Carbon monoxide (CO) is a colorless, odorless gas produced by incomplete combustion. It binds to hemoglobin in the blood far more strongly than oxygen does, starving the body of oxygen. In confined spaces it's especially dangerous because it can build up from equipment just outside the opening.
Exposure limits
| Organization | Limit |
|---|---|
| OSHA PEL | 50 ppm 8-hour time-weighted average |
| NIOSH REL | 35 ppm TWA (up to a 10-hour workday), 200 ppm ceiling |
| ACGIH TLV | 25 ppm 8-hour TWA |
| IDLH (NIOSH) | 1,200 ppm |
Most 4-gas monitors alarm at 35 ppm (some at 25 ppm). Many permits set acceptable entry conditions at or below the alarm level. Check readings with the gas reading checker.
Symptoms
OSHA describes early symptoms of CO exposure as headache, dizziness, drowsiness and nausea. Prolonged or higher exposure can lead to vomiting, confusion, collapse, loss of consciousness and muscle weakness, and can be fatal. Symptoms are easily mistaken for flu or fatigue, and confusion can keep a worker from recognizing the danger.
Common sources near confined spaces
- Gasoline-powered equipment: pumps, generators, compressors, saws and pressure washers near the opening.
- Vehicle exhaust: trucks idling near manholes or blower intakes.
- Welding, cutting and torch work inside the space.
- Fuel-burning heaters used to warm a space.
- Process sources: furnaces, boilers, incomplete combustion in industrial equipment.
Controls
- Keep combustion equipment out of confined spaces and away from openings and blower intakes; use electric or air-powered tools where possible.
- Ventilate with clean supply air to the work area.
- Test before entry and monitor continuously; entrants should wear personal monitors.
- Evacuate on alarm, find the source, and retest before re-entry.
- Train workers to recognize symptoms in themselves and co-workers.
See confined space hazards and atmospheric testing.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.