Is Carbon Dioxide Poisonous?

graphic representing carbon dioxide molecule


You probably know carbon dioxide is a gas that is present in the air you breathe. Plants "breathe" it in order to make glucose. You exhale carbon dioxide gas as a by-product of respiration. Carbon dioxide in the atmosphere is one of the greenhouse gases. You find it added to soda, naturally occurring in beer, and in its solid form as dry ice. Based on what you know, do you think carbon dioxide is poisonous or is it non-toxic or somewhere in between?

You Need Carbon Dioxide to Live

Ordinarily, carbon dioxide is not poisonous. It diffuses from your cells into your bloodstream and from there out via your lungs, yet it is always present throughout your body.

Carbon dioxide serves important physiological functions. As its level rises in the bloodstream, it stimulates the impulse to breathe. If the rate of breathing isn't sufficient to maintain the optimal level of CO2, the respiratory center responds by increasing the rate of breathing. Low oxygen levels, in contrast, do not stimulate increased rate or depth of breathing.

Carbon dioxide is essential for hemoglobin function. Carbon dioxide and oxygen bind at different sites on the hemoglobin molecule, but the binding of CO2 changes hemoglobin conformation. The Haldane Effect occurs when binding of carbon dioxide decreases the amount of oxygen bound for a particular partial pressure of the gas. The Bohr Effect occurs when rising CO2 partial pressure or decreased pH causes hemoglobin to offload oxygen to tissues.

While carbon dioxide is a gas in the lungs, it exists in other forms in the blood. The enzyme carbonic anhydrase converts about 70% to 80% of carbon dioxide into bicarbonate ions, HCO3-. Between 5% and 10% of carbon dioxide is a dissolved gas in plasma. Another 5% to 10% is bound to hemoglobin as carbamino compounds in red blood cells. The exact about of carbon dioxide varies according to whether blood is arterial (oxygenated) or venous (deoxygenated).

Too Much Carbon Dioxide Is Toxic

However, if you breathe high concentrations of carbon dioxide or re-breathe air (such as from a plastic bag or tent), you may be at risk for carbon dioxide intoxication or even carbon dioxide poisoning. Carbon dioxide intoxication and carbon dioxide poisoning are independent of oxygen concentration, so you may have enough oxygen present to support life, yet still suffer from the effects of rising carbon dioxide concentration in your blood and tissues.

The condition of excess carbon dioxide concentration in the blood is called hypercapnia or hypercarbia. Symptoms of carbon dioxide toxicity include high blood pressure, flushed skin, headache and twitching muscles. At higher levels, you could experience panic, irregular heartbeat, hallucinations, vomited and potentially unconsciousness or even death.

There are several potential causes of hypercapnia. It may result from hypoventilation, diminished consciousness, lung disease, rebreathing air, or exposure to an environment high in CO2 (e.g., near a volcano or geothermal vent or under in some workplaces). It can also occur when supplemental oxygen is administered to a person with sleep apnea.

Diagnosis of hypercapnia is made by measuring blood carbon dioxide gas pressure or pH. A blood gas concentration over 45 mmHg carbon dioxide combined with low serum pH indicates hypercarbia.

Fun Facts

  • The average adult human produces about 1 kg (2.3 lbs) of carbon dioxide per day. In other words, a person releases about 290 g (0.63 lbs) of carbon each day.
  • Breathing too quickly depletes carbon dioxide levels, causing hyperventilation. Hyperventilation, in turn, can lead to respiratory alkalosis. In contrast, breathing too shallowly or slowly eventually causes hypoventilation and respiratory acidosis.
  • You can hold your breath longer after hyperventilating than before it. Hyperventilation lowers the carbon dioxide concentration of arterial blood without having a significant impact on blood oxygen levels. The respiratory drive diminishes, so the urge to breathe is reduced. This carries a risk, however, since it's possible to lose consciousness before feeling an overwhelming urge to breathe.


  • Glatte Jr H. A.; Motsay G. J.; Welch B. E. (1967). "Carbon Dioxide Tolerance Studies". Brooks AFB, TX School of Aerospace Medicine Technical Report. SAM-TR-67-77.
  • Lambertsen, C. J. (1971). "Carbon Dioxide Tolerance and Toxicity". Environmental Biomedical Stress Data Center, Institute for Environmental Medicine, University of Pennsylvania Medical Center. IFEM. Philadelphia, PA. Report No. 2-71.
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Helmenstine, Anne Marie, Ph.D. "Is Carbon Dioxide Poisonous?" ThoughtCo, Oct. 29, 2020, Helmenstine, Anne Marie, Ph.D. (2020, October 29). Is Carbon Dioxide Poisonous? Retrieved from Helmenstine, Anne Marie, Ph.D. "Is Carbon Dioxide Poisonous?" ThoughtCo. (accessed August 3, 2021).