A family turns up at a clinic in February with headaches, fatigue and nausea. Nobody has a fever. They get told it is probably something going around, and they go home.
Home is the problem.
Carbon monoxide is a colorless, odorless gas made whenever a fuel burns incompletely. The Consumer Product Safety Commission calls it the invisible killer, and that is not marketing. Your nose cannot find it. Your eyes cannot find it. At low levels it produces exactly the symptoms of half the things going around in winter.
The Centers for Disease Control and Prevention estimates that more than 400 Americans die every year from unintentional carbon monoxide poisoning unrelated to fires, that more than 100,000 go to an emergency department, and that more than 14,000 are hospitalized.
An alarm that would have warned that family costs less than a tank of gas.
Where it comes from in an ordinary house
Anything that burns fuel makes some carbon monoxide. An appliance that is correctly installed, correctly vented and properly maintained sends it outdoors. The hazard appears when one of those three conditions fails. Group the sources by how they fail rather than by what they are.
Things that vent, until they do not. Furnaces, boilers, gas and oil water heaters, wood stoves and fireplaces all depend on a flue or a chimney. A blocked chimney, a disconnected vent pipe, a cracked heat exchanger or a flue sized wrong pushes combustion gases back into the house instead of out of it. EPA’s indoor air page also names backdrafting, where a house pulls exhaust back down a flue rather than letting it rise.
Things never meant to vent at all. Unvented kerosene and gas space heaters release their combustion products straight into the room by design. So do portable camp stoves and flameless chemical heaters, which is why they belong outside.
Things people bring inside during an emergency. Portable generators, charcoal grills and gasoline-powered tools. This group gets its own section below, because on a bad week it kills more people than the rest combined.
Engines next door to the living space. A car idling in an attached garage, or an engine running near an open window or a fresh air intake.
Gas cooking appliances also produce carbon monoxide in normal use, at levels EPA puts at 5 to 15 parts per million near a properly adjusted stove and 30 ppm or higher near a poorly adjusted one. What a stove emits during ordinary cooking is a separate subject with its own article. This one is the whole-house hazard and the emergency.
Why it looks like the flu, and the tell that gives it away
At the levels that make people sick rather than killing them outright, carbon monoxide produces headache, dizziness, weakness, nausea, vomiting, shortness of breath and confusion. CDC describes the picture as flu-like, with one difference a clinician notices: no fever.
CDC’s guidance for health care providers calls the findings variable and nonspecific, and tells clinicians to think about carbon monoxide when symptoms appear without fever, when there is a plausible exposure history, or when several people turn up with the same complaints at the same time.
That last clause is why whole families have been found together. A gas does not pick favorites. If the furnace is the source, everyone in the house is breathing it, so everyone gets sick on roughly the same schedule. A household that all comes down with something on the same evening looks like an ordinary February to the people living in it.
Here is the tell. The Consumer Product Safety Commission’s consumer guide puts it in one line, saying symptoms “lessen or disappear when you are away from home.” EPA’s indoor air page describes the same thing, naming “flu-like symptoms that clear up after leaving home.”
Think about what that means in practice. A virus travels with you. If your headache lifts an hour after you get to work, returns within an hour of walking in the front door, and repeats the next day and the day after, that is not a virus. That is a pattern tied to a building.
The same test applies to pets. Dogs and cats spend more hours in the house than you do, and they are often sick first.
One mechanism is worth knowing, because it explains a common false reassurance. Carbon monoxide binds to hemoglobin, the protein in red blood cells that normally carries oxygen, and it binds far more readily than oxygen does. The blood fills up with a molecule that cannot deliver oxygen to tissue. CDC’s clinical guidance notes that an ordinary two-wavelength pulse oximeter, the clip that goes on a fingertip, reads inaccurately when carbon monoxide is present. A reassuring oxygen number rules nothing out. Confirming carbon monoxide poisoning takes a specific blood measurement, which is a job for clinicians.
