Carbon monoxide poisoning happens when you breathe in enough of this odorless, colorless gas for it to displace oxygen in your bloodstream. Carbon monoxide binds to hemoglobin, the protein in red blood cells that carries oxygen, roughly 220 times more tightly than oxygen does. That extreme binding strength means even small amounts of the gas can quickly crowd out oxygen and starve your organs, especially your brain and heart.
What Happens Inside Your Body
When you inhale carbon monoxide, it passes through your lungs and latches onto hemoglobin, forming a compound called carboxyhemoglobin. Because the bond is so much stronger than the one oxygen forms, carbon monoxide doesn’t easily let go. It occupies the same binding sites oxygen needs, so your blood carries less and less oxygen with each breath. But the damage goes further than that: once carbon monoxide is attached, it also changes the shape of the hemoglobin molecule in a way that makes the remaining oxygen cling tighter. Your blood can still carry some oxygen, but it has a harder time releasing it to your tissues where it’s actually needed.
The effects don’t stop in the bloodstream. Carbon monoxide also enters your cells directly and interferes with the machinery that converts oxygen into energy. Specifically, it inhibits the final step of the energy-production chain inside mitochondria, the small power plants in every cell. When that step is blocked, cells can’t produce energy efficiently even if some oxygen does arrive. This is why carbon monoxide poisoning can cause severe damage at concentrations that might seem low on paper. Your cells are being hit from two directions: less oxygen delivered and less ability to use whatever oxygen gets through.
Where the Gas Comes From
Carbon monoxide is a byproduct of incomplete combustion, which means anything that burns fuel can produce it. The most common household sources are gas furnaces, water heaters, stoves, fireplaces, and dryers. The gas builds up when these appliances are improperly installed, poorly maintained, or inadequately ventilated. A cracked heat exchanger in a furnace, a blocked chimney flue, or a clogged dryer vent can all send carbon monoxide into living spaces instead of outside.
Some scenarios are particularly dangerous. Running a gasoline-powered generator or grill indoors, even in a garage with the door open, can produce lethal concentrations in minutes. After winter storms, snow can block exhaust vents on the outside of a house, causing carbon monoxide to back up inside. Even something as simple as lining the bottom of a gas oven with aluminum foil can restrict airflow enough to increase carbon monoxide production. Cars left running in attached garages are another well-known source. In 2021, the U.S. Consumer Product Safety Commission recorded an estimated 204 unintentional, non-fire carbon monoxide deaths linked to consumer products.
How Concentration and Time Determine Severity
The severity of carbon monoxide poisoning depends on two factors: how much gas is in the air (measured in parts per million, or ppm) and how long you’re breathing it. Low concentrations over many hours can be just as dangerous as high concentrations over minutes. OSHA data lays out the progression clearly:
- 35 ppm for 6 to 8 hours causes headache and dizziness.
- 100 ppm for 2 to 3 hours produces a slight headache.
- 200 ppm for 2 to 3 hours brings a more noticeable headache, impaired judgment, and vision problems.
- 400 ppm for 1 to 2 hours causes frontal headache and nausea, becoming life-threatening after 3 hours.
- 800 ppm causes dizziness, nausea, and convulsions within 45 minutes, with collapse possible within 2 hours.
- 1,600 ppm leads to confusion and staggering within 20 minutes, with death possible in under 2 hours.
- 3,200 ppm causes unconsciousness in 10 to 15 minutes and death within 30 minutes.
- 12,800 ppm causes unconsciousness after 2 to 3 breaths and death in under 3 minutes.
For context, OSHA’s maximum permissible workplace exposure is 50 ppm over an 8-hour period. A well-functioning gas appliance in a ventilated room produces concentrations far below that. Problems start when ventilation fails or a device malfunctions.
Why the Symptoms Are Easy to Miss
Early carbon monoxide poisoning feels a lot like the flu. Headache, nausea, dizziness, and fatigue are the first symptoms, and they come on gradually at lower concentrations. Because the gas has no smell, color, or taste, there’s no sensory warning that anything is wrong. People often assume they’re getting sick, especially in winter when both flu season and furnace use overlap.
One telling pattern is that symptoms improve when you leave the house and return when you come back. If multiple people or pets in a household feel ill at the same time, that’s another red flag. At higher concentrations, confusion and impaired judgment set in, which makes it harder to recognize the danger or take action to escape. This is part of what makes carbon monoxide so lethal: by the time the poisoning is severe, your ability to help yourself is compromised.
Damage That Shows Up Later
Even after someone survives an acute episode, carbon monoxide poisoning can cause neurological problems that appear days or weeks later. This is sometimes called delayed encephalopathy. Symptoms include memory loss, difficulty concentrating, personality changes, movement problems, and depression. The brain is especially vulnerable because it consumes a disproportionate amount of oxygen relative to its size, so it suffers the most when oxygen delivery and cellular energy production are disrupted simultaneously.
Severe poisoning is associated with carboxyhemoglobin levels above 20 to 25 percent, along with signs of serious oxygen deprivation to the brain or heart. But even moderate poisoning can leave lingering cognitive effects. The risk of delayed neurological symptoms is one reason carbon monoxide exposure is treated aggressively even when a person initially appears to recover.
How Poisoning Is Treated
The first step is always getting away from the source of carbon monoxide. Treatment then centers on breathing high-concentration oxygen, typically through a tight-fitting mask. Since carbon monoxide’s bond with hemoglobin is strong but not permanent, flooding the lungs with pure oxygen speeds up the process of displacing it. On room air, it takes several hours for your body to clear half the carbon monoxide from your blood. With high-flow oxygen, that time drops significantly.
In severe cases, hyperbaric oxygen therapy may be used. This involves breathing pure oxygen inside a pressurized chamber, which forces oxygen into the blood at much higher levels than a mask can achieve. It’s typically offered within 24 hours of exposure for patients with severe symptoms like loss of consciousness, significant heart involvement, or very high carboxyhemoglobin levels. The evidence on whether it reduces long-term neurological damage compared to standard oxygen therapy is still debated, and it’s not universally available.
How Carbon Monoxide Detectors Work
Residential carbon monoxide alarms are designed to alert you before concentrations reach dangerous levels, but they aren’t instant. Under the UL 2034 standard used in the U.S., alarms must sound within specific time windows depending on concentration: between 60 and 240 minutes at 70 ppm, between 10 and 50 minutes at 150 ppm, and between 4 and 15 minutes at 400 ppm. These response times are intentionally calibrated to avoid false alarms from brief, harmless spikes while still catching sustained dangerous levels.
This design means a detector won’t catch a very short burst of low-level carbon monoxide. It also means that at extremely high concentrations, like those from a generator running indoors, you could be incapacitated before the alarm activates. Detectors should be installed on every level of a home and near sleeping areas. They’re a critical safety layer, but they work best alongside proper appliance maintenance and ventilation, not as a substitute for it.