Definition - What does Negative-Pressure Respirator mean?
A negative-pressure respirator is a respirator in which the air pressure inside the facepiece is negative during inhalation in respect to the pressure outside. A negative-pressure respirator uses mechanical filters and chemical media.
Dust masks are also negative-pressure respirators if they are sealed. Note, however, that inexpensive pollen dust masks available at drug stores and other loose-fitting masks are not negative-pressure respirators.
Safeopedia explains Negative-Pressure Respirator
A negative-pressure respirator refers to any tight-fitting respirator in which the air pressure inside the face mask is negative during inhalation with respect to air pressure outside the respirator. A mechanical filter respirator is a classic example of a negative-pressure respirator. It retains particulate matters while the contaminated air is passing through it. These replaceable filters are basically made of wool, plastic, glass, cellulose, etc. HEPA filters are also used with negative-pressure respirators and can remove about 99.97% of all airborne particulates of 0.3 micrometers diameter or greater.