1. The physical state of the air contaminant(s)

2. Permissible exposure limit, threshold limit value, or other recognized exposure limit

3. Actual exposure measurement

4. Possibility of oxygen deficiency

5. Protection factor of the respirator

6. Toxic or irritant properties of contaminant, including routes of entry

7. Requirements of a specific OSHA standard

Suitable respirators are then selected from the following categories:

1. Negative pressure devices: negative pressure created in face piece or inlet covering of respirator during inhalation (e.g., air-purifying respirators, demand-type SCBAs, or air line respirators)

2. Positive pressure devices: positive pressure maintained in face piece or inlet covering at all times (e.g., self-contained air breathing apparatus operated in pressure demand mode, pressure demand air line respirators, or powered air purifying respirators)

3. Continuous air flow devices: atmospheric pressure is maintained inside the face piece or inlet covering of the respirator (e.g., air hats, hoods, or continuous flow respirators)