Why does CO₂ have a profound impact on the human body?

Daniel Lux
20 min readMar 24



There are two mechanisms which explain why we react to atmospheric CO2 levels.

1) Henry’s law states that CO2 is more than 22 times more soluble in water than Oxygen at body temperature. This is why our body has red blood cells, which are highly efficient at absorbing Oxygen
2) The red blood cells need a mechanism to release Oxygen where required. The Bohr effect states that the pH value around a red blood cell regulates the affinity towards Oxygen of the cell. When a muscle works hard, more CO2 is produced, thus lowering the pH and thus releasing Oxygen.

These two mechanisms work very well in a standard atmosphere. When, however, the CO2 level of the air surrounding our bodies increases, this decreases the efficiency of the oxygen transport in our bodies.

Our brains use 25% to 30% of the energy. When we decrease our bodies’ efficiency, we feel the effect very strongly in our brains. Symptoms are difficulty concentrating, fatigue, headaches and more.

This chapter takes a deep dive into our breathing mechanics.

What happens when we breathe

About 300 km northwest of Cameroon’s capital Yaoundé lies a body of water, and it bears the name Lake Nyos. The lake’s surroundings are lush and mountainous and are home to several small villages such as Cha, Nyos, Munji, Djingbe, and Subum. On certain days, blowing in a gentle wind, are calm fluffy clouds. They drift slowly above the landscape, covering the top of the green mountains in white, while on other days, an azure sky meets its blue image in the lake’s mirror.

Lake Nyos sits on top of a volcano, and the lake fills its circular mar or crater. The volcano is extinct, so it poses no threat of eruption to those living at its feet. Looking at the quiet agricultural landscape of grazing livestock and red dirt roads, one would not expect that a very different body exists below the lush, fertile soil and the blue water. Yet some 80 kilometers (50 miles) below Lake Nyos resides a pool of magma, slowly at work releasing carbon dioxide and other gases — as magma, does. However, the gases here in this specific geographic location travel upward through the Earth, and their fumes dissolve in the natural springs encircling Lake Nyos. As they rise to the surface, they leak into the lake, saturating its water with CO₂. It’s a slow process, invisible to the naked eye of those living as part of the land, in a quiet Northwest corner of the country. But on the night of August 21st, 1986, this calm blue lake literally turned on them, and the rare natural phenomenon of limnic eruption — or lake overturn — caused the death of 1,746 people and 3,500 livestock.

It is still unknown precisely what triggered that fateful event in 1986; most geologists suggest a landslide; some suspect a small volcanic eruption; others suggest the cause to be a fall of cool rainwater; while some consider a small earthquake to have triggered the overturn. Whatever the reason, the event caused a rapid mixing of the CO₂-saturated deep water with the upper layers of the lake. The result was a reduced pressure, triggering an eruption that suddenly released 100,000–300,000 tons of CO₂ from the lake.

Lake Nyos’ seemingly eternal blue turned deep red from risen iron-rich water, now oxidized by the air; the lake’s level dropped by about a meter, and the eruption knocked down trees in close vicinity to the lake. But the true unfathomable scale of the event unfolded as the released CO₂ — since the density of carbon dioxide is about 1.5 heavier than air — settled on the ground. The green landscape, usually only graced by white cumulus formations on its mountaintops, was now enveloped by an invisible cloud of highly concentrated carbon dioxide that moved along the ground, down the valleys, and into the villages where it suffocated the local people as they slept.

