A PROBABILITY MODEL FOR ASSESSING EXPOSURE AMONG RESPIRATOR WEARERS: PART II—OVEREXPOSURE TO CHRONIC VERSUS ACUTE TOXICANTS

Abstract

A model describing the lognormal total distribution of contaminant levels inside a respirator (C_I) is applied to assessing the probability of toxicant overexposure among a population of respirator wearers; the model accounts for between-wearer and within-wearer variability in ambient exposure levels (C_o) and decimal fraction respirator penetration (P) values. The three exceedance probabilities are defined as PrI, the proportion of all C_I levels over the permissible exposure limit (PEL); PrII, the proportion of wearers with an arithmetic mean C_I level over the PEL; and PrIII, the proportion of wearers with a 95th percentile C_I value over the PEL. PrII is considered that fraction of the population overexposed to a chronic toxicant; PrIII is considered that fraction overexposed to an acute toxicant. The behavior of PrII and PrIII over a range of exposure parameters is explored. An important observation is that a respirator-wearing population can have a substantial fraction of toxicant overexposure even though two conditions are met: (1) the P values for the population satisfy the criterion for the assigned protection factor (APF); and (2) the population arithmetic mean C_o level is at or below the maximum use concentration (MUC), defined as APF×PEL. The authors recommend that the current MUCs for air-purifying respirators be reduced by one-half to reduce the potential respirator-wearing population fraction of overexposure and that appropriate exposure surveillance programs for all wearers be mandated.