This article is not here to help you diagnose anyone. It is here to get you to make a phone call sooner.
Alarms: placement, expiry, and what they are not
A carbon monoxide alarm is not a smoke alarm. A smoke alarm senses particles from a fire. A carbon monoxide alarm senses a specific gas with a chemical sensor and is built to its own product standard, UL 2034, which CPSC names in its fact sheet. A fire can happen without carbon monoxide reaching alarm levels, and carbon monoxide can fill a house with no fire at all. Combination units exist and are a third product that does both jobs in one housing. Owning one kind does not cover you for the other.
Where they go. CPSC says put alarms on every level of the home and outside each sleeping area. Interconnected alarms, where one sounding sets them all off, are the best arrangement, because an alarm in a basement that nobody hears from a third-floor bedroom has solved nothing. Keep them where drapes and furniture cannot cover them, and away from heating vents. CPSC advises against kitchens and against mounting one directly above a fuel-burning appliance, where brief ordinary emissions cause nuisance alarms.
When they expire. This is the part that catches people. The sensor inside a carbon monoxide alarm degrades. It does not last the life of the house, and a dead sensor with a fresh battery will chirp happily and detect nothing. CDC advises replacing detectors according to the manufacturer’s instructions or every five years, and setting a reminder the day you install one. CPSC notes that alarms carry a recommended replacement age in the product literature and that newer models give an end-of-life signal.
Write the install date on the back of the unit in marker. That one habit is worth more than any feature on the box.
Batteries and testing. CPSC recommends replacing batteries annually and testing alarms monthly. CDC suggests checking batteries when the clocks change. Note what the test button does and does not do: CPSC’s fact sheet is explicit that pressing it checks the circuitry and the horn, not the accuracy of the sensor.
A note on which rules are rules. UL 2034 and the generator standards named below are private consensus standards, not federal regulations. Whether your home is legally required to have a carbon monoxide alarm is a question of state and local building code, and CPSC points readers to their local building official. No single national requirement covers existing homes.
The generator rule
This is the one rule here with no exceptions.
Never run a portable generator, a grill, or any fuel-burning engine inside a home, a basement, a garage, a crawlspace, a shed or a tent. Not with the door open. Not with the windows open. Not with a fan running. Not for a few minutes.
Run it outdoors only, at least 20 feet from the house, with the exhaust pointed away from windows, doors and vents.
The rule is absolute because of the quantity involved. CPSC states that a single portable generator can produce as much carbon monoxide as hundreds of cars. A garage with the door up is not outdoors. It is a box with one open side, and an engine running in it builds lethal concentrations faster than an open door can clear them. A fan moves the gas around rather than removing it.
These deaths cluster, and they cluster predictably. CPSC describes portable generators as one of the leading causes of post-storm carbon monoxide deaths, and more than 100 of the roughly 200 yearly deaths CPSC ties to consumer products involve them. The pattern is a hurricane, an ice storm or a derecho, a multi-day outage, cold dark houses, and people improvising.
A study in Pediatrics in 2009 shows how that plays out. In the first 36 hours after Hurricane Ike made landfall in 2008, one Houston hospital treated or triaged 37 people exposed to generator carbon monoxide across 13 separate incidents. Fifty-four percent were under 18. Eleven needed hyperbaric oxygen. One child died at the scene. Among the incidents where investigators learned why the generator was running, five of nine were powering video games or a television, and those accounted for three-quarters of the pediatric poisonings.
Nobody in those houses was being reckless as they understood it. They were keeping bored children occupied during a blackout.
The hardware has changed, too. Ahead of the 2026 hurricane season, CPSC pointed consumers toward portable generators with an automatic carbon monoxide shutoff, certified to PGMA G300-2023 or UL 2201-2023. These sense rising carbon monoxide and cut the engine. They back up the 20-foot rule rather than replacing it.
