenschaften und der unfallversicherungsträger der öffentlichen hand (dguv) () ukenglish201359the health and safety executive (hse) () ukenglish201629the uk nuclear industry good pracindustry radiological protection coordination group (irpcg) () irelandenglish201019the health and safety authority (hsa) () new zealandenglish199951occupational safety and health service (oshs) () chilespanish200940instituto de salud publica de chile (ispch) () spainspanish-16instituto nacional de seguridad, salud y bienestar en el trabajo (insht) () for standard filter classes used in respirators, see mechanical filter (respirator)#filtration standards. history earliest records to 19th century plague doctor the history of protective respiratory equipment can be traced back as far as the first century, when pliny the elder (c. 23 ad–79) described using animal bladder skins to protect workers in roman mines from red lead oxide dust. in the 16th century, 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. in 1785, jean-françois pilâtre de rozier invented a respirator. alexander von humboldt introduced a primitive respirator in 1799 when he worked as a mining engineer in prussia. practically all respirators in the early 18th century 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 allowed exhalation of used air. julius jeffreys first used the word "respirator" as a mask in 1836. the mask worked by capturing moisture and warmth in exhaled air in a grid of fine metal wires. inhaled air then was warmed and moistened as it passed through the same metal grid, providing relief to people with lung diseases. the respirator became popular, and was mentioned in the literature of the day, including in the writings of elizabeth gaskell, william makepeace thackeray and charles dickens. woodcut of stenhouse's mask "how a man may breathe safely in a poisonous atmosphere", an apparatus providing oxygen while using caustic soda to absorb carbon dioxide, 1909 in 1848, the first us patent for an air-purifying respirator was granted to lewis p. haslett 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 which became widespread in industrial use, and hurd's h.s. cover company was still in business in the 1970s. inventors in europe included john stenhouse, a scottish chemist, who investigated the power of charcoal in its various forms, to capture and hold large volumes of gas. he built 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. irish physicist john tyndall took stenhouse's mask, added a filter of cotton wool saturated with lime, glycerin, and charcoal, and in 1871 invented a 'fireman's respirator', a hood that filtered smoke and gas from air, which he exhibited 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 the brooklyn fire department among his customers. a predecessor of the n95 was a design by doctor lien-teh wu who was working for the chinese imperial court in the fall of 1910, which was the first that protected users from bacteria in empirical testing. subsequent respirators were reusable but bulky and uncomfortable. in the 1970s, the bureau of mines and niosh developed standards for single-use respirators, and the first n95 respirator was developed by 3m and approved in 1972. world war i the first recorded response and defense against chemical attacks using respirators occurred during the second battle of ypres on the western front in world war i. it was the first time germany used chemical weapons on a large scale releasing 168 tons of chlorine gas over a four-mile