Occupational Exposure Limits - TWA, STEL, Ceiling and IDLH
ASP written lesson, 13 min read. ASP Domain 1: Mathematical Calculations
Written with AI help from the rules it covers, then fact-checked against them. Study aid, not site-specific safety advice. The official rule controls.
What you'll learn
- Define TWA, STEL, ceiling, peak, action level and IDLH, and the time basis of each
- Tell legal OSHA limits apart from recommended limits
- Calculate an 8-hour TWA, including unsampled time
- Apply OSHA's mixture formula for additive exposures
- Explain how IDLH conditions change respirator selection
What this lesson covers
This lesson explains the four exposure limit types on the Associate Safety Professional (ASP) blueprint: the time-weighted average (TWA), the short-term exposure limit (STEL), the ceiling and the immediately dangerous to life or health (IDLH) level. It covers who sets limits, how to calculate a TWA and a mixture, and how each limit drives sampling and respirator decisions.
This lesson is included with a pass
Written lessons come with every ASP pass, along with the video lessons, the question bank, mocks and the tutor. $80 per exam for 6 months, or $143 for all three for 12 months.
Try the free exposure assessment and storage capacity calculations practice questions