Hazardous Substance Removal (HAZ) · Lesson 3 of 6

PELs, 8-Hour TWA and the Mixture Rule

"PELs, 8-Hour TWA and the Mixture Rule" is lesson 3 of 6 in the Hazardous Substance Removal (HAZ) Learning Hub. It opens with "PEL, STEL and Ceiling: Three Different Limits" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. PEL, STEL and Ceiling: Three Different Limits
  2. Reading Table AC-1
  3. The 8-Hour TWA Formula
  4. Mixtures: The D Value Must Not Exceed 1
  5. When You Are Over: Monitoring and Controls

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Check each against its own PEL" through "Check the mixture".

What you'll be able to do

  • Explain the main idea of "PELs, 8-Hour TWA and the Mixture Rule" in plain language you can use on the exam.
  • Apply this step with confidence: "The 8-Hour TWA Formula".
  • Apply this step with confidence: "Mixtures: The D Value Must Not Exceed 1".
  • Apply this step with confidence: "When You Are Over: Monitoring and Controls".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
PEL, TWA and the Mixture Rule (D Value)TABLE AC-1 LIMITS (ppm)SubstancePELSTELCeilingToluene10150500Ethylbenzene530--Xylene100150300Benzene (see 5218)15--TWA = (C1T1 + C2T2 + ...) / 88 is the denominator even on a 9-hour dayD = TWA1/PEL1 + TWA2/PEL2 + ...D must not exceed 1 (additive effects)9-HOUR SHIFT, PERSONAL SAMPLESToluene(32 + 18 + 4) / 8 = 6.75 ppm6.75 / 10 = 0.675Ethylbenzene(10 + 4) / 8 = 1.75 ppm1.75 / 5 = 0.350Xylene(80 + 50) / 8 = 16.25 ppm16.25 / 100 = 0.1625D = 0.675 + 0.350 + 0.1625= 1.1875Each TWA is under its own PELD = 1.19 is over 1.0: the mixed exposure is NOT acceptabletoluene 0.675ethylbenzene 0.350.16limit D = 1.0← xylene 0.1625Add the ratios: every TWA passes alone, but their sum does not8 CCR §5155(c), Table AC-1 and Appendix parts (A) and (B); §5192(g)(1)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: PEL, STEL and Ceiling: Three Different Limits

8 CCR §5155(b) defines the Permissible Exposure Limit (PEL) as the maximum permitted 8-hour time-weighted average concentration. The Short Term Exposure Limit (STEL) is a 15-minute time-weighted average that may not be exceeded at any time in the workday even if the 8-hour average is below the PEL. A ceiling limit is the maximum concentration an employee may be exposed to at any time. All three are checked separately, and §5155(c) requires every one to be met. On a HAZ job these limits drive respirator selection, air monitoring frequency and when work must stop.

Step 2: Reading Table AC-1

Table AC-1 in §5155 lists each chemical with its PEL in ppm and in mg/m3, a ceiling column, a STEL, and an S in the skin column for substances absorbed through skin, mucous membranes or eyes. Examples printed in Table AC-1: toluene PEL 10 ppm (37 mg/m3), STEL 150 ppm, ceiling 500 ppm, skin notation S; ethylbenzene PEL 5 ppm (22 mg/m3), STEL 30 ppm; xylene PEL 100 ppm (435 mg/m3), STEL 150 ppm, ceiling 300 ppm; hydrogen sulfide PEL 10 ppm, STEL 15 ppm, ceiling 50 ppm. Benzene is listed at PEL 1 ppm and STEL 5 ppm with skin notation S and a pointer to §5218. For S substances §5155(d) requires protective clothing as necessary to prevent skin absorption.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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