Carbon monoxide enters the body through the lungs, the same way oxygen does. When you breathe in air containing CO, the gas crosses from your lung tissue into your bloodstream in seconds, where it latches onto hemoglobin with roughly 200 times the strength that oxygen does. Because CO is colorless, odorless, and tasteless, you can inhale dangerous amounts without realizing anything is wrong.
From Lungs to Bloodstream
Carbon monoxide follows the exact same route as the oxygen your body needs. When you inhale, air travels down your windpipe and into smaller and smaller airways until it reaches tiny air sacs deep in the lungs. These sacs are wrapped in hair-thin blood vessels called capillaries, and gases pass freely across the membrane between the two. Oxygen normally crosses this membrane into your blood, while carbon dioxide crosses in the other direction to be exhaled. CO exploits this system perfectly because it’s a small, lightweight molecule that diffuses rapidly across the capillary membrane.
Once CO reaches the bloodstream, it binds to hemoglobin, the protein inside red blood cells responsible for carrying oxygen throughout your body. The resulting compound is called carboxyhemoglobin. The problem is that CO clings to hemoglobin about 200 times more tightly than oxygen does, so even small amounts of CO in the air can gradually take over a large share of your hemoglobin. Each hemoglobin molecule occupied by CO is one that can no longer carry oxygen to your brain, heart, muscles, and organs.
CO also does something subtler: it changes the shape of the hemoglobin molecule in a way that makes the remaining oxygen-carrying hemoglobin hold onto its oxygen more tightly, releasing less of it to tissues that need it. So CO doesn’t just block oxygen from loading onto red blood cells. It also prevents the oxygen that is loaded from being delivered effectively.
Why You Can’t Detect It
Unlike smoke, natural gas (which has an additive to make it smell), or chemical fumes, carbon monoxide gives no sensory warning at all. It doesn’t irritate your eyes, nose, or throat. It has no color and no taste. Your body’s normal breathing reflexes, the ones that make you cough around smoke or gag near strong chemicals, simply don’t activate. This is why CO is sometimes called the “silent killer.” You continue breathing normally, pulling more CO into your lungs with every breath, while oxygen levels in your blood steadily drop.
Where the Gas Comes From
Carbon monoxide is produced whenever fuel burns incompletely. In a well-functioning appliance with proper ventilation, the amount of CO generated is small and disperses safely. Problems arise when combustion is incomplete or exhaust can’t escape.
The most common household sources include:
- Gas furnaces and boilers that are worn, poorly maintained, or connected to blocked or leaking flues
- Gas water heaters that back-draft exhaust into living spaces instead of venting it outside
- Unvented kerosene and gas space heaters that release combustion byproducts directly into the room
- Gas stoves and ovens, especially when used for heating rather than cooking
- Fireplaces and wood stoves with leaking chimneys or poor draft
- Portable generators and gasoline-powered tools run indoors or in enclosed spaces like garages
- Car exhaust from vehicles left running in attached garages, which can seep into the home
The EPA notes that improperly sized, blocked, or disconnected flues are a particularly common culprit. Nearby roads and parking areas can also contribute, though outdoor CO typically disperses before reaching dangerous levels. The risk concentrates indoors, where gas can accumulate in enclosed spaces.
Symptoms at Different Blood Levels
The effects of CO poisoning track closely with how much of your hemoglobin has been converted to carboxyhemoglobin. Nonsmokers normally carry less than 1.5% carboxyhemoglobin from the body’s own metabolic processes. As that percentage climbs, symptoms progress in a fairly predictable pattern.
At 2 to 6%, people with existing heart disease may experience worsened chest pain and irregular heartbeats. Healthy adults typically start noticing reduced exercise stamina around 5 to 8%. Between 5 and 20%, cognitive and sensory changes set in: trouble with visual tracking, impaired attention, slower reaction times, and difficulty with tasks like driving. As levels rise from roughly 5 to 60%, the classic poisoning symptoms appear in escalating severity: headache, dizziness, drowsiness, nausea, vomiting, confusion, disorientation, visual disturbances, and eventually convulsions or coma. Above 50%, the risk of death is high.
Early symptoms like headache, nausea, and fatigue overlap heavily with the flu, food poisoning, and general tiredness, which is one reason CO poisoning is frequently misdiagnosed or recognized late. A useful clue: if multiple people in the same building develop these symptoms simultaneously, or if symptoms improve when you leave the building and return when you come back, CO exposure is a strong possibility.
How the Body Clears CO
The good news is that the same binding process works in reverse once you’re breathing clean air. Hemoglobin will eventually release CO and pick up oxygen again, but the timeline depends entirely on what you’re breathing. On regular room air (21% oxygen), the half-life of CO in your blood is about 320 minutes, meaning it takes over five hours to clear just half the CO from your system.
Breathing 100% oxygen through a mask drops that half-life dramatically to about 74 minutes. Under hyperbaric conditions, where a patient breathes pure oxygen in a pressurized chamber at three to five times normal atmospheric pressure, the half-life falls to roughly 20 minutes. The high oxygen pressure essentially floods the hemoglobin with so much competing oxygen that CO gets displaced much faster.
Hyperbaric oxygen treatment is generally reserved for severe cases: patients who have lost consciousness, show neurological symptoms, have heart-related complications, or present with very high carboxyhemoglobin levels (typically around 40% or above with symptoms). For milder exposures, high-flow oxygen through a mask is the standard approach, and most people recover fully over several hours.
Workplace Exposure Limits
OSHA sets the permissible exposure limit for carbon monoxide at 50 parts per million averaged over an eight-hour workday. That threshold applies to general industry and maritime operations alike. To put that number in perspective, a concentration of 600 ppm or higher can produce carboxyhemoglobin levels above 50%, the range associated with high risk of death. Even levels as low as 30 to 50 ppm sustained over hours can push carboxyhemoglobin to 5 to 8%, enough to reduce physical endurance in healthy people.
For homes, the Consumer Product Safety Commission recommends CO detectors on every level and near sleeping areas. Unlike smoke, which you’d wake up to, CO can incapacitate you in your sleep before you ever notice something is wrong. Battery-operated or plug-in detectors with battery backup typically alarm at concentrations well below dangerous thresholds, giving you time to ventilate and evacuate.