Chemistry and Science Behind Carbon Monoxide Poisoning

If CO is just a gas, why is it so dangerous — and why do symptoms look like the flu? The science behind carbon monoxide poisoning comes down to one molecule: hemoglobin. When you breathe CO, it binds to hemoglobin far more strongly than oxygen does, forming carboxyhemoglobin (COHb) and starving your tissues of oxygen from the inside out. Understanding how carbon monoxide poisoning works explains why a pulse oximeter can be misleading during exposure, why CO alarms matter more than intuition, and why fast evacuation saves lives. This page breaks down the chemistry in plain language.

Key Takeaways

• CO binds hemoglobin ~200x more strongly than oxygen, forming carboxyhemoglobin (COHb) and cutting oxygen delivery.
• CO also forces remaining hemoglobin to hold oxygen tighter (left shift), worsening tissue hypoxia.
• Early symptoms mimic the flu because reduced oxygen affects the brain first: headache, dizziness, nausea.
• Standard pulse oximeters can give falsely normal readings during CO exposure — blood testing is needed.
• The practical takeaway: you can't smell or see CO, so alarms + fast evacuation + professional help save lives.

CO in One Minute: Where It Comes From

Carbon monoxide is created when fuels don't burn completely. At home, that can involve heating systems, water heaters, fireplaces, gas stoves, and engines. Risk increases when combustion exhaust can't vent outdoors (blocked vents, backdrafting, poor maintenance), or when engines/generators run in enclosed or semi-enclosed spaces.

Related: Sources of CO poisoning.

Mechanism #1: CO Binds Hemoglobin (COHb)

Hemoglobin is the protein in red blood cells that carries oxygen. CO competes with oxygen for binding sites on hemoglobin and forms carboxyhemoglobin (COHb). CO binds much more strongly than oxygen, so even relatively small amounts can occupy hemoglobin and reduce how much oxygen the blood can carry. You can think of this as a form of "functional anemia": there may be plenty of red blood cells, but their capacity to deliver oxygen is reduced.

This is why the brain and heart are often the first organs to show serious effects — they rely heavily on steady oxygen delivery.

Mechanism #2: Oxygen Is Held Tighter (Left Shift)

CO doesn't only reduce oxygen capacity — it can also change how the remaining hemoglobin behaves. When hemoglobin is partly occupied by CO, the remaining binding sites tend to hold onto oxygen more tightly. That makes it harder for oxygen to be released to tissues, further worsening tissue hypoxia even when oxygen is present in the lungs.

Mechanism #3: Cellular Effects (High-Level)

Beyond hemoglobin, CO can interact with other heme-containing proteins involved in cellular energy processes. At a high level, the result is cellular stress and impaired energy production, which can contribute to neurological effects and delayed complications in some cases. The details are complex — the key point is that CO poisoning is not "just a headache"; it can disrupt oxygen delivery and cellular function.

Why CO Symptoms Often Look Like the Flu

Early CO poisoning symptoms are non-specific: headache, dizziness, nausea, fatigue, and difficulty concentrating. These symptoms overlap with viral illness, dehydration, and other common problems. That's why context is essential.

A strong warning pattern is when symptoms: (1) happen indoors, (2) affect multiple people or pets, and (3) improve in fresh air. If that pattern fits — or if a CO alarm sounds — treat it as a potential emergency.

Symptoms page: Symptoms of carbon monoxide poisoning.

Why Pulse Oximeters Can Be Misleading

A common misconception is that a normal pulse oximeter reading rules out CO exposure. Standard pulse oximeters estimate oxygen saturation based on light absorption patterns, but they are not designed to accurately distinguish oxyhemoglobin from carboxyhemoglobin in all situations.

That's one reason CO alarms and professional evaluation matter when exposure is possible — don't rely on a single consumer measurement to decide whether the environment is safe.

How Clinicians Confirm Exposure (High-Level)

In medical settings, CO exposure is typically evaluated using measurements that can quantify carboxyhemoglobin (COHb) and assess overall clinical status. This usually requires specialized testing (often called CO-oximetry) rather than standard oxygen saturation alone.

Important: If you suspect exposure, the first priority is stopping exposure (fresh air) and seeking appropriate help — not self-diagnosing at home.

Learn more: Carbon monoxide poisoning treatment.

What This Science Means for Real Life

The science points to practical safety rules:

• You can't smell CO — alarms and venting are essential.
• Symptoms are non-specific — treat exposure patterns seriously.
• CO can worsen over time indoors — leave first, investigate second.
• Many incidents involve predictable sources — maintenance + safe generator/garage habits prevent most cases.

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

Common Misconceptions About CO

• "A normal pulse oximeter reading means I'm fine." — Standard pulse oximeters cannot reliably distinguish COHb from oxyhemoglobin. A normal reading does not rule out CO exposure.
• "CO is heavier than air, so it sinks to the floor." — CO has a density very close to air and mixes readily throughout a space. Follow alarm manufacturer guidance for placement.
• "If I can't smell anything, the air is safe." — CO has no odor. The absence of a smell provides zero information about CO levels.
• "Low levels of CO aren't a big deal." — Prolonged low-level exposure can cause persistent symptoms and may be harder to detect. Any suspected exposure warrants investigation.

Glossary

• Carbon monoxide (CO): An odorless, colorless gas produced by incomplete combustion.
• Carboxyhemoglobin (COHb): Hemoglobin bound to CO; reduces oxygen delivery.
• Hemoglobin: Protein in red blood cells that carries oxygen.
• Hypoxia: Reduced oxygen delivery to tissues.
• Incomplete combustion: Fuel burning that produces CO instead of fully converting to CO₂.
• Backdrafting: Combustion gases flowing back into indoor space instead of venting outdoors.
• Venting: Pathway that moves combustion gases outdoors.
• CO alarm: Device designed to detect elevated CO and sound an alarm.
• Pulse oximeter: Device that estimates blood oxygen saturation; may be misleading in CO exposure.
• Exposure: Breathing in CO over time in a given environment.

Sources & References

• ATSDR/CDC — Toxicological Profile for Carbon Monoxide
• CDC — Carbon Monoxide Poisoning
• US EPA — Carbon Monoxide's Impact on Indoor Air Quality
• UKHSA — Carbon Monoxide Toxicological Overview

Published: January 15, 2024