OSHA and NFPA require responders that wear positive and negative pressure facepieces to be:

The requirement for responders who use positive and negative pressure facepieces to be fit tested arises from the necessity to ensure that these respiratory protective devices form a proper seal around the face. Fit testing helps to confirm that an individual can achieve an adequate fit with their respirator, which is critical for effective protection against airborne hazards, especially in hazardous materials operations.

When responders wear facepieces that do not fit properly, there is a risk of hazardous substances leaking into the respirator, which can compromise their safety and health. The fit testing is a critical component of respiratory protection programs mandated by OSHA and supported by NFPA guidelines. It ensures that the right type and size of the facepiece are chosen for each individual, accommodating different facial shapes and sizes.

While training, certification, and supervision are important aspects of a comprehensive safety program, the specific requirement in question focuses on the necessity of proper fit testing to ensure the efficacy of respiratory protection devices in high-risk environments.