Two more rules in the same family. Never burn charcoal indoors, in a garage, in a vehicle or in a tent, for any reason. And never heat a house with a gas oven or range.
Before the next outage
Storm preparation is where this gets fixed, because nobody makes good decisions by flashlight at two in the morning.
- Decide now where the generator will sit, measure the 20 feet, and look at what is downwind of that spot, including your neighbor’s bedroom window.
- Buy and test a battery-powered or battery-backup carbon monoxide alarm, because a plug-in alarm is useless in an outage.
- Get the extension cords you will need, so nobody is tempted to shorten the distance.
- Decide in advance what you will not power. Entertainment is not worth a hospital trip.
- Agree with the household that nobody starts or moves the generator without saying so out loud.
Cars, garages and the engine nobody turned off
Never leave a vehicle running in an attached garage, even with the garage door open. CPSC states this without qualification, and EPA lists attached garages among the routine sources of carbon monoxide indoors.
An attached garage shares walls, a ceiling and usually a door with the house. Pressure differences move air from the garage into the living space as a matter of routine, through that door, through gaps around it, and through the framing. Exhaust does not politely stay on the garage side.
Push-button ignition deserves its own mention, because it changed the way this fails. In an older car the engine stopped because you took the key out and the key went in your pocket. With a keyless system, the engine keeps running while the fob walks into the house with you. Modern engines are quiet, hybrids can be silent at a stop, and a car can sit running in an attached garage for hours with nobody noticing. The habit that fixes it is dull and effective: look at the dash before you close the car door, and make it a household rule that the garage door stays open until the engine is confirmed off.
Warming up a car in a garage on a cold morning is the same hazard with better intentions. Running a snowblower, pressure washer or gas-powered tool in a garage with the door up is the generator rule wearing different clothes.
When the alarm sounds, or someone feels sick
Read this part twice and tell your household.
If a carbon monoxide alarm sounds, or if anyone has symptoms you suspect could be carbon monoxide:
- Get everyone outside into fresh air immediately. Count heads. Take the pets.
- Call 911 from outside, on a cell phone or from a neighbor’s house.
- Do not go back in. Not to open windows, not to look at the furnace, not for a phone or a laptop or the cat you think you saw a second ago. Firefighters have meters and breathing apparatus. You have neither.
- Stay out until the fire department says it is safe to return.
CPSC’s fact sheet is unambiguous about the instinct to investigate, saying never to ignore an alarm and never to go looking for the source yourself. If the alarm sounds again within 24 hours of your return, go back outside, call the fire department again, and have a qualified technician inspect every fuel-burning appliance before anything is used again.
For a suspected poisoning or exposure where nobody is unconscious or struggling to breathe, call Poison help at 1-800-222-1222. The Health Resources and Services Administration runs that national line, which routes you to your regional poison center and is available 24 hours a day in more than 100 languages. Make the call from outside, in fresh air.
Call 911 instead, immediately, if anyone is unconscious, confused, seizing, having chest pain or struggling to breathe, or if you cannot wake someone.
Tell whoever answers, at either number, that you suspect carbon monoxide. Say it in those words. Naming the gas up front points the response toward the right tests.
Three things this article deliberately will not do. It will not help you diagnose anyone, because confirming carbon monoxide poisoning takes a blood test and a normal fingertip oxygen reading does not rule it out. It will not tell you how to treat anyone. And it will not tell you how to fix an appliance. If a system is suspected, have it serviced by a qualified technician.
Who gets hurt, and when
The pattern in the data is seasonal and it is not subtle.
A surveillance study published in Annals of Emergency Medicine in 2023, led by CDC researchers, pulled together poison center calls, emergency department visits, hospitalizations and deaths for unintentional carbon monoxide poisoning from 2005 through 2018. Across every one of those data sources, cases peaked in winter, with January and December standing out. Headache, nausea and dizziness were the symptoms most often reported to poison centers.
