Symptoms of Carbon Monoxide Poisoning

You wake up with a pounding carbon monoxide headache and nausea — and so does everyone else in the house. It feels like the flu, but the timing is suspicious. That pattern is one of the most common ways carbon monoxide (CO) poisoning is first noticed. The signs of carbon monoxide poisoning range from mild dizziness and fatigue to confusion, fainting, and seizures — and exposure while sleeping can be especially dangerous. This page explains the full range of CO poisoning symptoms, the red flags that demand emergency action, and why context matters more than any single symptom.

In 60 Seconds

• Early CO symptoms often mimic the flu: headache, dizziness, nausea, weakness, and fatigue.
• The key clue is context — multiple people (or pets) feel sick indoors and improve in fresh air.
• Confusion, fainting, seizures, chest pain, or trouble breathing are emergency warning signs.
• Nighttime exposure is especially dangerous; waking with a severe headache and nausea is a red flag.
• After an incident, watch for delayed symptoms days later and seek evaluation if new problems appear.

Common Early Symptoms

• Headache
• Dizziness or lightheadedness
• Nausea or vomiting
• Weakness / unusual fatigue
• Shortness of breath with exertion
• Blurred vision or trouble concentrating

A carbon monoxide headache is often described as a dull, persistent pressure — different from a typical tension headache in that it tends to affect multiple people in the same space at the same time. Dizziness may feel like lightheadedness or a sense that the room is unsteady. Nausea can range from mild queasiness to vomiting. The fatigue people report is often described as unusual — heavier than normal tiredness, sometimes making it hard to get off the couch or stay focused on simple tasks. Shortness of breath may only appear during physical activity at first, and blurred vision or difficulty concentrating can make it hard to recognize that something is wrong. The key pattern among these signs of carbon monoxide poisoning is that they tend to appear together, affect multiple people, and improve in fresh air.

Severe Red Flags — Seek Emergency Care

• Confusion, agitation, or inability to stay awake
• Fainting (syncope) or seizures
• Chest pain, palpitations, severe shortness of breath
• Severe weakness, loss of coordination, or sudden collapse
• Symptoms in a child, pregnant person, or someone with heart disease

The "Flu-Like" Pattern — The Key Clue

Because symptoms can mimic flu or food poisoning, the context matters. CO should be suspected when:

• More than one person in the same space develops similar symptoms.
• Symptoms improve when you go outside or spend time in fresh air.
• Symptoms are worse when heaters, hot water, fireplaces, or generators are running.
• Pets appear unusually lethargic or unwell at the same time as people.

If this pattern fits — or if a CO alarm sounds — treat it as a potential emergency: leave to fresh air and call for help.

Symptoms by Severity

CO affects oxygen delivery. Mild to moderate exposure may cause headache, nausea, dizziness, and difficulty concentrating. As exposure increases or continues, symptoms can progress to confusion, loss of coordination, fainting, and seizures.

CO can also stress the heart. Chest discomfort, shortness of breath, fainting, or unusual palpitations during or after a suspected exposure should be treated urgently — especially in older adults or people with existing heart disease.

Remember: symptoms are not specific to CO. The safest approach is to respond to the exposure risk first (fresh air and professional help), then seek medical evaluation when appropriate.

Low-Level and Chronic Exposure

Not all CO exposure is sudden or dramatic. Low level carbon monoxide poisoning symptoms can develop over weeks or months when a fuel-burning appliance has a slow, persistent leak. Because the symptoms are vague and come and go, people often don't connect them to CO at all.

What people describe in chronic low-level exposure situations often sounds like this: a headache that appears every evening and is gone by morning, fatigue that lifts on weekends or during vacations away from home, a "flu" that seems to return every winter when the heating is running, or brain fog and difficulty concentrating that has no other explanation. Some households go through an entire heating season attributing these symptoms to stress, poor sleep, or seasonal illness — never suspecting the furnace or water heater.

