SCBA stands for self-contained breathing apparatus. It’s a type of respirator that construction workers, firefighters and others wear to protect themselves from air contaminated by smoke or dangerous dust, fumes, vapors or chemicals. It includes its own source of compressed air, which wearers carry in a tank (or tanks) on their back. They breathe that air instead of outside air.

If SCBA sounds a bit familiar, it’s probably because it’s one letter away from SCUBA, which is an SCBA designed for underwater use.

The size of the tank, or cylinder, and how pressurized the air is determine how long the air will last.

Another type of respirator, known as a supplied-air respirator (SAR), uses a source of air that is not carried by the worker, and the respirator is therefore not self-contained.

According to the U.S. Department of Health and Human Services, there are four levels of personal protective equipment (PPE) to be used based on the severity of a chemical threat: A, B, C and D. An SCBA is used for the highest two threat levels. An SCBA is the only type of respirator that should be worn when the air is “immediately dangerous to life or health” (IDLH).

The most common type of SCBA is an open circuit, positive pressure demand SCBA. “Positive pressure” means the air pressure inside the facepiece is higher than the air pressure outside, both when the wearer inhales and when the wearer exhales. This helps prevent outside air from leaking in. Air pressure inside the facepiece is controlled by a regulator.

“Open circuit” means the air exhaled by the wearer is discharged into the atmosphere rather than being recycled. (Closed circuit SCBAs, which recycle the air, are used for longer operations.)

“Pressure demand” means the respirator admits breathing air to the facepiece when the pressure inside the facepiece is reduced by inhalation.

The facepiece, which needs to be tight fitting in order to form a good seal, should be fit-tested before use.

Employers are responsible for providing appropriate PPE and selecting the right PPE for the hazard. According to the Occupational Safety and Health Administration, if your employer requires you to wear a respirator on the job, they must select an appropriate respirator for you.

Marianne Wait is an editor and writer who creates content for Fortune 500 brands.

Image Credit: MSA