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What Is Carbon Monoxide Poisoning?

A boiler that has not been serviced in four winters. A generator on the porch during an ice storm, close enough to the window that the noise is annoying. A car left running in the garage while somebody goes back inside for a phone. In each case the fuel is burning the way it always has and the exhaust is going somewhere it should not. Carbon monoxide poisoning is the name for what that exhaust does to the people breathing it, and the reason it has its own name rather than being filed under fumes is that this particular gas gives the body nothing to react to until the damage is already being done.

This is general safety information, not medical advice. If you suspect active exposure or severe symptoms, move to fresh air and contact emergency services.

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In 60 Seconds

  • CO poisoning happens when fuel burns incompletely and the exhaust reaches people instead of the outdoors.
  • Early symptoms read as flu, and the giveaway is the pattern rather than the symptom: several people affected in one building, all improving in fresh air. Symptoms in detail.
  • You cannot smell it, taste it or see it. A working CO alarm is the only sense you have for this gas.
  • A silent alarm is not the same as clean air. Alarms are built to ignore low concentrations for a period by design.
  • Whether your building is legally required to have one is answered by your state and sometimes overridden by your city. The two do not always agree.

What the Term Covers

Carbon monoxide is produced whenever a carbon-based fuel burns without enough oxygen to burn completely: natural gas, propane, gasoline, kerosene, oil, wood, charcoal. Complete combustion sends carbon dioxide up a flue. Incomplete combustion sends carbon monoxide wherever the flue was supposed to be pointing, and if that is a room rather than the sky, the people in the room are the ventilation.

Breathing it displaces oxygen. Red blood cells take up carbon monoxide far more readily than they take up oxygen, and a cell carrying it cannot carry what it is supposed to. The result resembles the oxygen supply to every organ being turned down at once, which is why the brain and the heart, the two tissues least able to tolerate that, are the ones that show it first. It also explains the shape of the illness: it begins as something vague and worsens for as long as you stay where it is happening. Our page on the chemistry takes the mechanism further; carbon monoxide is not carbon dioxide, and the difference is the whole reason one of them is lethal at concentrations the other is harmless at.

Why You Cannot Be Your Own Detector

Most household hazards announce themselves. Smoke is visible, a gas leak is deliberately odorised, a hot surface hurts. Carbon monoxide has no colour, no smell and no taste, and it does not irritate the eyes or the throat on the way in. Nothing about it registers. The first information your body gives you about a carbon monoxide problem is a symptom, and by the time a symptom arrives the exposure has already happened.

That is what makes the alarm the load-bearing part of the whole subject, and it is also why the most dangerous belief about alarms is worth stating plainly: a quiet alarm is evidence about the alarm, not about the air. Residential CO alarms in the United States are listed to UL 2034, and that standard does not ask them to sound at the first molecule. It sets concentrations paired with times, so an alarm is required to stay silent below a level and required to sound within a window above one. The delay is deliberate, because an alarm that fired at every cooking spike would be unplugged within a month. The consequence for a household is that low-level exposure can run for a long time under a device that is working exactly as designed. Our guide to detecting a CO leak sets out those response windows and what testing beyond an alarm involves.

What It Looks Like From Inside the House

Headache, dizziness, nausea, unusual tiredness, trouble concentrating, breathlessness on mild effort. Every one of those belongs to a dozen ordinary illnesses, which is the practical difficulty with recognising this at all. Severe exposure adds confusion, an inability to stay awake, fainting, seizures, chest pain and collapse, and those need emergency care rather than a decision about whether to open a window.

The useful signal lives in the pattern around the symptoms. Several people in one building feeling ill at once, pets included, and everyone improving after time outdoors, is a pattern that infections rarely produce and that a badly vented appliance produces routinely. Symptoms that return each evening when the heating comes on, or each morning after a night in the same room, carry the same information. Our symptoms page takes the progression, the higher-risk groups and the delayed-effect question in full, and the long-term effects page covers what can follow a serious exposure.

