Carbon monoxide detectors cannot detect smoke

A carbon monoxide detector is designed to sense only carbon monoxide gas, which is colorless and odorless. It will not alert you to smoke, flames, or the presence of fire. The two devices work on completely different detection methods and protect you from different hazards. If you need both protections in your home, you need both devices.

This is a critical distinction because the two gases require separate sensors. A carbon monoxide detector uses an electrochemical cell or biomimetic sensor that reacts only to carbon monoxide molecules. A smoke detector uses either an ionization chamber or a photoelectric sensor that responds to smoke particles. Neither sensor can do the other's job.

Key Takeaways

  • Carbon monoxide detectors sense only carbon monoxide gas and will not detect smoke, fire, or flames.
  • Smoke detectors and carbon monoxide detectors use different sensor technologies and cannot substitute for each other.
  • You need both a working smoke detector and a working carbon monoxide detector in your home for complete protection.
  • Combination detectors that sense both smoke and carbon monoxide exist, but they are less common and more expensive than separate units.
  • Test each detector monthly and replace batteries twice a year to may support both are functioning when you need them.

How carbon monoxide detectors work

Carbon monoxide detectors contain a chemical or electrochemical sensor that changes color or triggers an alarm only when exposed to carbon monoxide. The most common type uses an electrochemical cell filled with a special solution. When carbon monoxide enters the cell, it reacts with the solution and produces an electrical signal that triggers the alarm.

Some detectors use a biomimetic sensor, which contains a gel that changes color when it absorbs carbon monoxide. The color change is detected by a light sensor, which then triggers the alarm. Both methods are highly specific to carbon monoxide and will not respond to smoke, dust, cooking fumes, or other household gases.

Carbon monoxide is produced by fuel-burning appliances like furnaces, water heaters, gas stoves, and car engines. It is dangerous because it binds to hemoglobin in your blood and prevents oxygen from reaching your organs. A detector alerts you to this invisible threat before you develop symptoms like headache, dizziness, or nausea.

How smoke detectors work

Smoke detectors use one of two sensor types: ionization or photoelectric. An ionization detector contains a small radioactive source that ionizes air molecules inside a chamber. When smoke particles enter the chamber, they disrupt the ionization and trigger the alarm. This type is more sensitive to fast-flaming fires.

A photoelectric detector uses a light source and a light sensor. Normally, the light beam travels straight across the chamber without hitting the sensor. When smoke particles enter, they scatter the light beam, which then hits the sensor and triggers the alarm. This type is more sensitive to smoldering fires that produce thick smoke.

Neither of these mechanisms responds to carbon monoxide gas, which is invisible and does not scatter light or disrupt ionization in a way that triggers the alarm. A smoke detector will not warn you of carbon monoxide, even if the gas is present in dangerous concentrations.

Why you need both detectors

Carbon monoxide and smoke are separate hazards that require separate detection. A fire produces smoke but may not produce carbon monoxide in the concentrations that trigger a carbon monoxide detector. Conversely, a malfunctioning furnace or water heater can leak carbon monoxide without producing any smoke at all.

The National Fire Protection Association (NFPA) recommends that every home have both working smoke detectors and carbon monoxide detectors. Smoke detectors should be installed on every level of your home, inside each bedroom, and outside sleeping areas. Carbon monoxide detectors should be placed near bedrooms and on each level, particularly near fuel-burning appliances.

If you have only one type of detector, you are leaving yourself unprotected against half of these common household dangers. The cost of adding a second detector is far lower than the risk of missing an early warning.

Combination smoke and carbon monoxide detectors

Some manufacturers sell combination detectors that contain both a smoke sensor and a carbon monoxide sensor in a single unit. These devices can reduce clutter and simplify installation, but they are typically more expensive than buying separate detectors and may be harder to troubleshoot if one sensor fails.

A combination detector will alert you to either smoke or carbon monoxide, but the alarm sound is usually the same for both hazards. This means you will need to investigate which threat is present when the alarm sounds. Some combination models display which sensor was triggered, which can help you respond faster.

Combination detectors are a valid option if you prefer a single device, but they are not necessary. Two separate, well-placed detectors are just as effective and often more reliable because a failure in one sensor does not disable the other.

Testing and maintaining both detectors

Test your smoke detector by pressing and holding the test button for three to five seconds. You should hear a loud alarm. Test your carbon monoxide detector the same way. Both should be tested at least once a month to confirm they are working.

Replace the batteries in both detectors twice a year. Many people do this when daylight saving time changes occur in spring and fall, which makes it straightforward to remember. If a detector chirps or beeps at regular intervals (usually every 30 to 60 seconds), the battery is low and needs replacement.

Replace the entire detector every 5 to 7 years for smoke detectors and every 5 to 10 years for carbon monoxide detectors, depending on the manufacturer's instructions. Check the label on your detector for the replacement date. Even if the detector still works, the sensors degrade over time and become less reliable.

What to do if either detector goes off

If your smoke detector goes off, assume there is a fire. Leave your home when ready through the nearest safe exit. Do not stop to gather belongings or investigate the source. Once outside, call 911 from a safe location and do not re-enter the building.

If your carbon monoxide detector goes off, leave your home when ready and move to fresh air outside or in another building. Call 911 or your local poison control center from outside. Do not re-enter your home until emergency responders have cleared it and identified the source of the carbon monoxide. If you experience symptoms like headache, dizziness, nausea, or chest pain, tell the emergency responder that you may have been exposed to carbon monoxide.

If a detector goes off and then stops, do not assume it was a false alarm. Test the detector again. If it goes off a second time, follow the same evacuation procedure. A detector that goes off repeatedly may indicate a real hazard that comes and goes, such as a furnace that leaks carbon monoxide intermittently.

Frequently Asked Questions

Can I use a carbon monoxide detector instead of a smoke detector?

No. A carbon monoxide detector will not detect smoke or fire. You need both devices for complete protection. Using only a carbon monoxide detector leaves you unprotected against fire, which is one of the most common household emergencies.

Do I need a carbon monoxide detector if I don't have a gas furnace?

Yes. Carbon monoxide can be produced by any fuel-burning appliance, including gas water heaters, gas stoves, fireplaces, and even charcoal grills used indoors. Even if your primary heating is electric, you may have other sources of carbon monoxide in your home.

Why does my carbon monoxide detector go off when I cook?

Carbon monoxide detectors should not go off from normal cooking. If yours does, the detector may be too close to your stove or there may be a problem with your gas appliance. Move the detector away from the kitchen and have your stove inspected by a professional. A detector that goes off repeatedly near the stove may indicate incomplete combustion.

Can I install a smoke detector and carbon monoxide detector in the same location?

You can install them near each other, but they should not be in the exact same spot. Smoke detectors work best away from kitchens and bathrooms where steam and cooking fumes can trigger false alarms. Carbon monoxide detectors should be near bedrooms and fuel-burning appliances. Placing them a few feet apart in the same room is acceptable.

What should I do if both detectors go off at the same time?

Leave your home when ready and call 911 from outside. A simultaneous alarm could indicate a fire that is also producing carbon monoxide, or it could be a coincidence. Either way, evacuate first and let emergency responders investigate. Do not re-enter until they have cleared the building.