Welding (C-60) · Lesson 2 of 6

Welding Fume: Calculating the 8-Hour TWA and the Additive Limit

"Welding Fume: Calculating the 8-Hour TWA and the Additive Limit" is lesson 2 of 6 in the Welding (C-60) Learning Hub. It opens with "Fume Limits Are Written as Numbers" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Fume Limits Are Written as Numbers
  2. The Time-Weighted Average Formula
  3. Compare the TWA to the PEL, and Check the Short-Term Limit
  4. When Two Fumes Are Present, Add the Fractions
  5. If the Number Is Over, Control the Source

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Multiply each concentration by its hours" through "Add the fractions".

What you'll be able to do

  • Explain the main idea of "Welding Fume: Calculating the 8-Hour TWA and the Additive Limit" in plain language you can use on the exam.
  • Apply this step with confidence: "Compare the TWA to the PEL, and Check the Short-Term Limit".
  • Apply this step with confidence: "When Two Fumes Are Present, Add the Fractions".
  • Apply this step with confidence: "If the Number Is Over, Control the Source".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Welding Fume: 8-Hour TWA Against the PELMANGANESE FUME, 9-HOUR SHIFT (mg/m³)TWA 0.219PEL 0.203 h x 0.354 h x 0.152 h x 0.050379hours into the shiftSum of C x T = 1.05 + 0.60 + 0.10 = 1.75TWA = (C1T1 + C2T2 + C3T3) / 8(0.35 x 3) + (0.15 x 4) + (0.05 x 2)= 1.75 / 8 = 0.219 mg/m³Divide by 8 even though the shift is 9 hoursPEL is 0.2 mg/m³: EXCEEDEDAfter local exhaust (levels halved)Mn TWA = 0.109; 0.109 / 0.2 = 0.547ZnO fume TWA 1.5; 1.5 / 5 = 0.300D = 0.547 + 0.300 = 0.847D is under 1: within limitsAdditive unless information shows otherwiseNo controls: 0.219 mg/m³ exceeds the 0.2 mg/m³ PEL for manganese fumeWith local exhaust: D = 0.847, under 1Source: 8 CCR 5155(c) and Appendix (TWA and D formulas), Table AC-1; 8 CCR 5150(a)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Fume Limits Are Written as Numbers

Title 8 does not just say "keep fumes low." 8 CCR Section 5155, Table AC-1 lists a permissible exposure limit (PEL) for each airborne contaminant. The rows a welder meets most often are: welding fumes, total particulates, 5 mg/m3 (the table adds "see also individual constituents"); manganese fume, 0.2 mg/m3 (short-term limit 3 mg/m3); zinc oxide fume, 5 mg/m3 (short-term limit 10 mg/m3); iron oxide fume, 5 mg/m3; and cadmium oxide fume, 0.005 mg/m3. Hexavalent chromium has its own rule, 8 CCR Section 1532.2(c): 5 micrograms per cubic meter as an 8-hour TWA.

Step 2: The Time-Weighted Average Formula

The PEL is an 8-hour time-weighted average (TWA). The Appendix to Section 5155 gives the formula: TWA = (C1T1 + C2T2 + ... + CnTn) / 8, where C is the concentration during each period and T is the duration in hours. The note under the formula is easy to miss: "Eight (8) is used as denominator regardless of total hours of workday." A 10-hour shift is still divided by 8, so overtime pushes the TWA up.

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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