Joseph Nkwain, from the village of Subum, miraculously lived through that night. But his words carry in them the disorienting and emotional trauma the event caused and also bore witness to the impact that CO₂ can have on both mind and body. Here is what Joseph Nkwain recounted to researcher Arnold H. Taylor from the College of Plymouth:

“I could not speak. I became unconscious. I could not open my mouth because then I smelled something terrible … I heard my daughter snoring in a terrible way, very abnormal … When crossing to my daughter’s bed … I collapsed and fell. I was there till nine o’clock in the (Friday) morning … until a friend of mine came and knocked at my door … I was surprised to see that my trousers were red, had some stains like honey. I saw some … starchy mess on my body. My arms had some wounds … I didn’t really know how I got these wounds … I opened the door … I wanted to speak, my breath would not come out … My daughter was already dead … I went into my daughter’s bed, thinking that she was still sleeping. I slept till it was 4.30 in the afternoon … on Friday. (Then) I managed to go over to my neighbors’ houses. They were all dead ….”

Even in an environment with sufficient Oxygen, high levels of CO₂ will kill us. It is not the lack of Oxygen that kills us, but the excess of CO₂. Luckily events such as what happened to Joseph Nkwain and his village are rare.

The rule of threes and breathing

Survival became Joseph Nkwain’s fate — with all that survival from such a tragic event brings with it. His tale leads us to the very notion of human survival and the Rule of Threes, the priority list that prepares people for emergencies. The Rule of Threes has been developed to aid decision-making when injured or in danger posed by the environment.

For the uninitiated, the Rules of Threes state that:

  1. You can survive 3 minutes without breathable air
  2. You can survive 3 hours in a harsh environment (extreme heat or cold).
  3. You can survive 3 days without drinkable water.
  4. You can survive 3 weeks without food.

For each line, the line(s) before need to be valid; for instance, you cannot survive in a harsh environment for three hours (Line Two) if you do not have access to breathable air (Line One), and so forth.

Why do we breathe?

But why do we have to breathe? To narrow in on the answer, let’s firstly ask ourselves the question of line three in the Rule of Threes; why do we drink?

The simplistic answer is that we drink because we use water to keep the body cool — we evaporate water by sweating and thus prevent the body from overheating. We also use water to process food: the body needs to flush food through the system, and so we drink to replenish the body with water.

Similarly, we can consider line four in the Rule of Threes and ask; why we eat?

Again, the simplistic answer is that the body is a machine that needs fuel, so we eat to supply the body with energy.

Coming back to line one; Breath is the first condition of survival in our rule of threes. Without the fulfillment of this line, survival is impossible. But the composition of the air we breathe also is of great importance. If we breathe air with more than 10% (100.000 ppm) of CO₂, we risk losing consciousness and death within minutes, even when there is enough Oxygen. At “only” 4% (40.000 ppm), we get dizzy, have trouble breathing, and risk our lives if we are exposed too long to such conditions.

To understand what happened to Joseph Nkwain, let’s look at an imaginary dialog. I have had similar dialogues with numerous people in real life and have condensed the essence of these discussions here for the book.

Me: “Why do we breathe?”

Other: “Because we need air.”

Me: “What do we need the air for?”

Other: “The air needs to be circulated in our blood.”

Me: “Why do we need air to be circulated in our blood?”

Other: “Because our cells need the air.”

Me: “What do our cells do with the air?”

Other: “The mitochondria in the cells use it to generate energy.”

Me: “How do the mitochondria generate energy?”

Other: “By burning carbohydrates, also called sugar, and for doing this, oxygen is needed.”

Me: “What happens if the cells don’t get oxygen?”

Other: “The cell will eventually die.”

To understand our body and its relationship to Oxygen, let’s zoom in on the chemical workings taking place within us when we breathe. Firstly, think of the body as a machine running on fuel. What does the body need to be able to burn this fuel? If you’re still grappling with the answer, think of another analogy, a burning fire. What happens if we close off all ventilation to a fireplace? A fire starved of Oxygen will die. Similarly, the body needs Oxygen to burn the fuel that keeps it running.

Unlike plants that gain energy from sunlight and convert this into carbohydrates — or sugar — to live, we humans gain our power by consuming these carbohydrates from green plants or animals that have eaten green plants. So, we breathe because we need to burn fuel: every single cell in the body needs Oxygen to burn the sugars from the food we consume. The burnt sugars generate energy and chemicals that are necessary to keep our bodies going. Simplified, we breathe because the burning of the fuel that keeps our bodies running requires Oxygen.

