A study of aerosol penetration through surgical masks was conducted to determine if surgical masks protect the medical staff from aerosols containing pathogens. Two surgical masks were evaluated: a thin molded cone with a coarse molded structure (mask-A) and a flat mask that contained a filter layer (mask-B). The masks were exposed to a corn-oil test aerosol at flow rates of 5 to 100 liters per minute (l/min). Aerosol number concentrations inside and outside the mask were measured by an aerodynamic particle sizer and a laser aerosol spectrometer. Mask performance was evaluated by calculating the extent of aerosol particle penetration from the data. Filter quality factors of the masks were computed. The data were compared with similar data obtained for nuisance dust (ND), dust mist, dust mist fume, and high efficiency particulate industrial respirator masks. Mask-A passed about 80% of the aerosol particles and showed a low airflow dependency for particles in the submicrometer range. For aerosol particles having diameters of 4 microns or larger, penetration depended on air flow, ranging from about 10% at 100l/min to 55% at 5l/min. This corresponded to collection efficiencies of 90 and 45%, respectively. Penetration through mask-B was less than through mask-A. The penetration of 0.3 micron particles, regarded as the most penetrating size, varied from 25% at 5l/min to 65% at 100l/min. The filter quality factors were characteristics of mask-A at 30l/min were similar to those of ND respirator mask. The penetration characteristics of mask-B were lower than those of the ND mask, but higher than those of the other respirator masks. The authors conclude that mask-B effectively removes large aerosol particles at all air flow rates and serves its purpose for removing bacteria expelled by the wearer. Neither surgical mask can effectively remove submicrometer sized aerosol particles such as aerosolized bloodborne pathogens.
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