Mixture of air contaminants
Used on the ASP, CSP and CHST. The worked example below is computed by code and checked against the source.
Em = C1/L1 + C2/L2 + … + Cn/Ln
Em must not exceed 1
C = measured concentration of each substance; L = its exposure limit from Subpart Z.
Worked example
OSHA example: 500 ppm (limit 1,000), 45 ppm (limit 200), 40 ppm (limit 200).
0.500 + 0.225 + 0.200 = 0.925, under 1, so within limits
The source prints this result, and our code reproduces it.
Where it appears on the exam
ASP: ASP11 Domain 1, Mathematical Calculations
Blueprint: ASP11 Domain 1, task 14: exposure assessments (TLV, TWA)
Practice: Exposure Assessment and Storage Capacity Calculations
CSP: CSP11 Domain 6, Occupational Health and Applied Science
Blueprint: CSP11 Domain 6, task 1: occupational exposures (hazardous chemicals)
Practice: Exposure, Noise, Containment and Lifting Calculations
CHST: CHST5 Domain 1, Hazard and Risk Identification and Control
Blueprint: CHST5 Domain 1, knowledge 21: chemical exposure
Source
The formula comes from
29 CFR 1910.1000(d)(2)(i)(opens ecfr.gov in a new tab)“In case of a mixture of air contaminants an employer shall compute the equivalent exposure as follows: Em = (C1 ÷ L1 + C2 ÷ L2) + . . .(Cn ÷ Ln) Where: Em is the equivalent exposure for the mixture. C is the concentration of a particular contaminant. L is the exposure limit for that substance specified in subpart Z of 29 CFR part 1910. The value of Em shall not exceed unity (1).”
Questions
- What is the formula for mixture of air contaminants?
- Em = C1/L1 + C2/L2 + … + Cn/Ln; Em must not exceed 1. C = measured concentration of each substance; L = its exposure limit from Subpart Z.
- Will this formula be given on the BCSP exam?
- BCSP says the formula is included in the question whenever one is needed, unless the question is testing whether you know the formula. You can't bring a formula sheet into the exam, so learn what each formula does and when to use it.
BCSP on formulas at the exam
BCSP, What to Expect When Sitting for a BCSP Exam (item 7)(opens bcsp.org in a new tab)“For any question requiring a mathematical formula, the formula will be included within the exam question (unless the question is specifically testing your knowledge of a formula). You will not be provided or permitted to bring a formula sheet.”
Related
- Time-weighted average (TWA)
- 8-hour TWA exposure (air contaminants)
- Dilution ventilation
- Velocity from a smoke tube
- ASP formula sheet
- CSP formula sheet
- CHST formula sheet
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.