The most common symptoms of carbon monoxide poisoning are headache, dizziness, nausea, and weakness. Because carbon monoxide is colorless and odorless, these symptoms often arrive without any obvious explanation, and many people initially mistake them for the flu. The key difference: carbon monoxide poisoning does not cause a fever. If multiple people in the same household develop flu-like symptoms at the same time, especially during heating season, carbon monoxide exposure should be the first suspicion.
How Carbon Monoxide Harms the Body
Carbon monoxide does its damage by hijacking your red blood cells. Hemoglobin, the protein in red blood cells that carries oxygen, has a much higher affinity for carbon monoxide than for oxygen. When you breathe in CO, it locks onto hemoglobin and forms a compound called carboxyhemoglobin, which is more stable than the normal oxygen-hemoglobin bond. That means your blood can no longer deliver oxygen to your organs, even though you’re still breathing.
The heart is especially vulnerable. Carbon monoxide binds to the oxygen-carrying protein in heart muscle at roughly 60 times the rate oxygen does. This can starve the heart of fuel, trigger inflammation, and disrupt the electrical signals that keep your heartbeat regular. Many people with moderate to severe poisoning develop rapid heart rate, irregular rhythms, or chest pain, sometimes without any prior heart problems.
Early and Moderate Symptoms
At lower exposure levels, symptoms tend to be vague and easy to dismiss. The earliest signs include:
- Headache, typically dull and persistent
- Fatigue or general weakness
- Dizziness
- Nausea or vomiting
- Shortness of breath, particularly during physical effort
As exposure continues or concentrations rise, the symptoms become harder to ignore. Blurred vision, confusion, difficulty concentrating, and unusual sleepiness are signs that the brain is losing oxygen. Some people describe feeling “off” or disoriented without being able to pinpoint why. Loss of muscle coordination can make walking unsteady, similar to being intoxicated.
Severe Poisoning
At high concentrations or with prolonged exposure, carbon monoxide poisoning becomes life-threatening. Severe symptoms include altered mental status, inability to walk, seizures, loss of consciousness, and coma. Chest pain and difficulty breathing may signal that the heart muscle itself is being injured. Irregular heart rhythms, including dangerous ones like ventricular fibrillation, can develop because CO lowers the threshold for electrical instability in the heart.
A carboxyhemoglobin level of 30% in the blood indicates severe exposure, though significant poisoning can occur well below that number. Blood levels alone are not a reliable predictor of how sick someone will get. The severity of symptoms, how long the exposure lasted, and the person’s underlying health all matter more than a single lab value.
Chronic Low-Level Exposure
Not all carbon monoxide poisoning is a sudden emergency. Weeks or months of exposure to low levels, from a slow furnace leak or a poorly vented appliance, can produce symptoms that build so gradually you never connect them to a source. Persistent headaches, fatigue, a general feeling of being unwell, trouble sleeping, difficulty remembering things, unexplained vision problems, and numbness are all associated with chronic low-level exposure.
These symptoms are easy to attribute to stress, poor sleep, or aging. One telling pattern: the symptoms improve when you leave the house and return when you come back. If that matches your experience, have your home checked for CO immediately.
Delayed Neurological Symptoms
One of the more concerning aspects of carbon monoxide poisoning is that some people develop new symptoms days or weeks after they seemed to recover fully. This is known as delayed neuropsychiatric sequelae, and it can appear within six weeks of the initial poisoning. The risk is higher in older adults and in anyone who lost consciousness during the exposure.
These delayed symptoms include memory problems, personality changes, difficulty with movement or coordination, speech difficulties, seizures, Parkinson-like symptoms, and urinary incontinence. Some of these resolve over time, but others can be long-lasting. This is why follow-up monitoring after a significant exposure matters, even when someone initially feels fine.
Pregnancy and Carbon Monoxide
Pregnant women face a unique and serious risk. Fetal hemoglobin binds carbon monoxide nearly three times more readily than adult hemoglobin, which means the fetus accumulates higher levels of CO than the mother does. On top of that, the fetus clears carbon monoxide up to five times more slowly.
A mother’s blood levels of carboxyhemoglobin do not accurately reflect what is happening in the fetus. A pregnant woman with mild symptoms may still have a fetus experiencing significant oxygen deprivation. The fetal brain, particularly in late pregnancy, is especially sensitive to CO injury. Outcomes tend to correlate more with the severity of the mother’s symptoms and how far along the pregnancy is than with any single lab measurement. Neurological damage to the fetus can occur even when the mother survives with relatively mild effects.
Why Home Detectors Have a Gap
Standard residential CO detectors certified under UL 2034 standards are designed to alarm at concentrations between 30 and 70 parts per million. They will not alert at levels below 70 ppm, and they won’t sound at 30 ppm even after 30 continuous days of exposure. This means low-level chronic exposure, exactly the kind that causes subtle but real symptoms, can go completely undetected by a standard alarm.
If you suspect low-level exposure based on your symptoms, a standard detector reading “normal” does not rule it out. Low-level CO monitors with digital readouts that display real-time concentrations are available and provide a more complete picture. Your local fire department or gas utility may also be able to test your home with more sensitive equipment.
What to Do If You Suspect Exposure
If you or anyone in your household has symptoms consistent with carbon monoxide poisoning, move everyone (including pets) outside into fresh air immediately. Call emergency services. Do not re-enter the building to look for the source. Once at a hospital, the standard treatment is breathing high-concentration oxygen, which accelerates how quickly your body clears carbon monoxide from the blood. For more serious cases, particularly those involving loss of consciousness, confusion, chest pain, or pregnancy, hyperbaric oxygen therapy (breathing pure oxygen in a pressurized chamber) may be recommended to reduce the risk of lasting neurological damage.
Recovery from mild poisoning is usually quick once the CO source is removed and oxygen is restored. Moderate and severe cases may require monitoring for weeks afterward because of the possibility of delayed neurological symptoms. Infants exposed to CO in the womb are typically followed with brain imaging even if they appear normal at birth, since damage may not be immediately apparent.