D. Hazardous Leakage Through Filters—Aerosol leakage through filter media is dependent on at least five types of independent variables (84):
- The leakage function for each make and model filter.
- The size distribution of the aerosol.
- The linear velocity through the filtering material, which is a function of the total filtering area and the volumetric flow rate through the filter(s).
- The filter loading (i.e., amount of contaminant deposited on the filter).
- Any electrostatic charges on the filter and on the aerosol.
Respirator filter media other than HEPA filters (e.g., surgical masks, dust and mist filters, or fume filters) have widely varying efficiencies against aerosols less than about 2 to 4 µm (85,86,87,88). Only HEPA filtersare certified to provide to provide the highest possible efficacy against aerosols smaller than 2 to 4 μm. For HEPA respirator filters, the NIOSH certification performance standard requires these filters be at least 99.97% efficient (i.e., leakage must be less than or equal to 0.03%) against the most filter-penetrating aerosol size (approximately 0.3 µm) (80). NIOSH certifications for dust and mist filters, and fume filters, do not permit their use for protection against highly toxic substances (i.e., those substances with exposure limits less than 50 micrograms per cubic meter) (80). In contrast, HEPA filters have been previously recommended for general ventilation air that is recirculated from the rooms of known or potential tuberculosis transmitters (15) and general-use areas in health-care facilities (10).
When HEPA filters are used on an air-purifying respirator, filter efficiency can be reliably assumed to be effectively 100% and hazardous filter leakage is not a consideration. Hence, for all HEPA-filter respirators, the potential for inward hazardous leakage of droplet nuclei
