(1) The average air-borne exposure of a worker in any eight-hour shift of a 40-hour work week. A formula is used to calculate the permissible exposure limit (PEL) using a time-weighted average when a worker is exposed in a hazardous environment for more than eight hours in a given day: PEL = 400/hours worked in the day. For example, if the PEL is 50 μg/m3 for an eight-hour workday, it is reduced to 40 μg/m3 in the case of a 10-hour workday (400/10 = 40);