The act of breathing is central to our being, to our being alive. We intuitively, implicitly know this; think only of the first question that goes through our mind when a child is born, “Is the child breathing?”. Similarly, at the other station in life, the moment of death is defined by the same question, “Is the patient breathing?”.

Let’s begin by considering how humans differ from plants in our relationship with Oxygen and CO₂. Plants obtain energy from the sun, releasing Oxygen as a waste product — the process of photosynthesis. Humans differ in that they use that very Oxygen to fuel our metabolism — and as a waste product, we release CO₂.

Our bodies use energy previously stored by plants. Even when we eat meat, ultimately, the power we use comes from plants when looking at the food chain. Now, what does that mean exactly?

We digest carbohydrates and proteins in our gut into small molecules that pass into the blood when we eat. The blood transports the sugar molecules to the cells. Here the mitochondria — the double membrane bounds organelles found inside cells — break up the sugar using Oxygen, resulting in a release of energy.

Why is this important, and what does it say about our relationship to Oxygen?

Every single cell in our body needs energy. Our muscles cannot move without it. Our brain cells use electricity to function. These very cells require Oxygen to be able to carry out the process that releases this vital energy. So, the body relies on a supply of Oxygen to burn the sugar that creates energy. This is also why we breathe.

Breathing mechanics

Next, let’s examine the process that happens along the way to supply each cell with Oxygen. Our breathing, scientifically speaking, is governed by physical chemistry laws.

The first step in our breathing cycle is the active part of exchanging the air in our lungs with the atmosphere surrounding our bodies. This first step is the only step we can control and that our mind consciously registers.

Inhalation and exhalation: The lungs, chest wall, and diaphragm are all involved in respiration, both (a) inhalation and (b) expiration. (1)

Boyle’s law governs our breathing process in this step. Boyle’s law is a physics law, first published in 1662 and it is still used today in the teaching of physics and medicine. The law accounts for how pressure and volume are always inversely proportional given a specific gas temperature. If, for example, you decrease the volume of a container, the pressure of the gas within the container will increase. We experience this in our everyday life when we inflate or deflate a balloon. When we pump the balloon up, it increases in size because there needs to be an equilibrium in pressure with the surrounding air.

When we deflate a balloon, we know that the size of the balloon will decrease. The rubber is pressing on the air inside, thus increasing the pressure. Since there is a connection between the inside of the balloon and the outside, the air will go out. If we squeeze the balloon, we decrease the volume further, thus increasing the pressure, and the air will escape faster.

The same principle is at work when we take a breath. Let’s for a moment engage with breathing on a conscious level: Take a deep breath and observe your body as it goes through this very process.

With the intake of breath, you have felt two muscle groups at work — the diaphragm and the external intercostal muscles. The diaphragm contracts and moves inferiorly toward the abdominal cavity, creating a larger thoracic cavity — this is the chest chamber, protected by the rib cage. This movement makes more space for the lungs as you inhale. Contraction of the external intercostal muscles moves the ribs upward and outward, causing the rib cage to expand, increasing the thoracic cavity volume.

Now, take another deep breath and pay close attention to the difference between inhalation and exhalation. What you will notice is that inhalation requires the workings of muscles and the use of energy. On the other hand, exhalation is passive, meaning that no effort is needed to push air out of the lungs.

This act of breathing is also called pulmonary ventilation. Simply put, it consists of two steps: inspiration and expiration — or more commonly known as inhalation and exhalation. Inhalation is the process that causes air to enter the lungs, and exhalation is the process that causes air to leave the lungs. The two processes are collectively known as the respiratory cycle. A newborn child goes through 30–60 respiratory cycles per minute. In other words, a newborn inhales and exhales about every second while, in comparison, an adult will go through 12–18 respiratory cycles per minute.

