Employees need to wear respirators whenever engineering and work practice control measures are not adequate to prevent atmospheric contamination at the worksite. Strategies for preventing atmospheric contamination may include enclosing or confining the contaminant-producing operation, exhausting the contaminant, or substituting with less toxic materials.

Respirators have their limitations and are not a substitute for effective engineering and work practice controls. When it is not possible to use these controls to reduce airborne contaminants below their occupational exposure levels, such as during certain maintenance and repair operations, emergencies, or when engineering controls are being installed, respirator use may be the best or only way to reduce worker exposure. In other cases, where work practices and engineering controls alone cannot reduce exposure levels to below the occupational exposure level, respirator use is essential.

Where respirators are required to protect worker health, specific procedures are necessary to ensure the equipment's effectiveness.