With regard to hazardous face-seal leakage, all non-powered filter masks (e.g, surgical masks and disposable PRs including disposable HEPA-filter respirators) have an inherent deficiency that markedly reduces the level and reliability of personal protection these devices can deliver even when correctly used under ideal conditions. During each inhalation by a wearer, a negative pressure (relative to the workplace air) is created inside the facepiece of this type of respirator. Due to this negative pressure, air containing aerosolized droplet nuclei can take a path of least resistance into the respirator—through leaks at the face-seal interface—thus avoiding the higher-resistance filter material. Additionally, the filter material creates a resistance to the wearer’s breathing, which results in physical discomfort, perceptible increases in the work of breathing, and impaired verbal and nonverbal communications (83).
In contrast to non-powered filter respirators, powered respirators (with HEPA filters) have a design advantage that markedly increases the level and reliability of personal respiratory protection these respirators can deliver under real-world conditions. A powered filter respirator produces a positive pressure inside the facepiece under most conditions of use. These respirators deliver a forced airstream to the facepiece using a battery-powered blower. The blower forcibly draws ambient air through HEPA filters, then delivers the filtered air to the facepiece. This air is blown into the facepiece at volumetric flow rates ranging from 115 to 170 L/min (4.1 to 6.0 cubic feet per minute). These flow rates exceed the vast majority of inhalation flow rates expected in workers needing personal protection against droplet nuclei. The small positive pressure inside the facepiece reduces face-seal leakage to very low levels, particularly during the relatively low inhalation rates expected in healthcare settings. NIOSH conservatively estimates that these respirators have less than 2% face-seal leakage under routine conditions (57). Thus, a powered filter respirator offers substantially higher and more reliable levels of personal respiratory protection than any non-powered filter mask can provide. Examples of this respirator type are given in Figures 1 and 2 on pages 34 and 35.
