What Is Carbon Monoxide Poisoning?

A furnace that hasn't been serviced, a generator running during a power outage, a car warming up in the garage — these are the kinds of everyday situations where carbon monoxide poisoning begins. CO poisoning happens when you breathe carbon monoxide (CO) from incomplete combustion, and because the gas is invisible and has no smell, exposure can escalate before anyone realizes what's happening. What causes carbon monoxide poisoning in a house is usually a fuel-burning appliance or engine that isn't venting properly. This page explains what carbon monoxide poisoning is, its most common symptoms and causes, and the safest next steps if you suspect a problem.

Key Takeaways

• CO poisoning happens when fuel-burning appliances, engines, or fires produce exhaust that isn't properly vented.
• Early symptoms mimic the flu — headache, dizziness, nausea — making it easy to miss without a CO alarm.
• If multiple people feel sick indoors and improve in fresh air, suspect CO and leave immediately.
• Common causes: furnaces, water heaters, fireplaces, generators, and vehicle exhaust in garages.
• Prevention starts with working CO alarms on every level, annual appliance maintenance, and safe generator use.

What Is Carbon Monoxide Poisoning?

Carbon monoxide poisoning is a form of toxic exposure. When you breathe CO, it interferes with oxygen delivery in the body. The result can range from mild, flu-like symptoms to severe neurological and cardiac effects.

CO is produced when fuels such as natural gas, propane, gasoline, wood, charcoal, kerosene, and oil burn incompletely. In normal conditions, combustion gases should vent outdoors. Poisoning risk increases when exhaust is trapped indoors or enters living areas.

Why Is It Dangerous?

CO is dangerous for two main reasons: you usually can't sense it, and it can affect vital organs quickly. At a high level, CO binds to hemoglobin and reduces oxygen delivery to tissues. The brain and heart are especially sensitive to reduced oxygen. Unlike carbon dioxide (CO₂), carbon monoxide is toxic even in small concentrations, and because it is completely odorless, you cannot rely on your senses to detect it.

This is why early recognition and rapid removal from exposure are so important. If you stay in the environment, symptoms can worsen as CO accumulates.

In simple terms, when you breathe in carbon monoxide, it takes the place of oxygen in your blood. Your red blood cells pick up CO much more readily than oxygen — and once CO is attached, those cells can no longer carry oxygen to your brain, heart, and other organs. The result is like slowly turning down the oxygen supply while the body tries to compensate by working harder. This is why symptoms can start subtly (headache, fatigue) and escalate quickly if exposure continues.

Common Symptoms: Early to Severe

Symptoms vary and are not specific to CO, which is why context matters. Common early symptoms include:

• Headache
• Dizziness or lightheadedness
• Nausea or vomiting
• Weakness / unusual fatigue
• Trouble concentrating
• Shortness of breath with activity

Severe symptoms that require urgent care include:

• Confusion or inability to stay awake
• Fainting or seizures
• Chest pain or severe shortness of breath
• Loss of coordination, collapse
• Severe symptoms in children, pregnancy, older adults, or heart disease

More detail and patterns: Symptoms of carbon monoxide poisoning.

Common Causes and Where It Happens

CO poisoning most often happens in predictable situations where combustion exhaust is not vented properly:

• Home heating systems (furnaces/boilers), water heaters, and other fuel-burning appliances with venting problems.
• Fireplaces and wood stoves with blocked chimneys or poor draft.
• Gas stoves/ovens used with poor ventilation or used incorrectly (never use an oven to heat a room).
• Attached garages — vehicle exhaust can build up and seep into the home.
• Portable generators used in garages, basements, or too close to windows/doors during outages.
• Travel scenarios (rentals, cabins, hotels, RVs, boats) with faulty appliances or inadequate alarms.

In many cases, what causes carbon monoxide poisoning is not a dramatic failure but a gradual one: a furnace heat exchanger develops a small crack over years, a bird nest partially blocks a chimney flue before heating season, or a water heater vent pipe comes loose at a joint and nobody notices because the utility closet is rarely visited. These problems build slowly and may only become dangerous when conditions are right — a cold night when the furnace runs continuously, a house sealed tight against winter air, or negative pressure from exhaust fans pulling combustion gases backward into living spaces.

Full source list: Sources of carbon monoxide poisoning.

Vehicle scenarios: Carbon monoxide in your car. Home scenarios: Carbon monoxide in your home.

How Exposure Typically Happens at Home

Understanding what is carbon monoxide poisoning in a practical sense means looking at the everyday situations where it develops. Here are common real-world scenarios:

• The overnight furnace run: A furnace with a cracked heat exchanger or blocked vent runs all night during a cold snap. CO accumulates slowly while the family sleeps. By morning, everyone wakes with headaches and nausea — if the CO alarm sounds, it may be the only warning.
• The garage warm-up: Someone starts the car in the attached garage to warm it up on a cold morning, planning to come back in "just a few minutes." Exhaust seeps through the door connecting the garage to the house, building CO levels in the kitchen and living room.
• The storm outage generator: Power goes out during an ice storm. A portable generator gets placed in the garage or on a covered porch near an open window. Within minutes, CO levels in the house can become dangerous.
• The blocked chimney: A fireplace or wood stove is lit for the season without checking the flue. A nest, debris, or collapsed liner blocks the chimney, and smoke and CO back-draft into the room instead of venting up and out.
• The forgotten water heater: A gas water heater in a tight utility closet runs for years without inspection. The vent pipe corrodes and disconnects at a joint, releasing CO into the closet and adjacent rooms every time hot water is used.

Each of these scenarios involves a fuel-burning source, a venting problem, and an enclosed space. That combination is at the core of most CO incidents. For a complete list of sources and risk patterns, see Sources of carbon monoxide poisoning.

What to Do If You Suspect Exposure

If you suspect CO exposure or a CO alarm sounds, get everyone (including pets) to fresh air right away and call your fire department or emergency services for guidance. Do not delay — even mild symptoms warrant immediate action because CO levels can rise quickly in enclosed spaces.

Full step-by-step response guide: What to do if you suspect a carbon monoxide leak. Detection methods: How to detect a carbon monoxide leak.

Quick Prevention Steps

Prevention is practical and usually inexpensive compared to the risk. Use working CO alarms on every level and near sleeping areas, maintain fuel-burning appliances, keep venting clear, never run generators indoors or in garages, and never idle vehicles in attached garages.

Full checklist: Prevention of carbon monoxide poisoning.

Myth vs. Fact

Sources & References

• CDC — Carbon Monoxide Poisoning (overview)
• MedlinePlus (NIH) — Carbon Monoxide Poisoning
• US EPA — Carbon Monoxide's Impact on Indoor Air Quality
• NHS — Carbon Monoxide Poisoning
• US CPSC — Carbon Monoxide Information Center

Last updated: February 15, 2026