If a carbon monoxide alarm sounds or anyone has possible CO poisoning symptoms, leave the building immediately, call emergency services from outside, and do not re-enter until qualified professionals declare the property safe.
What Is Carbon Monoxide?
Carbon monoxide is a poisonous gas created when carbon-based fuels do not burn completely. Common sources include natural gas, propane, heating oil, wood, charcoal, kerosene, gasoline, and diesel fuel. Furnaces, boilers, water heaters, fireplaces, stoves, generators, and vehicles can all produce CO under unsafe conditions.
CO can build up when an appliance is damaged, poorly maintained, incorrectly adjusted, or inadequately ventilated. Unlike smoke, it generally does not irritate the eyes or throat, so occupants may not recognize exposure until symptoms appear. The CDC’s carbon monoxide poisoning guidance explains how CO prevents the body from using oxygen effectively.
Carbon Monoxide Versus a CO2 Detector
The terms carbon monoxide detector and CO2 detector are sometimes confused, but they identify different gases. CO means carbon monoxide, a toxic combustion byproduct. CO2 means carbon dioxide, a naturally occurring gas produced by respiration and combustion. A household CO alarm detects carbon monoxide, not carbon dioxide.
When shopping for protection from combustion-related poisoning, choose a device labeled carbon monoxide alarm, carbon monoxide detector, or CO alarm. Look for products certified to applicable safety standards, such as UL 2034 in the United States.
How Carbon Monoxide Enters a Home
Malfunctioning Combustion Appliances
Dirty burners, incorrect fuel-to-air ratios, damaged heat exchangers, improper gas pressure, blocked burner ports, soot, cracked components, poorly adjusted controls, and inadequate combustion air can increase CO production. Fuel-burning equipment should be serviced by a qualified professional according to the manufacturer’s instructions.
Blocked or Damaged Vents
Furnaces, boilers, water heaters, and fireplaces rely on vents and flues to carry combustion gases outdoors. Bird nests, leaves, snow, ice, insects, rust, debris, collapsed masonry, deteriorated liners, and improperly installed caps can restrict airflow. Even a partial blockage can interfere with draft and cause exhaust to spill indoors.
Backdrafting and Negative Pressure
Backdrafting occurs when combustion gases move backward through a vent instead of exiting outdoors. Kitchen and bathroom exhaust fans, clothes dryers, central vacuums, tight building envelopes, whole-house fans, wood-burning appliances, and unbalanced ventilation can create negative pressure that contributes to backdrafting.
Attached Garages and Generators
Vehicles and gasoline-powered equipment can release CO into a home through door gaps, wall penetrations, ductwork, foundation cracks, framing cavities, or open doors. Never run a vehicle, lawn mower, snow blower, or generator in an attached garage or other enclosed space. Operate portable generators outdoors at least 20 feet from the home, with exhaust directed away from windows, doors, and vents. The U.S. Consumer Product Safety Commission’s generator safety alert provides additional guidance.
Combustion Appliances That Need Attention
A standard home inspection can identify visible safety concerns, but it is not generally a substitute for specialized appliance servicing, chimney evaluation, combustion analysis, or flue-gas testing.
Gas Furnaces and Boilers
Inspectors should note soot, rust, corrosion, unusual odors, yellow or flickering flames, excessive condensation, damaged vent connectors, missing panels, improper clearances, water leakage, and possible heat-exchanger damage. A yellow flame does not prove that CO is present, but it may indicate incomplete combustion and requires prompt professional evaluation.
Gas Water Heaters
Warning signs include a discolored draft hood, rust around the flue, soot above the unit, loose or disconnected vent pipes, improper vent slope, missing fasteners, inadequate clearance from combustibles, and installation in an unsuitable location. A vent connector should generally rise toward the chimney or exterior vent; a downward-sloping or poorly supported connector can create a serious safety concern.
