At a glance
| Chemical family | Inorganic gas (CO) — a product of incomplete combustion of any carbon fuel. An acute chemical asphyxiant, not a cumulative-exposure toxicant. Distinct from carbon dioxide (CO2), which is the complete-combustion product and far less acutely dangerous. |
| CAS number | 630-08-0 |
| Classification | Not classified as a carcinogen (IARC). It is a criteria air pollutant with an EPA National Ambient Air Quality Standard of 9 ppm (8-hour average) and 35 ppm (1-hour average). The hazard is acute poisoning via carboxyhemoglobin formation, not long-term chemistry. |
| Where you encounter it | Faulty or unvented fuel-burning appliances (furnaces, gas/oil water heaters, gas ovens used for heat), blocked or leaking chimneys and flues, portable kerosene heaters — and, most lethally, generators, running cars in attached garages, and charcoal grills used indoors. |
| Sleep micro environment relevance | Direct and decisive. CO poisoning most often becomes fatal while people are asleep, because the early symptoms (headache, drowsiness, confusion) go unnoticed by someone already unconscious. A CO alarm near the bedroom is the single most important air-safety device in the home. |
| Activated carbon capture | Not applicable — CO is not captured by ordinary activated-carbon air filters, and filtration is not the control. The control is prevention and detection: correctly vented appliances plus a CO alarm. Inferred from CO's chemistry (poorly adsorbed by standard carbon) and the established prevention-and-detection safety model |
Regulatory & certification status
Carbon monoxide is regulated as an air pollutant and, in homes, through building codes that increasingly mandate CO alarms. The rows below are the health-based standards and the detection requirement, not product certifications.
| United States (air) | EPA sets National Ambient Air Quality Standards for CO of 9 ppm over 8 hours and 35 ppm over 1 hour, neither to be exceeded more than once per year. These are outdoor-air standards, but they anchor what "elevated" means indoors. Regulatory — US EPA |
| United States (alarms) | Most US states now require carbon monoxide alarms in residential dwellings by statute or building code, and the CPSC recommends a CO alarm on every level and near sleeping areas. Home CO alarms are certified to the UL 2034 standard. Regulatory — US CPSC |
| World Health Organization | The WHO Guidelines for Indoor Air Quality set short-term CO guideline values (for example 10 mg/m³ over 8 hours) specifically to prevent carboxyhemoglobin from exceeding the level associated with health effects. Regulatory — WHO |
| International | CO is a recognized criteria/priority air pollutant across regulatory systems (EU ambient air quality directives, Health Canada residential guidance). It is universally treated as an acute-exposure hazard managed by combustion control and detection, not by a long-term chemistry limit. Regulatory — ATSDR |
| Certifications | Not a product-certification topic the way foam or textiles are — there is no "CO-free" label because CO is a combustion event, not an ingredient. The relevant certification is on the detector (UL 2034), and on correct installation and venting of fuel-burning appliances. Inferred — CO is generated in use, not present as a material ingredient, so consumer-product content certifications do not apply |
| The 72-hour test window | Not applicable. CO is not a material emission measured in a product chamber test; it is a combustion gas detected in real time by a CO alarm. Inferred — from the exposure mechanism (real-time combustion gas) versus the VOC/material focus of product emissions testing |
What it is
Carbon monoxide is a simple molecule — one carbon, one oxygen — produced whenever a carbon-based fuel burns without enough oxygen to convert fully to carbon dioxide. A perfectly tuned, well-vented appliance produces very little; a starved, dirty, or unvented one produces a lot. The gas is colorless, odorless, tasteless, and slightly lighter than air, so it mixes readily through a home and gives no sensory warning whatsoever. That absence of warning is the whole danger.
The toxicology is fast and mechanical. Carbon monoxide binds to hemoglobin — the oxygen-carrying protein in blood — with an affinity more than 200 times that of oxygen, forming carboxyhemoglobin. Every hemoglobin molecule holding CO is a molecule that cannot carry oxygen, so the blood's oxygen-delivery capacity falls even though the person is breathing normally. The tissues most sensitive to oxygen starvation — the brain and the heart — are affected first. This is why the early symptoms are neurological (headache, dizziness, confusion) and why judgment is impaired exactly when a person most needs to recognize the problem and leave.
