CO and CO₂: Two Gases, Two Risks, Two Biological Mechanisms

CO and CO₂ molecules compared at the level of their structure and biological effects — The amount of CO₂ produced by several people sleeping in a closed room is far from negligible. It is enough to confine the air quickly and reach high concentration levels. Above 1,500 to 2,000 ppm: the air is considered poorly ventilated or confined (common in a closed bedroom in the early morning).
Image source: astronoo.com (new window) - AI-generated image, public domain.

Scientific Summary

The Astronoo article compares carbon monoxide (CO) and carbon dioxide (CO₂), two molecules that are close in composition but opposite in their biological effects. It explains that the addition of a single oxygen atom changes the geometry, polarity, and chemical reactivity. CO binds to hemoglobin with extreme affinity and blocks oxygen transport, while CO₂ remains a normal product of metabolism, regulated by respiration and blood pH. The article finally illustrates how tiny structural differences can produce major physiological consequences.

Why is CO deadly at low doses while CO₂ is tolerated by the body?

This Astronoo article answers a simple question: how can two gases made of carbon and oxygen have such different effects? Carbon monoxide (CO) and carbon dioxide (CO₂) differ only by one oxygen atom, but this difference changes everything. CO is a polar molecule that binds to hemoglobin about 230 to 300 times more strongly than oxygen, forming carboxyhemoglobin and causing rapid hypoxia. CO₂, on the other hand, is continuously produced by cells, transported in the blood as bicarbonate, and exhaled by the lungs. It does not block hemoglobin in the same way and only becomes dangerous at high concentrations, generally above 2 to 3% in inspired air. Molecular structure, polarity, and biochemical interactions therefore explain this vital difference. The article also shows that isomeric or similar molecules, such as ethanol and dimethyl ether, can have radically opposite properties.

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Two gases, a single atom of difference

A deceptive chemical proximity

Carbon monoxide (CO) and carbon dioxide (CO₂) are two simple compounds of carbon and oxygen, but their physiological properties differ radically. While CO is a highly toxic gas at very low concentrations, CO₂ is a gas continuously produced by our metabolism, tolerated within certain limits. Yet the only structural difference between these two molecules is the presence of an additional oxygen atom in CO₂.

Comparison of bonds in carbon monoxide and carbon dioxide

Bond lengths and energies

The table below replaces the image by comparing the bond length and bond energy in CO and CO₂. These two quantities help explain why the C≡O bond of carbon monoxide is shorter and stronger than the C=O bonds of carbon dioxide.

Comparison of bond lengths and energies in carbon monoxide and carbon dioxide
Molecule	Formula	Representation	Bond length	Bond energy
Carbon monoxide	CO	C≡O	112.8 pm	~1,076 kJ/mol
Carbon dioxide	CO₂	O=C=O	116.3 pm	~805 kJ/mol

N.B.:
In CO, the C≡O bond is triple; in CO₂, the two C=O bonds are double. Bond length is measured between carbon and oxygen. The energy indicated corresponds to the bond dissociation energy of the C-O bond in each molecule.

Physics of the chemical bond: difference in structure

Geometry and polarity

CO contains only two atoms (carbon and oxygen), so its geometry is linear (a single alignment axis). The atoms are aligned in a straight line: C≡O (two aligned atoms).

CO₂, for its part, is also linear but symmetric and nonpolar, with two double bonds: O=C=O (three atoms in a straight line).

Electronegativity and polarity

The greater the difference in electronegativity, the more the electron cloud is distorted toward the most electronegative atom, generating a polar bond.
The bonds in CO and CO₂ have the same electronegativity difference (0.89), but the molecular geometry makes all the difference. CO is polar (net dipole moment), CO₂ is nonpolar (zero dipole moment by symmetry).

Bond length and strength

The shorter a bond, the stronger it is, that is, the more energy is needed to break it.
In CO, the C≡O bond is very short (~112.8 pm) and has a high bond energy (~1076 kJ/mol), characteristic of a triple bond.
In CO₂, the C=O bonds are longer (~116 pm) and weaker than those in CO, although still relatively stable (energy ~805 kJ/mol).

Consequences for chemical reactivity

This structural difference modifies the chemical reactivity, the interaction with biomolecules, and therefore the physiological toxicity.

Toxicity linked to affinity for hemoglobin

Affinity of CO for hemoglobin

The toxicity of CO comes from its very strong affinity for hemoglobin, about 230 to 300 times greater than that of oxygen. Once bound to hemoglobin to form carboxyhemoglobin (HbCO), CO prevents O₂ from binding and alters its release into tissues, causing severe hypoxia. At 0.1% (1000 ppm) CO in air, a person can lose consciousness within minutes.

By contrast, CO₂ does not bind to hemoglobin in the same way. It is produced in the mitochondria during cellular respiration, transported partly as bicarbonate \( \text{HCO₃⁻} \), and exhaled by the lungs. This gas even plays a key role in regulating blood pH.

CO₂: a physiological gas… at controlled doses

A regulated metabolic waste

Although it is a metabolic waste, carbon dioxide is not intrinsically toxic at low concentrations. Nevertheless, when it exceeds 2 to 3%, it disrupts the acid-base balance by forming carbonic acid \( \text{CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻}\), which leads to respiratory acidosis. Only above 5 to 8% CO₂ in air are serious neurological effects and a life-threatening risk observed.

Why can several people sleep without problem in a closed room?

CO₂ concentration in a closed room

CO₂ becomes toxic to the human body starting at 2 to 3%, with serious and potentially fatal effects above 5% in the air breathed.

