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: For standard filter classes used in respirators, see Mechanical filter (respirator)#Filtration standards. History Earliest records to 19th century The history of protective respiratory equipment can be traced back as far as the first century, when Pliny the Elder (–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. 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