1917 Subpart B
The carbon monoxide concentration must be kept at or below 50 parts per million (ppm) as an eight‑hour average, and employees must be removed if the concentration exceeds a ceiling of 100 ppm. See 1917.24(a) for the exact limits.
The eight‑hour average area level means the carbon monoxide concentration averaged over an eight‑hour work period in the area where employees work, and that average must not exceed 50 ppm. Employers should use measurements that represent the atmosphere in the room, building, vehicle, railcar, or enclosed space over that eight‑hour period. See 1917.24(a).
You must remove employees from the enclosed space if the carbon monoxide concentration exceeds the 100 ppm ceiling. The standard requires employee removal at concentrations above the ceiling to protect worker health. See 1917.24(a).
Tests must be made whenever necessary to ensure that employee exposures do not exceed the limits in paragraph (a). That means testing whenever there is reason to suspect CO could be high — for example after equipment changes, complaint or symptoms, visible exhaust or smoke, start‑up of engines in an enclosed space, or other changes in conditions. See 1917.24(b).
Designated persons must perform the tests using either NIOSH‑certified gas detector tube units (under 30 CFR part 11) or other measuring instruments whose accuracy is as great or greater. Employers should ensure the designated testers are trained to use the chosen instruments and understand their accuracy limits. See 1917.24(c).
The standard specifies area testing by designated persons using certified or equivalently accurate instruments, but it does not prohibit using personal monitors so long as the measuring method provides accuracy at least equal to the specified instruments and the tests are performed by designated persons. Employers should ensure whichever method they use reliably represents employee exposure and meets the accuracy requirement in 1917.24(c).
A record of the date, time, location, and results of carbon monoxide tests must be available for at least 30 days. Make sure records include those four elements so you can show compliance and track trends. See 1917.24(d).
You must record the date, time, location, and the test results for each carbon monoxide measurement and keep those records available for at least 30 days. Those are the explicit record elements required by the rule. See 1917.24(d).
You do not have to remove employees solely because an eight‑hour average exceeds 50 ppm, but you must take action to reduce and control exposures because the standard requires maintaining the eight‑hour average at not more than 50 ppm. Immediate removal is required only if the concentration exceeds the 100 ppm ceiling; however, if eight‑hour averages exceed 50 ppm, you must investigate, implement controls, and retest to comply with 1917.24(a) and 1917.24(b).
No. The standard requires maintaining the carbon monoxide content of the atmosphere in the specific room, building, vehicle, railcar, or enclosed space at or below the limits, so measurements should be taken in the actual enclosed space to determine compliance. Tests done elsewhere may not represent the atmosphere inside the space. See 1917.24(a) and 1917.24(b).
Yes — continuous electronic monitors are acceptable if they have accuracy equal to or better than the NIOSH‑certified gas detector tube units referenced in the standard. The rule allows "other measuring instruments whose accuracy is as great or greater." Ensure the instrument is operated by designated persons and the data are documented. See 1917.24(c).
The employer must designate qualified persons to perform CO tests; those individuals should be trained in use of the chosen instruments, understand instrument accuracy and limitations, and know how to record and interpret results to protect employees. The requirement that tests be made by "designated persons" is in 1917.24(c).
Testing frequency is "as necessary" to ensure employee exposures do not exceed the limits — that is, repeat testing after any change that could affect CO levels (new equipment, altered ventilation, employee complaints, unusual readings, or after corrective actions) and periodically as part of a monitoring plan. See 1917.24(b) and 1917.24(d) for recordkeeping of tests.
An instrument that is not NIOSH‑certified may still be acceptable if its accuracy is as great as or greater than the NIOSH‑certified gas detector tube units referenced in the rule. Employers must ensure designated persons properly operate and maintain the instrument and can demonstrate its accuracy. See 1917.24(c).
No. The standard explicitly requires that employees be removed from the enclosed space if the concentration exceeds the 100 ppm ceiling. Relying solely on respirators does not satisfy the removal requirement in 1917.24(a). Employers should instead reduce the atmospheric concentration and follow applicable testing and recordkeeping requirements in 1917.24(b)–(d).
You must investigate and implement corrective measures (improve ventilation, change equipment or work practices, limit time in the space) to bring the eight‑hour average back to 50 ppm or below, and continue testing to verify control effectiveness. The standard requires maintaining the eight‑hour average at not more than 50 ppm and testing as necessary under 1917.24(a)–(b).
No — certain facilities designated as "designated waterfront facilities" are not covered by Part 1917. OSHA's letter of interpretation explains that Part 1917's specific requirements do not apply to such facilities, so you must confirm whether your operation falls within Part 1917's scope before relying on 1917.24. See the OSHA interpretation at https://www.osha.gov/laws-regs/standardinterpretations/2017-05-31 for details.
Employers must keep a record of the date, time, location, and results of CO tests and make that information available for at least 30 days; employees or their representatives should be able to review those records on request. See 1917.24(d).
Yes, provided the gas detector tube units are NIOSH‑certified under 30 CFR part 11 or have accuracy equal to or better than the certified tubes; they must be used by designated persons and handled per manufacturer and certification guidance. See 1917.24(c).
While 1917.24(d) requires recording date, time, location, and results of tests for at least 30 days, employers should also document corrective actions and follow‑up testing to demonstrate they addressed exceedances and brought atmospheres back into compliance with 1917.24(a) and 1917.24(b). Recording corrective actions is a practical compliance step even if not listed word‑for‑word in 1917.24(d).