Ceiling limit (C)
An exposure limit that must never be exceeded at any time during the workday.
Definition, in the source's words
29 CFR 1910.1000(a)(1)(opens ecfr.gov in a new tab)“Substances with limits preceded by “C”—Ceiling Values. An employee's exposure to any substance in Table Z-1, the exposure limit of which is preceded by a “C”, shall at no time exceed the exposure limit given for that substance. If instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time during the working day.”
What it means
A ceiling is not averaged over the shift. If you can't measure instantaneously, OSHA has you assess it as a 15-minute TWA, and no 15-minute period may exceed it.
Example
A substance has a limit of C 50 ppm. Its 8-hour TWA works out to 20 ppm, but one 15-minute sample during a tank cleanout averages 55 ppm. The exposure violates the ceiling even though the shift average is low.
Where it appears on the exam
ASP: ASP11 Domain 1, Mathematical Calculations
Practice: Exposure Assessment and Storage Capacity Calculations
CSP: CSP11 Domain 6, Occupational Health and Applied Science
Questions
- How is a ceiling measured if direct reading isn't possible?
- As a 15-minute time-weighted average, which must not be exceeded at any time during the working day (1910.1000(a)(1)).
Related
- Time-weighted average (TWA)
- Maximum use concentration (MUC)
- Formula: 8-hour TWA exposure (air contaminants)
- Regulation: 29 CFR 1910.1000
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.