used in the 1970s and 1980s
Airborne toxic materials may be gaseous (for example, mustard gas and chlorine gas) or particulates (such as biological agents). Many filters include protection from both types.
The first gas masks mostly used circular lenses made either of glass, mica or cellulose acetate. With the exception of the latter, these materials were quite brittle and needed frequent replacement. Later on, the Triplex lens style (two layers of glass and one layer of cellulose acetate in between)[1] became more popular, and alongside the simpler cellulose acetate they became the standard into the 1930s. Panoramic lenses were not popular until the 1930s, but there are some examples of those being used even during the war (Austro-Hungarian 15M). Later, polycarbonate started being used for its strength.
Some have one or two filters screwed (via inlets) onto to the gas mask while others have a large filter (coffee can filter) connected to the gas mask with a hose that is sometimes confused with an air-supplied respirator in which an alternate supply of fresh air (oxygen tanks) is delivered.
