Air
Carbon Monoxide
CO
A colorless, odorless gas produced by incomplete combustion. It displaces oxygen on hemoglobin, and unlike other common indoor air pollutants it can reach concentrations in a house that are acutely dangerous.
In more detail
Anything that burns fuel can produce it: furnaces, boilers, water heaters, gas ranges, fireplaces, space heaters, generators, and vehicles in an attached garage. Complete combustion yields carbon dioxide; incomplete combustion — insufficient air, a dirty or misadjusted burner, a blocked or backdrafting flue — yields carbon monoxide instead.
The reason it is treated differently from every other indoor pollutant is physiological. Carbon monoxide binds to hemoglobin far more readily than oxygen does, forming carboxyhemoglobin and reducing the blood's capacity to carry oxygen. EPA describes the resulting effects at elevated indoor concentrations, ranging from headache, dizziness, fatigue and nausea to loss of consciousness and death at high exposures. Because the gas has no color, odor or taste, none of this announces itself, and early symptoms are readily mistaken for illness.
Detection is therefore not an optimization but a requirement. Listed carbon monoxide alarms are the standard measure; NFPA publishes guidance on where they belong — outside each sleeping area and on every level — and on replacing them at the interval the manufacturer states, since the sensors have a finite life. Household air-quality monitors that display a CO figure are not a substitute for a listed alarm.
There is a separate and less settled question about chronic low-level exposure, which is an active research area rather than a matter of established residential guidance.
Why it matters
This is the one indoor air subject where the goal is assurance rather than improvement. Everything else in a house is a matter of degree — a bit more filtration, a bit more outdoor air — and this is binary. It also has a house-level cause that is easy to miss: a large exhaust fan in a tight enclosure can depressurize the building enough to reverse the draft on a naturally vented appliance, which turns a correctly installed water heater into a source. That is why combustion safety testing and makeup air belong in the same conversation as the alarm.
The common misunderstanding
That a smoke alarm covers it. They detect different things by different means, and a smoke alarm will not respond to carbon monoxide. Combination units exist and are clearly labeled as such; a device that is only a smoke alarm provides no carbon monoxide protection at all, and an air quality monitor showing a CO reading is a display, not a listed alarm.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.