Fireplaces, Wood Stoves, and Gas Ranges
Blocked chimneys, damaged flues, creosote buildup, deteriorated mortar, cracked flue tiles, smoke staining, poor draft, excessive ash, and unapproved modifications can make fireplaces and wood stoves hazardous. Gas ovens and ranges can also produce CO when burners are dirty or improperly adjusted. Never use a gas oven as a heating source. Outdoor-rated heaters must never be used indoors.
Inspection Warning Signs of CO Risk
Visible conditions do not necessarily prove that carbon monoxide is present, but they indicate that the combustion or venting system may not be operating safely.
- Soot, smoke staining, or black deposits around an appliance, vent, wall, ceiling, or draft hood.
- Persistent yellow, orange, or flickering burner flames.
- Rust, corrosion, water damage, or condensation near an appliance or flue.
- Gaps, missing screws, loose pipes, crushed sections, improper slope, or disconnected venting.
- Blocked chimneys, flues, exterior exhaust openings, or deteriorated liners.
- Fuel-burning appliances installed in a confined room, bedroom, bathroom, or unsuitable closet.
- Missing combustion-air openings or signs of negative pressure.
- Recalled, modified, poorly maintained, or aging equipment.
- A carbon monoxide alarm that activates, even if occupants feel well or the alarm later stops.
Do not clean away soot, repair gas equipment, alter combustion-air openings, or reconnect venting without qualified professional advice.
Symptoms of Carbon Monoxide Poisoning
The symptoms of carbon monoxide poisoning can resemble the flu, food poisoning, exhaustion, or a migraine. Common symptoms include headache, dizziness, weakness, nausea, vomiting, shortness of breath, chest pain, confusion, blurred vision, drowsiness, and loss of coordination. Severe exposure can cause fainting, seizures, unconsciousness, coma, cardiac arrest, or death.
Symptoms that improve after leaving the building and return indoors are especially concerning. Multiple occupants or pets becoming ill at the same time is a major warning sign. Infants, children, older adults, pregnant people, individuals with heart or respiratory conditions, people with anemia, and sleeping or impaired occupants may face increased risk.
What to Do if CO Exposure Is Suspected
- Leave the building immediately.
- Take everyone and pets outside if it is safe.
- Call emergency services from outside.
- Tell responders that carbon monoxide exposure is possible.
- Seek medical evaluation, especially if symptoms occurred.
- Do not re-enter until emergency personnel or qualified professionals confirm that it is safe.
- Do not reset, silence permanently, or ignore the alarm.
Do not delay evacuation to search for the source, open windows, collect belongings, or reset the detector.
Proper Carbon Monoxide Detector Placement
Install CO alarms on every level of the home, outside each sleeping area, and near bedrooms where occupants can hear them. Homes with attached garages may also need an alarm near the interior entry from the garage. Follow the manufacturer’s instructions and local requirements.
- Keep alarms where they can be heard while sleeping.
- Avoid placing them directly beside fuel-burning appliances unless the manufacturer permits it.
- Keep them away from windows, doors, strong drafts, cooking appliances, bathrooms, and high humidity.
- Do not cover, paint, obstruct, or install an alarm behind furniture.
- Keep each unit accessible for testing and battery replacement.
- Use the manufacturer-recommended mounting height.
There is no universal rule that every CO detector must be installed near the floor or ceiling. Manufacturer instructions and local requirements should control placement. Large homes may need additional alarms so occupants can hear warnings throughout the building.
Best Types of Carbon Monoxide Detectors
The best carbon monoxide detector is certified, correctly installed, regularly tested, and maintained. Battery-powered models are flexible and continue operating during power outages. Plug-in models are convenient, but a battery backup is important. Hardwired alarms may provide reliable power and interconnection, although professional installation may be needed.
Combination smoke and CO alarms can reduce the number of devices, while smart alarms may provide phone notifications and maintenance reminders. However, smart features should supplement, not replace, a loud local alarm. Digital displays and peak-level memory can provide useful information, but occupants should never wait for a numerical reading before leaving.
Check the package or device for certification marks, manufacturing and replacement dates, battery information, installation instructions, test functions, and warranty details. Avoid secondhand alarms with unknown age or maintenance history.
