1. Modifying OELs for Extended Work Shifts The best models for extrapolating TLVs are complex. Must be done substance by substance. Based on critical health effects, pharmacokinetics and biological half- life. Often, detailed information is not sufficient for the models.

2. Extended Work Shifts Guidance Exposure limits whose goals are to avoid excessive irritation or odor will, in general, not require modification to protect persons working unusual work shift. Adjustments to TLVs are not generally necessary for unusual work shifts if the biological half-life of the toxicant is less than 3 hours or more than 400 hours.

3. Adjusting TLVs OSHA Model Brief and Scala Model Hickey and Reist Model

4. OSHA MODEL Equivalent PEL T is the duration of exposure

5. Brief and Scala Model T is the duration of exposure

6. HICKEY and REIST MODEL t 1 = hours worked per day on unusual schedule t 2 = 24 times days worked/week on unusual schedule

7. Example Assuming that 1,2-trichloroethane has a biologic half-life of 16 hours in people, what modified TLV or PEL would be appropriate for persons who wished to work 3 days, 12 hours per day for the work week. The present ACGIH TLV and OSHA PEL for 1,2-trichloroethane is 10 ppm.

8. HICKEY and REIST MODEL t 1 = hours worked per day on unusual schedule t 2 = 24 times days worked/week on unusual schedule

9. Modified OEL Solution