Typical scenarios include a furnace with a small crack in the heat exchanger that only leaks when the unit runs for extended periods, a water heater with a partially disconnected vent in a tight closet, or a gas fireplace with a slow draft problem that worsens when the house is sealed up in cold weather. In each case, CO levels may be too low to trigger an alarm immediately but high enough to cause ongoing symptoms.

The difficulty with low level carbon monoxide poisoning symptoms is that no single symptom points clearly to CO. The clearest clues are the patterns: symptoms that track with time spent in the building, that improve away from home, and that affect multiple household members or pets. If you notice this kind of pattern, have your home tested by a professional with calibrated equipment — and consider installing or upgrading CO alarms. See our guide on how to detect a carbon monoxide leak.

Children, Pregnancy, and Higher-Risk Groups

Some people have less reserve and can become seriously ill faster. Seek urgent evaluation for suspected exposure in:

• Infants and children (they may not describe symptoms clearly).
• Pregnant people (reduced oxygen delivery can affect the fetus).
• Older adults and people with heart disease or lung disease.
• Anyone who lost consciousness or had neurological symptoms during the incident.

In children, signs of carbon monoxide poisoning can be especially hard to spot because young children may not be able to describe what they're feeling. Parents and caregivers may notice unusual fussiness or irritability, refusal to eat, unexplained lethargy or sleepiness during the day, or a child who seems "off" in a way that's hard to pinpoint. If a child is unusually cranky or listless and other family members also feel unwell, consider CO as a possible cause — especially if there are fuel-burning appliances in the home or the symptoms improve outdoors.

Nighttime and Sleeping Exposure

Many CO incidents happen at night when people are asleep and may not notice symptoms progressing. Waking with a severe headache, nausea, confusion, or unusual fatigue — especially if others in the home feel similar — should prompt caution. Working CO alarms near sleeping areas are a critical layer of protection. If your alarm wakes you with beeping or chirping, never ignore it — follow the alarm response steps first.

What NOT to Do

When CO symptoms appear, some common reactions can actually make the situation worse. Here are mistakes to avoid:

• Don't ignore recurring symptoms. If headaches, nausea, or fatigue keep coming back — especially in a pattern tied to being at home or to heating season — don't dismiss them as "just stress" or "just the flu." Persistent, patterned symptoms deserve investigation.
• Don't go back inside until cleared. After evacuating for a CO alarm or suspected exposure, do not re-enter the building until a professional has tested the air and confirmed it's safe. CO levels can remain elevated or rise again if the source is still active.
• Don't disable or ignore a CO alarm. If your alarm sounds repeatedly, the correct response is to evacuate and call for help — not to remove the batteries or unplug the unit. Repeated alarms may indicate a real but intermittent problem.
• Don't assume "it's just the flu" when there's a pattern. If multiple people in the household get "flu-like" symptoms at the same time, and symptoms improve when they leave the building, CO should be suspected even if nobody smells anything unusual.
• Don't rely on opening windows as a solution. Ventilation may temporarily lower CO levels, but it does not fix the source. Leaving and getting professional help is the safe response.

Delayed Symptoms After Exposure

Some people develop new or worsening symptoms days to weeks after an exposure event (for example, memory problems, mood changes, or balance issues). If symptoms return after an initial improvement, seek medical evaluation.

Learn more: Long-term effects and delayed symptoms of carbon monoxide poisoning.

What to Do If Symptoms Suggest CO

If these symptoms point to possible CO exposure, leave the building with everyone (including pets) and get into fresh air before doing anything else. Call emergency services for air testing and guidance — and seek urgent medical care if anyone shows severe signs such as confusion, chest pain, or loss of consciousness.

Full emergency checklist: What to do if you suspect a carbon monoxide leak.

Symptom Severity at a Glance

Sources & References

• CDC — Carbon Monoxide Poisoning
• MedlinePlus (NIH) — Carbon Monoxide Poisoning
• NHS — Carbon Monoxide Poisoning
• ATSDR/CDC — Toxicological Profile for Carbon Monoxide
• UKHSA — Carbon Monoxide Toxicological Overview

Last updated: February 15, 2026