Air Purifying Respirators or Gas Masks are commonly used to filters out the harmful components of a non-IDLH atmosphere. They are best used when there is at least 19.5% oxygen in the atmosphere.
Powered air-purifying respirators are for use by emergency responders in atmospheres that contain chemical, biological, radiological, and nuclear (CBRN) respiratory hazards. These are a step above APR masks in that they use a battery powered blower that creates a positive air pressure inside the mask facilitating better sealing around the face.
Self-Contained Breathing Apparatus contain its own atmosphere; therefore, they can be used in IDLH atmospheres. SCBAs come in a variety of flavors such as cylinder type and harness and respirator.
You may never know when an emergency will occur and that is where escape masks come into play. Escape Masks are used to get out of the immediate hazardous area.
While it looks impressive in movies when people come storming in with gas masks on without canisters attached; in reality, without the canisters/cartridges attached to the gas masks, they will suffer from the gas like everyone else. Do not forget these small but vital part of the gas mask.
Most of these accessories are needed in order to keep your repirators fully functional.