Computing combined exposures for multitask jobs using the ACGIH TLV© for mono task hand work

Introduction
The ACGIH TLV© for manual work is based on Hand Activity Level, HAL, and peak finger force, Fp, for mono-task work for 4 or more hours per day. Construction work often involves multi tasks jobs. This paper investigates ways of evaluating the TLV© for multi task jobs that may be performed more longer than 4 hours per day.

Methods
We examine three methods for evaluating the TLV© for multitask jobs. The first method is based on a dose, D, which is computed for each task as the ratio of the observed force to the acceptable force for the corresponding HAL value multiplied by the task duration and then divided by 4 hours. The total D is computed as the sum of Ds for each task that makes up the job and should not exceed 1. We compared exposures based on a TWA of HAL and in one case on a TWA of Fp for each task and in a second case on the overall peak force of all tasks.

Results
The overall TLV© based on the TWA of HAL and Fp for each task of a multitask job will not be exceeded if the TLV© is not exceed for any of the individual tasks. The TLV© may be exceed if it is based on the TWA of HAL and on highest Fp for the individual tasks. Using the Dose concept, it can be shown that if a worker works at the TLV© for 4 hours, then any additional exposure will result in exceeding the TLV©. The exposure for that task must be reduced if the worker is to work longer than 4 hours.

Discussion
The dose concept provides guidance for applying the TLV© to jobs that entail multiple tasks and may be performed for extended work shifts are needed in construction work.