Where the Exhaust Comes From

The list is short and it repeats: heating appliances and water heaters whose venting has failed, fireplaces and wood stoves behind a blocked chimney, gas ranges used as room heaters, vehicles running in attached garages, portable generators placed under cover during an outage, and the travel version of all of the above in cabins, rentals, boats and recreational vehicles. The common factor is the combination rather than any one appliance: something burning fuel, a venting path that has stopped working, and an enclosed space for the result to collect in.

Failures of that kind are usually gradual rather than dramatic. A heat exchanger cracks over several seasons, a nest goes into a flue between one heating season and the next, a vent joint corrodes in a closet nobody opens. Our sources page covers each of these with the federal figures on which of them kills people, and there are dedicated guides for the home and for vehicles.

Whether Your Building Has to Be Able to Warn You

Every institutional page on this subject ends in the same place: fit a carbon monoxide alarm. None of them mentions that across most of the United States a statute already says so, with conditions attached, and that those conditions are set in two places at once.

The state answer is the one most people find, and it is usually the right one. The exception is that a city can adopt its own building or fire code, amend the state's, or publish its own guidance about it, and the result does not always match. We have read the municipal layer as well as the state layer: 93 city and county codes across all 51 US jurisdictions, with 42 of those records carrying at least one municipality written up and 9 recording that a search found nothing local to add. Those write-ups include clean confirmations as well as differences, which is worth saying because "a city was examined" and "a city changed the rule" are not the same claim and folding them together would overstate what the reading found.

What the reading did establish is a specific trap, and San Francisco is the clearest instance of it. The city's fire department publishes guidance stating that San Francisco requires carbon monoxide alarms in all dwelling units. No section of the fire code as adopted in San Francisco says that, and department guidance is not an ordinance. The reason is sharper than a wording difference: San Francisco never wrote a carbon monoxide section at all. Its fire code is an amendment document, printing only what the city changes from the California Fire Code, and in the current edition on American Legal, Chapter 9 runs 901, 902, 903, 904, 905, 907, 909, 912, 913, 914 and stops before 915, while Section 1103 carries subsections .1, .4, .5 and .7 and no .9. The phrase carbon monoxide returns nothing in either chapter. So the sections that answer a San Francisco building are the state ones, unamended, which is why they read the same as Los Angeles. The operative requirement in San Francisco is California's own statute and its fuel-burning appliance, fireplace and attached-garage triggers, which is a narrower rule than the guidance describes. Fitting an alarm on the strength of the city page harms nobody. Telling a landlord they are legally obliged to, on the same strength, is a claim the code does not support.

The transferable lesson is the one worth carrying away from a definitional page: a fire department web page describes what a city would like you to do, and an adopted code section describes what it can require. Where they differ, the code governs. Our state-by-state law records give the statute and section for each jurisdiction and record the municipal reading underneath it, and the rental hub covers the landlord and tenant split where the appliance belongs to somebody else.

If You Think It Is Happening Now

Get everyone out, including animals, into open air, and call the fire department or emergency services from outside. Do not open windows first and do not go back in to look for the source. Ventilating a room lowers the reading without touching the cause and it destroys the evidence the responding crew needs to find it. Mild symptoms are still a reason to leave; concentrations in an enclosed space can climb faster than the decision to go.

Afterwards, the appliance has to be found and fixed before anyone sleeps in the building again. Prevention is the same short list every time and it is cheap against the alternative: working alarms on every level and near sleeping areas, an annual service on anything that burns fuel, chimneys and vents kept clear, generators outdoors and well away from openings, and no vehicle left idling in an attached garage. Our emergency guide has the full sequence, the prevention page the full checklist, the treatment overview what happens after medical help arrives, and our safety checks will walk you through your own building a room at a time.

Sources & References

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Last updated: August 20, 2026

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