Testing and Maintaining CO Alarms
- Test alarms according to the manufacturer’s instructions.
- Replace batteries when the low-battery warning sounds or as directed.
- Never remove a battery without installing a replacement immediately.
- Replace each alarm at the end of its service life, commonly five to ten years depending on the manufacturer.
- Learn the different sounds for an emergency, low battery, malfunction, and end of service life.
A test confirms that the alarm can produce a warning sound, but it does not necessarily confirm accurate sensing at every concentration.
Are Rental Properties Required to Have CO Detectors?
Whether a rental property must have a carbon monoxide detector depends on state, municipal, fire, housing, and building requirements. Rules may vary based on construction date, fuel-burning appliances, attached garages, sleeping rooms, heating systems, number of units, and whether the property is a single-family, multifamily, or short-term rental.
Many jurisdictions require landlords to install CO alarms when a rental has a fuel-burning appliance, fireplace, or attached garage. Requirements may also address locations, testing, battery replacement, tenant notification, and maintenance. Owners and tenants should consult the local building department, fire marshal, housing agency, landlord-tenant agency, applicable codes, and lease documents.
Landlord and Tenant Responsibilities
Depending on local law, landlords may need to provide, install, maintain, test, and replace alarms; service hazardous appliances; and respond promptly to alarm reports. Tenants may need to keep alarms installed, test them, replace batteries, report faults, avoid blocking devices, and allow safety inspections. No one should disable or remove an alarm because of nuisance alerts. The underlying cause should be investigated.
When Immediate Professional Evaluation Is Necessary
Arrange immediate professional evaluation after an alarm activation, possible CO poisoning symptoms, unexplained illness affecting multiple occupants, soot or smoke staining, abnormal flames, damaged or disconnected venting, a blocked chimney, suspected heat-exchanger damage, generator or vehicle operation near the home, repeated unexplained alarms, unpermitted appliance modifications, or recent air-sealing and remodeling.
The appropriate professional may be a licensed HVAC contractor, plumber, chimney specialist, gas utility technician, fire department, or other qualified service provider. A home inspector can document visible conditions and recommend further evaluation, but many inspections do not include combustion-efficiency testing, flue-gas analysis, pressure testing, or internal heat-exchanger inspection.
Common Mistakes That Increase CO Risk
- Using a gas oven to heat the home.
- Operating a generator or outdoor heater indoors or in a garage.
- Running a vehicle in an attached garage.
- Ignoring a low-battery, end-of-life, or nuisance alarm.
- Installing an alarm behind furniture or blocking combustion-air openings.
- Assuming an open window makes generator operation safe.
- Performing unqualified gas-appliance repairs or altering vents.
- Confusing a carbon dioxide monitor with a carbon monoxide alarm.
- Re-entering a home after an alarm without professional clearance.
Carbon Monoxide Safety Checklist
- Install certified alarms on every level and outside sleeping areas.
- Test alarms regularly and replace batteries and expired units.
- Schedule professional service for heating equipment.
- Have chimneys and fireplaces inspected as recommended.
- Keep vents, flues, and exterior exhaust openings clear.
- Operate generators at least 20 feet from the home.
- Identify every fuel-burning appliance and inspect visible venting.
- Ask about appliance and chimney service history when buying a home.
- Review local CO alarm requirements for rental properties.
- Know the symptoms and practice an evacuation plan.
Conclusion
Carbon monoxide is a hidden household danger from furnaces, boilers, water heaters, fireplaces, stoves, generators, vehicles, and other combustion sources. Because it cannot be seen or smelled, prevention depends on safe venting, adequate combustion air, proper appliance maintenance, and correctly installed carbon monoxide alarms.
Install and maintain certified alarms, understand local rental-property requirements, and treat any alarm, abnormal flame, soot, damaged vent, or possible symptom as an emergency. A carbon monoxide detector is not a substitute for professional inspection, and a CO2 detector is not the same device. When warning signs appear, leave the property and obtain immediate professional evaluation.
