[A proposed method for evaluating short-term exposure condition].

Abstract

High short-term exposure to toxic chemicals can occur during a workday, even if the daily average exposure is lower than the permissible exposure limit, because the exposure concentration varies from minute to minute. To protect workers from acute health effects due to high short-term exposure, the Japan Society for Occupational Health recommends that the maximum value for 15-min time-weighted average (15-min TWA) exposure during a workday should not exceed 1.5 times the occupational exposure limit for 8-hr TWA, and the American Conference of Governmental Industrial Hygienists issues the threshold limit value-short-term exposure limit (TLV-STEL), that is a 15-min TWA exposure which should not be exceeded at any time during a workday. A workday (8 hr) consists of thirty two 15-min periods. If the thirty-two 15-min TWAs are measured, the short-term exposure situation can be appropriately evaluated by comparing the highest measured value with the standard value (e.g. TLV-STEL), but such continuous monitoring consumes a lot of cost and time. In this paper, we propose a method for evaluating short-term exposure by using three or more measured values. This evaluation method corresponds to two different types of selection of sampling periods. One is a random selection of three or more 15-min periods among the 32 periods. If this selection is adopted, a comparison between the 98.44 percentile of the within-day distribution of 15-min TWAs and the standard value can be made by using one-sided tolerance factors, KI, KII and KIII, and the exposure situation is classified into four exposure classes at 95% and 50% confidence levels. Another is a random selection among high exposure periods. If this selection is adopted, a comparison between the specific percentile of the distribution and the standard value can be made with modified one-sided tolerance factors, and the exposure class is determined similarly. This method can provide a precise evaluation of exposure, so that it is useful in the industrial hygiene field.