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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.