It is worth stating plainly what CO is not: it is not a carcinogen, not a hormone disruptor, not a slow accumulator. It does not build up in the body over years. Its half-life in the blood is a matter of hours once the person is in fresh air. That makes it categorically different from almost everything else in this Atlas — and it means the entire risk-management strategy is different too. You don't reduce CO exposure gradually; you prevent the source and you detect the event.
Where you encounter it
From everyday fuel-burning appliances
The routine sources are the fuel-burning appliances a home already relies on: the furnace, the water heater, a gas range, a fireplace or wood stove. When these are correctly installed, maintained, and vented, they release their combustion products safely outdoors. Problems arise when a heat exchanger cracks, a flue or chimney is blocked (by debris, snow, or a bird's nest), a vent is disconnected, or an appliance is simply old and burning inefficiently. An annual inspection of fuel-burning appliances and their venting is the maintenance action that heads this off.
From the lethal shortcuts
The fatal cases cluster around a few specific mistakes, often during power outages or cold snaps: running a portable generator in a garage, basement, or near an open window; warming up or leaving a car running in an attached garage; using a charcoal or gas grill indoors; or using a gas oven or range to heat a room. Each of these can drive CO to fatal concentrations quickly. Generators in particular must run far outdoors, away from doors, windows, and vents — never in any enclosed or partly enclosed space.
The pattern that should make you suspect it
Because the symptoms are non-specific, the tells are situational: several people (and pets) in the home feeling ill at the same time, symptoms that ease when everyone leaves the house and return on coming back, and vague flu-like illness without a fever. If that pattern appears — or if a CO alarm sounds — the response is to get everyone into fresh air immediately and call emergency services, not to investigate first.
What the research says
The scale of the harm
The US Centers for Disease Control and Prevention attributes approximately 400 deaths per year in the United States to unintentional, non-fire-related carbon monoxide poisoning, along with tens of thousands of emergency-department visits and thousands of hospitalizations. Regulatory These are largely preventable events, and the seasonal peak in winter — heating systems, generators during outages, cars in garages — points directly at the interventions.
The mechanism and the standard
The ATSDR Toxicological Profile for Carbon Monoxide details the carboxyhemoglobin mechanism and the dose-response, and the WHO indoor air quality guidelines set short-term exposure limits explicitly to keep carboxyhemoglobin below the threshold for cardiovascular and neurological effects. Regulatory The EPA outdoor standards (9 ppm / 8 hr; 35 ppm / 1 hr) are built on the same endpoint. The science here is old, settled, and unambiguous — which is unusual for this Atlas and worth saying.
The lasting effects in survivors
Beyond the acute event, a body of research documents delayed and persistent neurological effects — problems with memory, concentration, and mood — in some survivors of significant CO poisoning, sometimes emerging days to weeks after apparent recovery. Peer-reviewed This is part of why any suspected poisoning warrants medical evaluation rather than "we feel better now, we're fine."
What helps
Install CO alarms — on every level and near every sleeping area. This is the entire headline. The near-the-bedroom placement matters because poisoning turns fatal during sleep. Test alarms monthly and replace the unit on its stated schedule (the electrochemical sensor wears out, typically in 5–7 years). Interconnected alarms that all sound together are better in a multi-level home.
Have fuel-burning appliances and venting inspected annually. A qualified technician checking the furnace, water heater, chimney, and flues before heating season catches the cracked heat exchangers and blocked vents that cause the routine cases.
Never run a generator, car, or grill in or near the enclosed home. Generators run well outside, away from windows, doors, and vents. Never idle a car in an attached garage, even with the door open. Never use a charcoal or gas grill, or a gas oven, to heat indoor space.
Treat an alarm as real, every time. If a CO alarm sounds, get everyone (including pets) into fresh air and call emergency services. Do not go back in to "check." Do not assume it's a false alarm.
What does NOT help
- Relying on your nose. CO is odorless. "I'll smell it if something's wrong" is the belief that kills people — you will not smell it.
- Air purifiers and carbon filters. Ordinary HEPA and activated-carbon air cleaners do not remove CO and are not a safeguard against it. Only combustion control and a CO alarm are.
- Cracking a window near a strong source. A little ventilation will not reliably protect against an actively producing source like a generator or a running car; the concentration can outpace it. Remove the source; don't try to out-ventilate it.
- A smoke alarm. A smoke detector does not detect CO (unless it is a specifically combined smoke/CO unit). They are different sensors for different hazards.
- An expired alarm. A CO alarm past its replacement date may not sound at all. The sensor degrades; the date on the unit is real.
