Glossary · Testing & air leaks

Carbon monoxide (CO)

Also called: CO alarm · carbon monoxide alarm · CO detector · carbon monoxide detector

A colorless, odorless gas from incomplete burning of fuel; it reduces the blood's ability to carry oxygen and can kill at high levels.

What it means

Furnaces, boilers, water heaters, gas stoves, fireplaces, wood stoves, generators and cars in attached garages can all produce CO. Levels are measured in parts per million (ppm) with a CO meter or alarm. Because you can't see or smell it, working CO alarms near sleeping areas are the main everyday defense.

Why it matters in an audit

Measuring CO is a core part of a combustion safety test, and air sealing or new exhaust fans can turn a marginal appliance into a dangerous one.

Typical numbers

Reference values with their sources. Your auditor’s report gives the figure for your own house.

What	Value	Source
Typical US homes (EPA)	0.5–5 ppm without gas stoves; often 5–15 ppm near properly adjusted gas stoves; 30 ppm or more near poorly adjusted ones	epa.gov ↗
US EPA outdoor air standard (NAAQS)	9 ppm over 8 hours; 35 ppm over 1 hour	epa.gov ↗

In the United States

EPA's outdoor standards are often used as a yardstick indoors; state rules on CO alarms vary.

Sources

Learn more here

Related terms

ppm (parts per million) A way to express tiny concentrations: 1 ppm is one part of a gas in a million parts of air, used for carbon monoxide readings.
Combustion safety test A check that furnaces, boilers, water heaters and stoves burn cleanly and vent their exhaust outside, even with fans running.
Backdrafting and spillage When exhaust from a furnace, boiler, water heater or fireplace comes back into the house instead of going up the chimney or vent.
Combustion appliance zone (CAZ) The room or area holding a fuel-burning appliance such as a furnace, boiler or water heater; it's the focus of combustion safety testing.
Indoor air quality (IAQ) How clean and healthy the air inside a home is, affected by ventilation, humidity, combustion gases, particles, chemicals and radon.

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