Simple physical modeling for comparing respirator breathing resistance standards

Air purifying respirators are designed to reduce the number of hazardous particles respired by wearers. One barrier to respirator use is their inherent increase in breathing difficulty. The National Institute for Occupational Safety and Health (NIOSH) evaluates respirators using Standard Test Procedures (STP) for various factors, including measuring the breathing resistance as the pressure drop across the filter at a certain relatively high flow rate. The International Standards Organization (ISO) has a competing standard based on a ‘Work of Breathing’ (WOB) concept, which considers Pressure-Volume (PV) work for a series of sinusoidal simulated breaths with frequencies and magnitudes reflecting a variety of occupational scenarios, from resting to intense physical exertion. We compare the WOB and NIOSH breathing resistance limits using a simple linear model, which reflects the most common type of particulate filtering respirators. We find that the NIOSH result is more stringent than the ISO standard for most of the ISO work rates, Resting, W1, W2, and W4, but is less stringent for the moderately high W3 work rate. The effect of the non-linear flow characteristics associated with exhalation valves will be discussed along with the results of a less rigorous treatment.