Open questions
- The long-term health significance of chronic low-level CO exposure (below acute-poisoning thresholds) — for example from minor appliance leakage or traffic infiltration — is less characterized than the acute picture. Speculation re: chronic low-dose effects; the acute toxicology is settled
- Which survivors develop delayed neurological sequelae, and why, is not fully predictable, and optimal treatment (including hyperbaric oxygen) for preventing those effects remains an area of active clinical research. Speculation
- Real-world alarm coverage and maintenance — how many homes have a working, in-date CO alarm near sleeping areas — remains a public-health gap that drives preventable deaths despite the technology being cheap and effective. Inferred from the injury-prevention literature
Where you meet Carbon Monoxide across your home
The same compound turns up in more than one place you live. Here's where it shows up in Embr — each links to the full breakdown for that part of your home.
Citations
- US Environmental Protection Agency. National Ambient Air Quality Standards for Carbon Monoxide — 9 ppm (8-hour), 35 ppm (1-hour). epa.gov Regulatory
- US Centers for Disease Control and Prevention. Carbon Monoxide Poisoning — prevention guidance; ~400 US deaths/year from unintentional non-fire-related CO poisoning. cdc.gov Regulatory
- Agency for Toxic Substances and Disease Registry (2012). Toxicological Profile for Carbon Monoxide. atsdr.cdc.gov Regulatory
- US Consumer Product Safety Commission. Carbon Monoxide Information Center — CO alarms and placement. cpsc.gov Regulatory
- World Health Organization (2010). WHO Guidelines for Indoor Air Quality: Selected Pollutants — carbon monoxide. who.int Regulatory
Frequently asked questions
Can you smell carbon monoxide?
No. Carbon monoxide is completely colorless and odorless — you cannot see it, smell it, or taste it. This is the entire reason it is dangerous: there is no sensory warning, so people are poisoned without knowing it, often while asleep. The "smell of exhaust" people notice is from other combustion compounds, not the CO itself, and a source can produce dangerous CO with little or no odor. The only reliable detection is a carbon monoxide alarm.
Where does carbon monoxide in a home come from?
From incomplete combustion of any fuel — natural gas, propane, oil, wood, coal, gasoline, charcoal. The common household sources are faulty, unvented, or poorly maintained fuel-burning appliances: furnaces, gas or oil water heaters, gas stoves and ovens (especially if used for heating), fireplaces and wood stoves with blocked or leaking flues, and portable kerosene heaters. The most lethal sources are generators run in or near the house, cars left running in an attached garage, and charcoal grills used indoors — these can produce fatal levels within minutes.
What are the symptoms of carbon monoxide poisoning?
Early symptoms mimic the flu without a fever: headache, dizziness, weakness, nausea, shortness of breath, and confusion. Higher exposures cause vomiting, loss of consciousness, and death. A telling pattern is that symptoms improve when you leave the house and return when you come back, and that multiple people (and pets) in the home feel ill at once. Because the early signs are vague and CO impairs judgment, people often don't realize what's happening — which is why it is especially dangerous during sleep.
Do I need a carbon monoxide alarm in the bedroom?
You need a CO alarm on every level of the home and near every sleeping area — that near-the-bedroom placement is the point, because CO poisoning most often turns fatal while people are asleep and cannot notice the early symptoms. Follow the manufacturer's placement instructions, test the alarm monthly, and replace the unit on its stated schedule (typically 5–7 years, as the sensor degrades). A CO alarm is a roughly $20–40 device that prevents the single most common home poisoning death.
Is carbon monoxide a long-term or cancer risk?
Carbon monoxide is not classified as a carcinogen — it is an acute asphyxiant, not a slow chemistry concern like formaldehyde or PFAS. The danger is immediate: CO binds hemoglobin far more tightly than oxygen does, starving tissues of oxygen. That said, survivors of significant CO poisoning can have lasting neurological effects (memory, concentration, mood), and there is research interest in chronic low-level exposure. But the headline is acute: CO belongs in a different mental category from the cumulative-exposure compounds — it is a same-day hazard, and detection is the whole game.
Related compounds
Embr researches the chemistry of where you live — including the acute hazards, not just the slow ones. See the methodology page for how this Atlas tags claims by evidence strength, and the indoor-air source hub for how carbon monoxide fits alongside radon and the other things in your air.
Last reviewed 2026-07-12. This page is general information, not emergency guidance — if a CO alarm sounds or you suspect poisoning, get into fresh air and call emergency services. If you find a factual error, contact us.