Carbon monoxide detectors work by sensing CO molecules in the air and converting that detection into a measurable signal, most commonly through a chemical reaction that generates a tiny electrical current. The most widely used type, the electrochemical sensor, can detect CO concentrations as low as 30 parts per million. Different detector technologies take slightly different approaches, but all of them continuously sample the surrounding air and trigger an alarm when CO reaches a dangerous level over a set period of time.
Electrochemical Sensors: The Most Common Type
The majority of home carbon monoxide detectors use electrochemical sensors. Inside the detector, electrodes sit submerged in a liquid called an electrolyte, all housed in a small gas-permeable compartment. When carbon monoxide drifts into that compartment, it reacts chemically with the electrolyte and produces a surge in electrical current flowing between the electrodes. The size of that current surge tells the detector exactly how much CO is present in the air.
This is a proportional system: a small amount of CO produces a small current, and a large amount produces a large one. The detector’s processor reads that signal continuously and compares it against built-in thresholds. If the concentration stays high enough for long enough, the alarm sounds. Electrochemical sensors are popular because they’re accurate, use very little power (making them suitable for battery-operated units), and respond reliably at the concentrations that matter for human health.
Metal Oxide Semiconductor Sensors
A second technology uses a thin film of metal oxide, typically tin dioxide, deposited on a small chip inside the detector. In clean air, oxygen molecules stick to the surface of this film and pull electrons out of it, creating a high-resistance barrier to electrical flow. Think of it like a partially blocked pipe: current has a hard time getting through.
When carbon monoxide reaches the sensor, it reacts with those surface oxygen molecules and knocks electrons back into the metal oxide. This widens the path for electrical flow and drops the material’s resistance. The detector measures that resistance change and calculates the CO concentration from it. Some sensors use tiny amounts of platinum or palladium on the surface to speed up this reaction, making them more sensitive at lower CO levels. These sensors tend to draw more power than electrochemical ones, so they’re more common in plug-in models than battery-operated detectors.
Why Detectors Don’t Alarm Immediately
Carbon monoxide detectors are deliberately designed to respond based on both concentration and time, not just the presence of CO. This mirrors how CO actually harms people: it’s the combination of how much you’re breathing and for how long that determines poisoning risk. A brief whiff of moderate CO from opening a garage door, for example, isn’t dangerous, but a steady 70 ppm leak for several hours is.
The safety standard used in the U.S. (UL 2034) sets specific alarm windows based on this principle:
- 400 ppm: must alarm between 4 and 15 minutes
- 150 ppm: must alarm between 10 and 50 minutes
- 70 ppm: must alarm between 60 and 240 minutes
Detectors are also required to resist false alarms at low concentrations. They must not alarm below 30 ppm for at least 30 days of continuous exposure, and must not alarm below 70 ppm for at least 60 minutes. These thresholds were chosen to align with the level of CO exposure that produces early signs of poisoning in the bloodstream.
Why Those Thresholds Matter for Your Health
Carbon monoxide is dangerous because it binds to hemoglobin in your blood roughly 200 times more readily than oxygen does, gradually starving your tissues of oxygen. The symptoms progress predictably with concentration and exposure time. According to OSHA data, 35 ppm over six to eight hours causes headaches and dizziness. At 200 ppm for two to three hours, you can expect headache, impaired judgment, and vision problems. At 400 ppm, frontal headache and nausea set in within one to two hours, and the exposure becomes life-threatening after three hours. Above 800 ppm, collapse and death can occur within hours or even minutes.
This is why detectors are calibrated to sound faster at higher concentrations. At 400 ppm, you have a narrow window before serious harm, so the alarm must trigger within 15 minutes. At 70 ppm, the danger builds slowly, giving the detector up to four hours to confirm the reading isn’t a temporary fluctuation.
False Alarms and Cross-Sensitivity
Electrochemical CO sensors can react to gases other than carbon monoxide. Hydrogen gas is the most well-documented cause of false readings. Batteries that are actively charging (like in a forklift bay or a utility room with charging equipment) release hydrogen gas, which can register on a CO detector as if carbon monoxide were present. This cross-sensitivity between hydrogen and CO sensors is well known in industrial settings and occasionally affects home detectors placed near battery charging stations or certain cleaning products that release volatile compounds.
If your detector alarms and you suspect a false reading, ventilate the area and see if the alarm stops. But treat every alarm as real until you can confirm otherwise, because CO is colorless and odorless, and there’s no way to detect it with your senses alone.
How CO Detectors Differ From Smoke Detectors
Smoke detectors and carbon monoxide detectors protect against completely different threats using completely different technologies. Smoke detectors sense physical particles in the air, either by detecting how smoke scatters a beam of light (photoelectric) or by sensing disruption to a small ionized air chamber (ionization). They respond almost instantly to the presence of smoke.
CO detectors sense a specific invisible gas through chemical or electrical reactions, and they intentionally delay their response based on concentration, as described above. A smoke detector cannot detect carbon monoxide, and a CO detector cannot detect smoke. Combination units that do both contain two separate sensors in one housing. If your home has fuel-burning appliances (a gas furnace, water heater, stove, or attached garage), you need CO detectors regardless of whether you already have smoke detectors.
Placement and Mounting Height
Carbon monoxide has nearly the same density as regular air at room temperature, so it mixes evenly throughout a space rather than rising or sinking. According to NFPA guidelines, detector performance is not generally dependent on mounting height. You can mount a CO detector on a wall at any height or on the ceiling, and it will work effectively. Follow the manufacturer’s instructions for your specific model, but don’t worry about the common misconception that CO detectors need to be placed low because the gas is “heavy” or high because it “rises.”
Place detectors near sleeping areas so the alarm will wake you, and on every level of the home. Avoid placing them directly next to fuel-burning appliances, where brief, normal combustion byproducts could cause nuisance alarms, or in areas with high humidity like bathrooms.
Lifespan and Replacement
CO detectors have a lifespan of around seven years. The sensors inside gradually degrade as the chemical components that react with carbon monoxide get consumed or contaminated over time. An old detector may respond more slowly, read lower than the actual concentration, or fail to alarm entirely.
All CO alarms manufactured after August 2009 include an end-of-life warning, typically a distinct chirp pattern different from the low-battery alert, that tells you the unit needs replacing. If your detector is chirping and new batteries don’t fix it, check the manufacture date printed on the back. If it’s more than seven years old, replace it. Testing the alarm button only confirms that the speaker and battery work; it does not test whether the sensor can still detect carbon monoxide.