How Carbon Monoxide Detectors Function

Important Points to Remember

Referred to as "the silent killer," carbon monoxide (CO) is a colorless, tasteless, and odorless gas produced by incomplete combustion of fuel. Anything you burn around your home — whether it's gasoline, wood, coal, propane, natural gas, or oil — can generate carbon monoxide under certain conditions. Without proper ventilation, the gas can accumulate to fatal levels. And it only takes a small amount of fuel to become dangerous: even small equipment like chainsaws can emit enough CO to cause illness.

Carbon monoxide enters red blood cells, rapidly displacing the oxygen your body requires. Your blood then transports the CO to vital organs. Inhalation of small amounts of CO over extended periods might not be fatal, but it can result in permanent damage to the brain, lungs, and heart. Moderate CO exposure can cause symptoms like burning eyes, nausea, vomiting, dizziness, severe headaches, shortness of breath, and confusion. These symptoms are often confused with other common illnesses, such as food poisoning or the flu. If carbon monoxide begins to fill a home while people are sleeping, the exposure may be fatal before anyone realizes the danger. Infants, the elderly, and those with respiratory or circulatory conditions are at a higher risk of deadly CO poisoning.

You can lower your risk of carbon monoxide poisoning by keeping your fuel-burning appliances in good condition and only using combustible fuels in properly ventilated spaces. A key safety measure is installing a carbon monoxide detector in your home. In this article, we will explore how these detectors detect and respond to this deadly gas, and what to consider when purchasing one.

Components of a Carbon Monoxide Detector

You might be surprised to learn that a carbon monoxide (CO) detector is not as complex as it may seem. Inside a plastic chamber about 3 inches (7.6 centimeters) in diameter, components work together to alert homeowners when dangerous levels of gas are detected. The specific components may differ by model, but here's an overview of the essential parts:

In the U.S., home-use sensors are calibrated to detect CO concentrations according to Underwriters Laboratories (UL) safety standards. When dangerous levels of CO are detected, the sensor sends an electronic pulse to activate the alarm. The higher the concentration, the faster the alarm responds. For instance, the alarm reacts to levels of about 70 ppm within an hour, but will respond to 400 ppm in just four minutes [source: UL 2034]. The alarm sounds at around 85 decibels (dB), slightly quieter than a lawnmower, but often very shrill.

CO detectors can employ various sensor types to detect carbon monoxide in the air. Biomimetic sensor technology mimics CO’s impact on hemoglobin in the blood. In a biomimetic sensor, a gel changes color when it absorbs CO. A separate sensor detects this color change and triggers the alarm. Once the gel has changed color, it must be placed in a CO-free environment to reset. Biomimetic sensors are also found in portable CO detector cards — if the disc turns dark, it indicates dangerous CO levels in the air.

Metal oxide semiconductor sensors utilize circuits applied to a silica chip. When CO interacts with the circuitry, it lowers the electrical resistance. The processor detects this change and triggers the alarm. These sensors consume a significant amount of electricity, so they are typically plugged into wall outlets rather than relying on batteries.

Like metal oxide sensors, electrochemical sensors detect carbon monoxide by monitoring electrical current changes. Instead of using an integrated chip, these sensors rely on electrodes submerged in a chemical solution. Electrochemical instant detection and response (IDR) sensors are typically found in professional equipment and can quickly identify hazardous CO levels.

Installing a Carbon Monoxide Detector

In the United States, approximately 20,000 individuals visit emergency rooms each year due to carbon monoxide (CO) poisoning, and about 400 fatalities occur annually from CO exposure [source: CDC]. The majority of non-fatal exposures happen within homes, underscoring the importance of having a CO detector alongside, or as part of, your smoke detector.

Before choosing a carbon monoxide (CO) detector, take time to research your options. Check whether local regulations mandate a specific type or configuration of CO detectors. Decide whether you prefer a battery-powered model or one that plugs into the wall. A plug-in detector is portable and can be moved to different rooms, but without a battery backup, it leaves you vulnerable during power outages. Although short-term outages may seem insignificant, a prolonged winter power loss could force you to rely on fuel-burning devices like space heaters or wood stoves, which may emit CO.

Battery-powered detectors are typically fixed to a surface in your home. This usually isn’t an issue unless the unit mistakenly detects other gases as CO, leading to false alarms. Depending on your home’s size, as well as the number and location of fuel-burning appliances, a combination of detector types may offer the best protection. Be sure to follow the manufacturer’s guidelines for installation, upkeep, and periodic checks.

The National Fire Protection Agency (NFPA) advises installing carbon monoxide detectors in several key areas of the home:

The NFPA also suggests using interconnected detectors, so when one alarm sounds, all others are triggered, ensuring that everyone in the household is alerted.

While installing CO detectors is a vital part of protecting yourself from carbon monoxide poisoning, there are additional important steps to take, including:

Lastly, be sure to test your carbon monoxide detector every month — it's your last line of defense if a stove or heater fails.