Risk is not spread evenly by age. In that study, children under 10 had the highest rates of poison center calls and emergency department visits. Adults over 80 had the highest rates of hospitalization and of death. Deaths occurred more often among men and in the Midwest. Non-fire poisoning rates for hospitalizations and deaths declined over the study period.
CDC separately names the people more likely to become seriously ill at a given exposure: infants, older adults, people who are pregnant, and people with chronic heart disease, anemia or breathing problems. The reason is straightforward. Carbon monoxide starves tissue of oxygen, so anyone whose oxygen delivery already has less margin feels it sooner and worse.
One honest note about the numbers. CDC’s figure of more than 400 deaths a year covers unintentional non-fire poisoning from all sources. CPSC’s figure of more than 200 counts only deaths tied to the consumer products it regulates, which leaves out motor vehicles. Both are correct. They count different things, and a reader comparing headlines will otherwise assume one is wrong.
Three tasks, in order of payoff
Each one is small. They are ordered by how much risk each removes per minute spent.
One. Find every carbon monoxide alarm in your house and turn it over. If there is no date on it, if the date is more than five years old, or if you cannot find an alarm at all, that is this weekend’s errand. You want one outside each sleeping area and one on every level, with battery backup.
Two. Settle the generator plan while the power is on. Mark where it goes, buy the cords, and tell everyone the rule. Twenty feet is easy to measure in daylight and nearly impossible to negotiate on the second night of an outage.
Three. Have your heating system, water heater and any other fuel-burning appliance checked annually by a qualified technician, and have chimneys and vents inspected and cleaned. CDC recommends exactly this, and it is the step that prevents the quiet chronic exposure an alarm eventually catches but should never have to.
Carry the two phone numbers in your head rather than in a drawer. 911 when someone is in danger right now. Poison help, 1-800-222-1222, when you suspect an exposure and nobody is in crisis. Both calls get made from outside, in fresh air, with every person and every pet already out of the house.
Keep exploring: plastic resin codes, clothing chemicals and houseplants air quality.
Sources
- Centers for Disease Control and Prevention. “Carbon Monoxide Poisoning Basics.” cdc.gov
- Centers for Disease Control and Prevention. “Clinical Guidance for Carbon Monoxide Poisoning.” cdc.gov
- U.S. Consumer Product Safety Commission. “Carbon Monoxide Information Center.” cpsc.gov
- U.S. Consumer Product Safety Commission. “Carbon Monoxide Fact Sheet.” cpsc.gov
- U.S. Consumer Product Safety Commission. “Carbon Monoxide Questions and Answers (The Invisible Killer), CPSC Document 464.” cpsc.gov
- U.S. Consumer Product Safety Commission. “CO Alarms.” cpsc.gov
- U.S. Consumer Product Safety Commission. “CPSC Warns of Generator, Carbon Monoxide and Fire Hazards Ahead of Hurricane Season.” cpsc.gov
- U.S. Environmental Protection Agency. “Carbon Monoxide's Impact on Indoor Air Quality.” epa.gov
- U.S. Fire Administration. “Carbon Monoxide Poisoning Prevention.” usfa.fema.gov
- MedlinePlus, U.S. National Library of Medicine. “Carbon Monoxide Poisoning.” medlineplus.gov
- Health Resources and Services Administration. “Poison Help.” poisonhelp.hrsa.gov
- PubMed. “Shin M et al. "Morbidity and Mortality of Unintentional Carbon Monoxide Poisoning: United States 2005 to 2018." Annals of Emergency Medicine, 2023. PubMed. DOI: 10.1016/j.annemergmed.2022.10.011.” doi.org
- PubMed. “Fife CE et al. "Dying to play video games: carbon monoxide poisoning from electrical generators used after hurricane Ike." Pediatrics, 2009. PubMed. DOI: 10.1542/peds.2008-3273.” doi.org