A male adult’s total lung capacity is approximately 6 liters, and a female’s lung capacity is about 4.2 liters. When we breathe normally, approximately 10% of the air in our lungs gets exchanged, approximately 0.5 liters. The air we exhale contains CO₂, a waste product from the burning process that happens every second of our lives in our cells. The percentage of CO₂ in our lungs and, therefore, in the air we exhale is about 4% or 40.000 ppm. The percentage of CO₂ in the atmosphere and, thus, in the air we inhale is currently at 0,04% or 400 ppm. If we do a bit of math, we can calculate how much the CO₂ content in the lungs decreases with each breathing cycle.

9/10th of the air in our lungs contains 4% of CO₂. When we then add 1/10th of air with 0,04% to our lungs, the new mixture of gases in our lungs is as follows:

90% * 4% + 10% * 0,04% = 3,6%

So, the CO₂ content in our lungs after a fresh breath is 3,6% or 36000 ppm.

For Oxygen, we can a similar calculation can be set up. The Oxygen content in our lungs is 16%, and the Oxygen content in the atmosphere is 21%. Thus, the calculation for Oxygen is as follows:

90% * 16% + 10% * 21% = 16,5%

So, the Oxygen content in our lungs after a fresh breath is 16,5%.

It is important to note that a change in the atmosphere around us will merely have a linear effect on the composition of the air in our lungs. In other words, a shift in concentration has little or no impact on our bodies concerning the respiratory cycle when looking at breathing in and out specifically.

Remember taking those deep breaths? Our bodies sometimes tell us to do this. The urge to take a deep breath can occur because we exercise or concentrate or even because the air surrounding us is somewhat stale. When we take a deep breath, we exchange far more air than only 10% of our lung volume. We can exchange up to 80% of the air in our lungs by breathing in deeper and forcing more air out of our lungs. Taking deep breaths, however, has a price. It costs more energy.

Let’s revisit the above calculations for the concentration of CO₂ and Oxygen in the lungs after we take a deep breath:

CO₂: 20% * 4% + 80% * 0,04% = 0,8%

Oxygen: 20% * 16% + 80% * 21% = 20%

Now let us compare the Oxygen level inside our lungs for a normal breath versus a deep breath. The Oxygen level for a deep breath is significantly increased, from 16,5% to 20%. At the same time, the CO₂ level, when taking a deep breath, is reduced substantially from 3,6% to 0,8%.

Inside our lungs

According to Fick’s law, gases must first dissolve to diffuse across a membrane. Therefore, our lungs contain a high percentage of water vapor. You have experienced the proof of this on a cold clear winter day when a mist is forming every time you breathe out. This water vapor fulfills a helping function in the next step in our respiratory cycle. For our body to get Oxygen inside our bloodstream, Oxygen needs first to get solved in water. The alveoli are membranes that have a moisture lining. The water vapor and the moisture lining collaborate to improve the efficiency of gas exchange in our lungs. Oxygen can more easily enter the bloodstream, and CO₂ more easily leaves our bodies.

Illustration of Oxygen moving into the bloodstream, CO₂ moving out. (2)

There is a second physical chemistry law at work at the specific step of solving Oxygen and CO₂ into the water. The law I am referring to is called Henry’s law, and it describes that the amount of dissolved gas in a liquid is proportional to the partial pressure of the gas above the liquid. For every combination of liquid and gas, we can find a constant that describes how much gas will be in the liquid at given pressures above the liquid.

Next, let us put Henry’s law to work in the context of the pulmonary system, the lungs. Our body is mainly composed of water, and therefore the processes happening in our lungs are very well matched by choosing water as the reference liquid. The gasses of interest are the main components of our atmosphere: Nitrogen (78%), Oxygen (21%), Argon (1%), and CO₂ (0,04%).

