Expedient methods of respiratory protection. III. Submicron particle tests and summary of quality factors

The efficacy of readily available materials, such as cotton fabrics, toweling, a surgical mask, and a single-use respirator, for providing emergency respiratory protection was evaluated by determining the filtration efficiency as a function of aerosol particle size over the size range of 0.001 to 5.0 μm and as a function of filtration face velocity. Filtration face velocity was set at 1.5, 5.0, and 15.0 cm/s. This report describes the equipment and procedures used to obtain efficiency measurements for particles 0.5 μm in diameter and smaller, and summarizes the results of all three phases of this research. Particles with diameters from 0.10 to 0.50 μm proved to be the most difficult sizes of particles to remove. Particles smaller than 0.10 μm were removed due to diffusion while particles larger than 0.50 μm were removed due to inertia and gravitational settling. Deposition of the smallest particles was favored by the use of low face velocities. A fractional efficiency curve was determined for each material at each velocity for comparison. Values of the quality factor [-1n(penetration]/(pressure drop), were calculated. Quality factors were less for wet materials than for dry; less at high velocities rather than low; and best for the single-use respirator mask, next best for the surgical mask and often third best for the toweling (washcloth)