CO₂ production by one person

An adult at rest exhales about 200 mL of CO₂ per minute. Over 8 hours of sleep, each person generates about 90 to 100 L of CO₂ per night.

Estimate in a 15 m² room

Take a room of 15 m² with 2.5 m ceiling height, total volume = 37,500 L. The CO₂ exhaled by one person represents: \(\frac{100\ \text{L}}{37\,500\ \text{L}} = 0{,}0027 = 0{,}27\%\) Addition of 0.27% CO₂ in the room over a full night, without ventilation.

Real concentration with several occupants

Outdoor air contains ~0.04% CO₂. In a closed room, with 2 people, the level can rise between 0.2% and 0.5% (i.e., 2000 to 5000 ppm). This level remains below the pathological thresholds (the first symptoms of hypercapnia begin around 2 to 3%).

Why the risk remains limited

Even with several people, CO₂ remains well below the toxic thresholds during a typical night, unless the room is extremely small, completely airtight, and without any air leak (which is rare).

Minimal structural differences, radically different effects: illustrations

The table below gathers four cases where a tiny structural difference leads to opposite effects:

Comparison of four cases of similar molecules with opposite effects
Case	Molecules compared	Molecular formula	Structural difference	Opposite effects or properties
Ethanol and dimethyl ether	Ethanol (C₂H₅OH) and dimethyl ether (CH₃OCH₃)	C₂H₆O	Connectivity of oxygen: bound to a carbon and a hydrogen in ethanol, to two methyl groups in dimethyl ether	Ethanol drinkable and metabolized by the liver; dimethyl ether gaseous, anesthetic and toxic at high doses
Glucose and galactose	Glucose and galactose	C₆H₁₂O₆	Orientation of the -OH group at carbon 4: right in glucose, left in galactose	Glucose source of cellular energy; galactose converted by GALT, its accumulation causes galactosemia
Thalidomide (R and S)	R and S enantiomers of thalidomide	C₁₃H₁₀N₂O₄	Chirality: two enantiomers mirror images of each other	R enantiomer sedative and anti-nausea; S enantiomer teratogenic and causes congenital malformations
Ethambutol (S,S) and (R,R)	(S,S) and (R,R) enantiomers of ethambutol	C₁₀H₂₄N₂O₂	Spatial configuration: two enantiomers of the same chemical formula	(S,S) form antibiotic antitubercular; (R,R) form without antibacterial activity and can cause optic neuritis leading to blindness

When molecular chemistry becomes unpredictable!

Unpredictable biological consequences

An apparently minor modification in the molecular structure, such as the addition or removal of a single atom, can lead to profoundly different, even opposite, biochemical effects.

This shows how much chemical properties emerge from fine molecular structure. And it highlights why predicting chemical and biological interactions is complex: it depends not only on the elements present, but on their spatial arrangement, polarity, symmetry, molecular orbitals…

Homochirality: a choice of nature

Nature has indeed made a radical choice: living things are homochiral. Proteins are made almost exclusively of L-amino acids (left), while sugars like glucose and ribose are of D configuration (right). This asymmetry is not trivial: it probably results from an initial chance, followed by amplification by natural selection. Once a biochemical system has organized around one form, the other becomes unusable, or even toxic.

Enzymes, themselves chiral, recognize only the right form. This is why the S enantiomer of thalidomide is teratogenic while the R form is sedative, or why ethambutol (S,S) cures tuberculosis while the (R,R) form causes blindness. Natural selection has therefore frozen a choice of form, and this choice conditions all biological functioning.

Thus, the chemistry of life is not just a matter of atomic composition: it depends deeply on the geometry and chirality of molecules. What nature has selected is a set of forms compatible with life, and any deviation can have dramatic consequences. Predicting biological effects therefore remains inseparable from the spatial architecture of molecules.

References

FAQ: Everything you need to know about CO and CO₂

What is carbon monoxide (CO)?

Carbon monoxide (CO) is a colorless, odorless, and tasteless gas produced by the incomplete combustion of carbonaceous materials. It is dangerous because it binds to hemoglobin and prevents the transport of oxygen in the blood.

Why is CO more dangerous than CO₂?

CO has an affinity for hemoglobin about 230 to 300 times greater than that of oxygen. It then forms carboxyhemoglobin, which blocks the oxygenation of tissues. CO₂, on the other hand, does not bind in this way and remains a normal product of metabolism.

Can CO₂ become toxic?

Yes. At low concentration, CO₂ is tolerated, but above 2 to 3% in inspired air, it can cause headaches, drowsiness, and respiratory acidosis. Above 5 to 8%, the effects become serious and potentially fatal.

Why can one sleep in a closed room despite the CO₂?

Because the amount of CO₂ produced by one or more people during a night generally remains below toxic thresholds. An ordinary room is not perfectly airtight, and outdoor air contains about 0.04% CO₂. The concentration can rise to 0.2 to 0.5%, but it remains far from the dangerous 2 to 3%.

What is carboxyhemoglobin?

Carboxyhemoglobin (HbCO) is the compound formed when carbon monoxide binds to hemoglobin. This bond is reversible but very stable, and it prevents oxygen from binding, which leads to hypoxia.

Why does molecular structure change the properties of gases?

The geometry, polarity, and symmetry of a molecule determine how it interacts with other substances. CO is polar, while CO₂ is nonpolar by symmetry. This difference explains why their biological effects are so different despite a similar composition.