Carbon Monoxide (CO)
CO is a colorless, odorless gas produced by incomplete combustion. It binds hemoglobin and blocks oxygen transport in the bloodstream, making it acutely lethal at high indoor concentrations.
Reviewed by Hayden Williams. Last reviewed 2026-05-01. Unit: ppm.
CO is the silent killer of indoor air quality. It binds to hemoglobin 200x more strongly than oxygen, displacing oxygen from blood. At low outdoor concentrations it has minor cardiovascular effects, but at indoor concentrations from faulty heaters, generators, or unvented combustion, CO causes headaches, dizziness, unconsciousness, and death within hours. Outdoor CO is rarely a regulatory problem in modern economies.
Health effects
Headaches, dizziness, chest pain at moderate levels. At high indoor concentrations, unconsciousness and death within hours. Chronic low-level exposure may worsen cardiovascular disease.
Outdoor ambient CO concentrations in modern cities rarely exceed 1-2 ppm and are not a major public health concern in regulatory contexts. Indoor CO from unvented combustion (gas heaters, generators in enclosed spaces, blocked flues) regularly causes hundreds of accidental deaths globally each year, plus thousands of hospital visits. Always install battery-backed CO detectors near sleeping areas.
Vulnerable groups
Anyone in spaces with combustion appliances and inadequate ventilation - residents of homes with old heating systems, occupants near operating generators, people sleeping in vehicles with running engines.
Common sources
- Vehicle exhaust (cold-start engines especially)
- Faulty or improperly vented gas heaters and water heaters
- Portable generators (highest indoor risk)
- Wildfires (significant outdoor CO source)
- Tobacco smoke
- Charcoal grills used indoors (acute lethal risk)
Regional context
Outdoor CO is largely solved in EU/US/Japan since catalytic converters became universal. Higher levels persist in older vehicle fleets and during wildfires. Indoor CO incidents follow cold weather and power outages worldwide.
Regulatory thresholds
How CO is measured
Non-dispersive infrared (NDIR) absorption is the reference method. Affordable home CO detectors use electrochemical sensors and are required by code in most modern jurisdictions. Always have a working detector with battery backup.
How to reduce your exposure
- Install CO detectors near every sleeping area and replace batteries annually
- Never run generators, charcoal grills, or vehicles in enclosed spaces
- Service gas heating appliances annually
- Clear blocked flues and chimneys
- Outdoor exposure: catalytic converters and EVs have largely solved this
Trends
Outdoor ambient CO has fallen by 80%+ in the US since 1980 due to catalytic converters. EU, Japan, Korea show similar declines. Indoor CO incidents continue worldwide and follow seasonal heating patterns.
Cities where CO matters most
Live readings for cities historically affected by elevated CO:
Frequently asked
Is outdoor CO dangerous?
Outdoor concentrations in modern cities are usually well below health-effects thresholds. The main outdoor exposure spike is in heavy traffic with old vehicle fleets, or during wildfires.
Why is indoor CO so dangerous?
CO is colorless and odorless. At indoor concentrations from blocked flues or generators, lethal exposure can happen during sleep without warning. CO binds hemoglobin 200x stronger than oxygen, so the blood cannot carry oxygen even if you breathe normally.
Do I need a CO detector?
If you have any gas, oil, or wood appliance in your home, yes. Place detectors near every sleeping area and at least one per floor. Replace units every 5-10 years and batteries annually.
Sources + further reading
Track CO live across 217 cities on atmos.today. See methodology for how readings are sourced and calculated.