Carbon monoxide (CO) is a colorless, odorless, and highly toxic gas produced by incomplete combustion from fuel-burning appliances, generators, vehicles, fireplaces, and charcoal. Because it cannot be seen or smelled, a carbon monoxide detector is often the only warning before CO reaches dangerous levels.
In this guide, you’ll learn what different CO ppm (parts per million) readings mean, which exposure limits are considered safe, when carbon monoxide alarms typically activate, and the immediate actions to take if high CO levels are detected. Whether you’re a homeowner, facility manager, contractor, or MEP engineer, this reference provides a practical overview based on commonly accepted safety guidelines.
BuildMEP Quick Answer
Safe indoor CO levels are typically below 9 ppm. Readings above 35 ppm require investigation, while 70 ppm or higher should be treated as hazardous depending on exposure time. Always follow your local safety regulations and the instructions provided with your CO detector.
🚨 Safety Alert
If your carbon monoxide alarm sounds or anyone experiences symptoms such as headache, dizziness, nausea, or confusion, leave the building immediately, move to fresh air, and contact your local emergency services. Do not ignore a CO alarm or attempt to locate the source before ensuring everyone’s safety.
Carbon Monoxide Safe Levels at a Glance
| CO Level | Risk Level | Recommended Action |
|---|---|---|
| 0–9 ppm | ✅ Normal | Typical indoor background level. Monitor if persistent. |
| 10–34 ppm | ⚠ Elevated | Improve ventilation and investigate the source. |
| 35–69 ppm | ⚠ High | Avoid prolonged exposure. Inspect fuel-burning equipment. |
| 70–199 ppm | 🚨 Very High | Leave the area if symptoms occur. Treat as urgent. |
| 200+ ppm | ☠ Dangerous | Evacuate immediately and seek emergency assistance. |
Quick Tip: CO concentration alone does not determine the risk. Exposure time, ventilation, and the health of occupants all affect the severity of carbon monoxide poisoning.
What You’ll Find in This Guide
CO Exposure Limits
Public Health Reference (EPA)
- 9 ppm (8-hour average)
- 35 ppm (1-hour average)
WHO Indoor Air Guidelines
- 10 mg/m³ for 8 hours (~8.7 ppm)
- 35 mg/m³ for 1 hour (~30 ppm)
- 100 mg/m³ for 15 minutes (~87 ppm)
Occupational Limits
- OSHA PEL: 50 ppm (8-hour TWA)
- NIOSH REL: 35 ppm TWA, 200 ppm ceiling, 1200 ppm IDLH
CO Alarm Behavior
Many residential CO alarms reduce nuisance alarms by responding to exposure over time. Dangerous levels may exist before the alarm sounds.
Tip: Treat any confirmed CO reading as urgent even if the alarm hasn’t sounded.
Symptoms by PPM
CO poisoning often feels like the flu. Common symptoms include headache, dizziness, weakness, nausea, chest pain, confusion.
- ~200 ppm: headache, fatigue, dizziness, nausea after a few hours
- ~400 ppm: strong headache in 1–2 hours; life-threatening if exposure continues
What to Do if CO is High
- Evacuate immediately
- Call emergency services
- Account for everyone (children, elderly, pets)
- Do not re-enter until cleared
- Fix the source (appliance service, venting)
Common CO Sources
- Cracked heat exchanger (furnace)
- Backdrafting water heater
- Blocked chimney/flue
- Generators near windows/doors
- Vehicle exhaust in garages
- Charcoal grills / shisha indoors
- Poor combustion from burners
Prevention Checklist
- Install CO alarms near sleeping areas & each level
- Service fuel-burning appliances annually
- Ensure proper combustion air & venting
- Never run generators indoors / near windows
- Do not idle vehicles in garages
- Check kitchen exhaust/backdrafting issues
BuildMEP Tip: Integrate CO sensors into BMS for proper ventilation and monitoring.
FAQ
What CO ppm is considered safe?
Common references: 9 ppm (8-hr) and 35 ppm (1-hr). Persistent indoor readings require investigation.
Is 10 ppm dangerous?
Typically elevated. Ventilate and investigate if persistent indoors.
When does a CO alarm go off?
Alarms respond to dose over time. Evacuate immediately if symptomatic.
OSHA CO limit?
50 ppm TWA (8-hour).
Symptoms of CO poisoning?
Headache, dizziness, nausea, weakness, chest pain, confusion. Move to fresh air and seek help.
