Paint respirators are devices which have a face piece which fits to the face using straps. The face piece can cover either the lower half of the face (including the mouth and nose) or the entire face, with a plastic or glass visor to see through.
They are designed to stop the user breathing in harmful gases and particles when painting, either with brushes, rollers or spray painting. Although often used by professionals they are also very useful for home improvement. Indeed whenever you are painting with a spray gun full facepiece respirators may be used to reduce harmful effects of paint spray vapor.
The vast majority of paint respirator models on the market today use a cartridge system. The cartridge is replaceable and must be replaced regularly (the useful life of cartridges is usually printed on the cartridge or packaging). Depending on the type of paint used and size of airborne particles the cartridge can either be a chemical cartridge containing an absorption media such as activated charcoal, or a mechanical cartridge with a filling made of wool, plastic, glass or cellulose.
IMPORTANT: You must make sure you are using a cartridge designed specifically for the job you intend to use it for. Do not keep reusing cartridges once they have become saturated.
If you are just doing some light painting at home a small and cheap latex paint respirator will do the job fine, however for larger projects such as spraying a car a full face respirator will not only protect your lungs from organic vapor, but also stop particles from irritating your eyes. Popular models of full face respirators are available from lots of different manufacturers including 3M.