   frame|right|a half face particulate (air-purifying) mask is generally worn to protect the wearer from dust and paint fumes. a respirator is a device designed to protect the wearer from inhaling harmful dusts, fumes, vapors, or gases. respirators come in a wide range of types and sizes used by the military, private industry, and the public. respirators range from cheaper, single-use, disposable masks to reusable models with replaceable cartridges. 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 first century, when pliny the elder (circa a.d. 23-79) 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. 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. thumb|right|woodcut of stenhouse's mask the first us patent for an air purifying respirator was granted to lewis p. haslett in 1848{{cite book | last =christianson | first =scott | authorlink = | title =fatal airs: the deadly history and 
Filters Masks Protective_gear