Page:NIOSH DM DFM respirator evaluation draft.pdf/87

10—Evaluation factors for DM- and DFM-filter-leakage data.

  In order to evaluate the nature and extent of a possible hazard to respirator wearers due to contaminant leakage through NIOSH-certified DM and DFM filters, it is important to understand the factors affecting respirator-filter leakage. There are several major technical factors that determine the actual leakages through dust and mist (DM) and dust, fume, and mist (DFM) filter respirators. For over two decades it has been well known that these determinant factors include,^[1] but are not limited to:

• Leakage function for each make and model filter (i.e., filter leakage as a function of particle size and air velocity through a filter).
• Size distribution for airborne contaminant (i.e., both the range and relative frequencies of different particle sizes challenging the filtering material).
• Linear air velocity through the filtering material, which is a function of the total filtering area and the wearer's volumetric flow rate through the mask filter(s).
• Filter loading (i.e., amount of contaminant deposited on the filtering material during use).
• Electrostatic charge(s) on the filtering material and on an airborne contaminant or test aerosol. These filter and aerosol charges are affected by the humidity conditions in the workplace, where the filters are stored before use, and how the contaminant or test aerosol are generated.

  When comparing or evaluating leakage measurements (or APF values) from a filter-performance study, it is necessary to consider the effects from each of these five factors, particularly the first three. Regarding filter leakage functions for each make and model filter, note that the variability in observed leakage between different filter lots from the same manufacturer can be of comparable magnitude to that observed