Which type of respirator is best used when atmospheric hazards are unknown?

When atmospheric hazards are unknown, the best type of respirator to use is a self-contained breathing apparatus (SCBA). SCBAs provide the highest level of respiratory protection because they supply breathable air from a tank carried on the user's back. This capability is essential in environments where the air quality could be contaminated or hazardous, as it allows the wearer to operate safely regardless of what the potentially harmful substances might be.

In situations with unknown hazards, the user is not able to rely on air that may contain harmful toxins or insufficient oxygen levels, which is why relying on respirators that depend on the surrounding air—which include air-purifying respirators (APRs) or powered air-purifying respirators (PAPRs)—is not advisable. Air-supplied respirators (SARs) also rely on an external air supply, making them unsuitable for scenarios where the atmospheric threats cannot be determined. Therefore, the SCBA is the safest choice in unknown or unpredictable environments.