Page:NIOSH DM DFM respirator evaluation draft.pdf/79

9—Derivation and evaluation of two leakage-function models for describing a user's protection factor while wearing a DM- or DFM-filter mask.

  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 necessary to evaluate the combined effect of filter leakage and face-seal leakage on user protection factors (PFs) for filter-mask respirators. For the Institute to perform a quantitative evaluation of the efficacy of these filter masks, NIOSH examined two leakage-function models. These models describe a user's protection factor as a function of total user-inhaled leakage, filter leakage, and face-seal leakage. For both models the user protection factor for a given wearer shall be denoted by PF_user, total user-inhaled leakage by L_inhaled filter leakage by L_filter, and face-seal leakage by L_{face seal}, Where leakages are always given in fractional-leakage values, not percentage values. A protection factor is defined as the reciprocal of the corresponding fractional leakage (e.g., PF_user = 1/L_inhaled).
  A simple-additive model relating L_inhaled to the component leakages is given by

(1)

Since L_{face seal} = 1/PF_{face seal}, after rearranging the terms the simple-additive model becomes

(2)

where L_filter is given as fractional leakage (i.e., percentage leakage divided by 100). This is the same additive-leakage model used by a major respirator manufacturer.^[1] It is based on assumptions that filter leakage and face-seal leakage are independent and additive. For the simple-additive model, Figure II illustrates the predicted effects on PF_user resulting from PF_{face seal} values between 6 and 1,000 and L_filter values between 0.10 and 0.50.