Smoke and Carbon Monoxide Alarms Detect Different Things

Two alarms hang on ceilings and walls across American apartments and they are frequently assumed to be the same device. They respond to entirely different hazards and neither substitutes for the other.

Smoke alarms sense particles in the air

Ionization sensors detect the very small particles produced by fast, flaming fires. Photoelectric sensors detect the larger particles from slow, smoldering fires that produce heavy smoke.

Each type responds noticeably faster to its own kind of fire, which is why dual-sensor units exist and why guidance favors having both technologies present in a home.

Nuisance alarms from cooking are a photoelectric and ionization problem alike, and the answer is placement away from the kitchen rather than removing the battery.

Carbon monoxide is a gas with no warning signs

Carbon monoxide is produced by incomplete combustion in gas appliances, furnaces, water heaters, vehicles and any fuel-burning device. It is colorless and has no smell.

It binds to hemoglobin far more readily than oxygen does, which is the mechanism by which it becomes dangerous at concentrations that a person cannot perceive.

Symptoms of exposure resemble ordinary illness, which is precisely why a detector rather than attention is the safeguard. Suspected exposure means leaving the building and calling emergency services.

Placement follows the physics of each hazard

Smoke rises, so smoke alarms belong on ceilings or high on a wall, away from corners where air moves poorly and away from vents that dilute the smoke.

Carbon monoxide mixes fairly evenly with air rather than settling, so detectors work at varied heights, and manufacturers' instructions are the authority for a given model.

Both need one on every level of a dwelling and near sleeping areas, since the primary function is to wake someone who is asleep.

Sensors expire even when the battery is good

Both types of sensor degrade over time and carry a manufacturer's replacement interval printed on the housing, typically measured in years from the date of manufacture.

A unit past that date may still chirp, still respond to the test button and still be incapable of detecting what it was installed for.

The test button confirms the horn and the electronics, not the sensor. Checking the printed date is the only way to know the device is still within its service life.