Most smoke detectors do not detect carbon monoxide — they only sense smoke

A standard smoke detector is built to sense smoke particles in the air. It cannot detect carbon monoxide, which is an odorless, colorless gas. If your detector says "smoke detector" on the label or packaging, it will alert you to fire but will not warn you about carbon monoxide leaks from a furnace, water heater, or car running in an attached garage.

Carbon monoxide requires a separate detector with its own sensor. The two devices work on completely different principles: smoke detectors use either ionization or photoelectric sensors to spot particles, while carbon monoxide detectors use electrochemical or biomimetic sensors to measure gas concentration in parts per million. You cannot rely on one to do the job of the other.

Key Takeaways

  • Smoke detectors sense smoke particles only and will not detect carbon monoxide gas, no matter how sensitive they are.
  • A separate carbon monoxide detector is required to warn you about carbon monoxide from furnaces, water heaters, and vehicle exhaust.
  • Combination smoke and carbon monoxide detectors exist and are labeled as such on the packaging — check your detector's label to know what it covers.
  • If you have only a smoke detector, you have no protection against carbon monoxide poisoning and should install a dedicated carbon monoxide detector.

How to tell what your detector actually covers

The easiest way to know is to look at the label on the front or back of your detector. A smoke-only detector will say "Smoke Alarm" or "Smoke Detector." A combination unit will say "Smoke and Carbon Monoxide Alarm" or "Dual Sensor." If the label does not mention carbon monoxide, your detector does not sense it.

You can also check the packaging or manual if you still have it, or search the model number on the manufacturer's website. The manufacturer's spec sheet will list every type of hazard the detector is designed to sense. Do not guess based on price or appearance — a large or expensive smoke detector is still just a smoke detector unless it explicitly says it detects carbon monoxide.

Combination detectors: what they are and where to place them

A combination smoke and carbon monoxide detector contains two separate sensors in one unit. It will alert you to either smoke or carbon monoxide, whichever it detects first. These are sold under brand names like First Alert, Kidde, and others, and they cost roughly $25 to $60 depending on features like battery backup or voice alerts.

Combination detectors should be placed on every level of your home, with at least one near bedrooms so you hear the alarm while sleeping. For carbon monoxide specifically, place a detector near any fuel-burning appliance: furnace, water heater, gas stove, or fireplace. Do not place it directly above the appliance or in a kitchen, where cooking fumes can trigger false alarms. A hallway or bedroom near the appliance works better.

Why you need both types if you only have smoke detectors now

If your home has only smoke detectors, you are not protected against carbon monoxide poisoning. Carbon monoxide kills roughly 400 people per year in the United States, according to the CDC, and many more experience poisoning symptoms like headache, dizziness, and nausea without realizing the source. A smoke detector will not warn you.

The cheapest option is to buy a standalone carbon monoxide detector and place it near your furnace and water heater, plus one in a bedroom. This costs $15 to $30 per unit. If you prefer a single device on each level, replace your smoke detectors with combination units over time as the old ones reach the end of their life (usually 8 to 10 years). Both approaches work; the choice depends on your budget and how many detectors you need to replace.

What carbon monoxide detectors actually sense

A carbon monoxide detector measures the concentration of carbon monoxide gas in the air and sounds an alarm when it reaches a dangerous level. Most detectors are set to alarm at 70 parts per million after 60 minutes of exposure, or at 400 parts per million after 4 minutes. These thresholds are based on health standards from the National Institute for Occupational Safety and Health.

The detector does not tell you where the carbon monoxide is coming from — only that it is present. If your detector goes off, you should leave the house when ready, call 911 or your local fire department, and let them investigate. Do not assume it is a false alarm or try to find the source yourself.

Common mistakes that leave you unprotected

One mistake is assuming a "smart" or "advanced" smoke detector detects carbon monoxide. It does not unless the label says so. Another is placing a carbon monoxide detector in the wrong spot — too close to a stove or in a kitchen where it will alarm constantly, or too far from the appliances that produce it. A third is buying only one detector for a multi-level home; carbon monoxide can spread, but you want early warning on the level where it starts.

A fourth mistake is not replacing detectors when they expire. Smoke detectors and carbon monoxide detectors have a lifespan, usually printed on the back. After that date, the sensors degrade and may not detect hazards reliably. Check the date on your detectors now and replace any that are past their expiration date.

Frequently Asked Questions

Can I use a smoke detector in place of a carbon monoxide detector?

No. A smoke detector senses smoke particles and will not detect carbon monoxide gas. You need a separate carbon monoxide detector to be warned about carbon monoxide leaks. Using only a smoke detector leaves you unprotected against carbon monoxide poisoning.

What should I do if my carbon monoxide detector goes off?

Leave your home when ready and call 911 or your local fire department from outside. Do not re-enter the house to investigate or turn off the alarm. Let the fire department check for the source of the carbon monoxide. If the alarm was triggered by a malfunctioning appliance, they will tell you which one needs repair.

How often should I test my detectors?

Test both smoke and carbon monoxide detectors once a month by pressing the test button. The alarm should sound loudly. If it does not, replace the batteries (if it uses them) or replace the entire unit if it is hardwired. Also check the expiration date on the back and replace any detector that has expired.

Do I need a carbon monoxide detector if I have an electric furnace?

No. Electric furnaces do not produce carbon monoxide because they do not burn fuel. You only need a carbon monoxide detector if you have a gas furnace, gas water heater, gas stove, fireplace, or any other fuel-burning appliance. If you are unsure whether an appliance burns fuel, check the label or ask your utility company.