Permissible Exposure Time Calculator

TWA = Time-Weighted Average · STEL = Short-Term Exposure Limit. Calculates how long exposure at a given concentration can continue while remaining compliant with an 8-hour TWA, and checks short-term excursions against STEL/excursion limit guidance.

Permissible Exposure Time

STEL / Excursion Check

8-Hour TWA Limit (OEL/PEL/TLV)

Actual/Peak Concentration During Exposure

Background Concentration for Remainder of Shift Concentration during the rest of the 8-hour shift, once away from the peak exposure task. Use 0 if no further exposure occurs.

0.00 hr

MAX PERMISSIBLE TIME AT PEAK CONCENTRATION

8-Hour TWA Limit (OEL/PEL/TLV)

Measured 15-Minute Concentration

Does This Substance Have an Established STEL?

0×

Ratio to TWA

Applicable Limit

Formula Reference

Permissible Time = 8 × (TWA − Background Concentration) ÷ (Peak Concentration − Background Concentration)
(Simplifies to 8 × TWA ÷ Peak Concentration when background = 0)

ACGIH STEL Rules

A TLV-STEL is a 15-minute TWA that must not be exceeded at any time, even if the 8-hour TWA is within limits — permitted up to 4 times per day, with at least 60 minutes between successive exposures.

ACGIH Excursion Limit Guidance (No Established STEL)

For substances with a TWA but no STEL, ACGIH guidance allows exposure to exceed the TWA by:

Up to 3× TWA	For no more than a total of 30 minutes during a workday (max 4 occasions, 1 hour apart)
Never exceed 5× TWA	As a 15-minute TWA, under any circumstances
8-hour TWA	Must still not be exceeded regardless of excursion compliance

Based on ACGIH TLV guidance (STEL and excursion limit rules) and standard OSHA/ACGIH TWA methodology. This tool assumes stable, known concentrations for the calculation period — real exposures vary continuously, so use conservative (upper-bound) concentration estimates. This does not replace calibrated exposure monitoring or a certified industrial hygienist's assessment, and does not apply to substances with a Ceiling limit (which must never be exceeded, even instantaneously). For customized calculators tailored to your organization, please contact Impact Tech Solutions.