There are two basic categories of air purifying respirators: non-powered air purifying respirators (no mechanical power source) and powered air purifying respirators (mechanical source present for use).  Air-purifying respirators can protect in different ways, including particulate filtering, gas/vapor removing, and combination gas/vapor and particulate filtering respirators.  Air-purifying respirators are only safe for use in atmospheres that contain sufficient oxygen levels to support life.  They do not provide breathing air (oxygen) to the user.

Air-purifying respirators remove contaminates from the air by absorbing and trapping the chemical and/or by a filtration media, but because they do not provide air, these respirators cannot be used in oxygen-deficient atmospheres.  NIOSH defines oxygen-deficient atmospheres as any atmosphere containing oxygen at a concentration below 19.5% at sea level.  NIOSH certification of supplied-air or air- purifying respirators is limited to those respirators used in atmospheres containing at least 19.5% oxygen, except for those supplied-air respirators equipped with auxiliary self-contained breathing apparatus (SCBA).  The minimum requirement of 19.5% oxygen at sea level provides an adequate amount of oxygen for most work assignments and includes a safety factor.  The safety factor is needed because oxygen-deficient atmospheres offer little warning of the danger, and the continuous measurement of oxygen-deficient air is difficult.