Definition - What does Positive-Pressure Respirator mean?
A positive-pressure respirator is a type of respiratory protection device in which positive air pressure is maintained within the facepiece while the device is in use and being worn properly. Air pressure within the facepiece is defined as positive when it is greater than the ambient air pressure within which the mask exists.
The most common type of positive-pressure respirator is a pressure-demand respirator, which supplies air to the facepiece in response to a decrease in pressure due to breathing or a breach in the facepiece. Positive-pressure respirators provide a higher degree of protection than other types of respirators, as they prevent the intake of contaminants into the facepiece due to negative-pressure airflow.
Safeopedia explains Positive-Pressure Respirator
Occupational health and safety agencies require the use of respirators in a variety of contexts where workers face exposure to hazardous substances or oxygen-deficient environments. The use of positive-pressure respirators is required for work that takes place in the most hazardous environments, such as those described as being “Immediately Dangerous to Life or Health” (IDLH). Although many high-performance positive-pressure respirators may experience brief instances of negative pressure due to overbreathing, these instances should not result in a significantly increased exposure risk if worn properly.
The rate at which pressure within a facepiece is reduced due to breathing depends on the wearer's work rate—the rate at which his or her breathing moves airflow through the system. This means that positive-pressure respirators must be able to replace air at different rates—depending on how much breathing the worker is doing during a given time period—or provide air at a constant rate designed to compensate for high work rates.
NIOSH testing requirements for positive pressure only require respirators to be able to maintain pressure during moderate work-rate situations. The NFPA’s respirator guidelines for firefighters require the ability to maintain pressure under much higher work rates—around 12 cubic feet per meter (cfm) maximum—and most respirators can provide airflow rates that exceed the NIOSH minimum.
Positive-pressure respirators include different types of respirators, including self-contained breathing apparatuses (SCBAs), supplied air respirators (SARs), and powered air-purifying respirators (PAPRs). Within these categories, positive-pressure respirators include pressure-demand devices as well as continuous-flow (CF) devices that move air through the respirator at a constant rate regardless of the wearer's work rate. For CF devices, positive pressure is often defined in terms of its flow rate (cfm), while pressure-demand devices determine pressure via the use of a gauge.