Let’s take a closer look at Henry’s constants for these gases at 25 degrees Celsius (77 F)

It is essential to realize that higher values for the constant H imply higher gas solubility in a liquid. The liquid, in our case, being water. Furthermore, we can see that CO₂ is 26 times more soluble than Oxygen (0.034/0.0013 ≈ 26.15), 24 times more soluble than Argon (0.034/0.0014 ≈ 24.29), and a staggering 55 times more soluble than Nitrogen (0.034/ 0.00061 ≈ 55.74).

To give a correct picture of what happens when Oxygen enters our bloodstream, we also need to consider the temperature. A gas´ solubility depends on the temperature of the liquid. The highest solubility is found just above freezing, and when a liquid is boiling the solubility is close to zero. In the case of our bodies, with a temperature of 37 degrees Celsius (98.6F), the difference in solubility for Oxygen compared to CO₂ decreases, so that CO₂ is “only” 22 times more soluble than Oxygen.

For the interested reader, van ‘t Hoff equations describe this relationship between Henry’s constant and temperatures.

To translate this into an everyday example, think of a bottled carbonated soft drink. Most of us enjoy this best when it’s very cold and at its most fizzy. The refreshing fizz in the soft drink is achieved by dissolving carbon dioxide into the liquid and placing a lid onto it to keep the CO₂ inside the solution. Before you open the bottle, a gas fills the space between the drink itself and the lid — this is almost pure carbon dioxide. This CO₂ below the cap is at a pressure higher than the atmospheric pressure. You will know from experience that when you open the bottle, this gas escapes — giving off the characteristic hiss we all know. The fizzy, delicious drink in your hand is Henry’s Law at work: While pressure has little effect on the solubility of solids or liquids, it has a significant effect on the solubility of gases — in this case, CO₂. The amount of solved CO₂ inside the bottle increases when the partial pressure of the CO₂ above the liquid increases. In short, the carbonated soft drink is at its most fizzy, just when you open it, because of the high pressure of the CO₂ trapped between the liquid and the lid.

After opening the bottle and the trapped CO₂ below the lid escapes, the partial pressure of the carbon dioxide above the liquid is much lower. Some dissolved carbon dioxides will come out of the solution in bubbles as it begins to equalize pressure with the atmosphere. Finally, you will also know that if the drink is left to stand, it will eventually lose its fizz entirely.

These different stages of fizziness happen because the carbon dioxide concentration in the drink over time will come into equilibrium with the carbon dioxide in the air — and the result? The drink will eventually go flat — CO₂ will finally have evaporated from the beverage as its pressure equalizes with the CO₂ pressure in the atmosphere — the 0.04 % that CO₂ makes up in our atmospheric composition of gases.

O₂ and CO₂ in the bloodstream

Once CO₂ and Oxygen have entered our bloodstream, Dalton’s physical chemistry law becomes relevant. Dalton’s law states that when looking at a mixture of different gases, these gases behave independently, and the gases move as if the other gases did not exist.

Imagine two rooms. One room has a higher pressure of air than the other room. When the door between these two rooms is open, the air will now move from the room with the higher pressure to the other room, the one with the lower pressure. We experience this phenomenon as a draft of air.

The same principle is at work inside of our bloodstream. In the bloodstream close to the lungs, our respiration has created a pressure difference for CO₂ and Oxygen. The red blood cells, also called erythrocytes, are specially designed to transport Oxygen. Each erythrocyte cell contains approximately 270 million molecules of hemoglobin, a molecule that contains iron. Each of the hemoglobin molecules again contains four subunits with an iron atom each. The iron helps the cell bind the Oxygen to transport it to all the cells in the body.

Hemoglobin has a very high affinity to Oxygen. We need these specialized cells because Oxygen is not very soluble in liquids. Other components than the erythrocytes only contain about 1.5% of the Oxygen that we have in our bodies. When all four hemoglobin iron atoms are bound to Oxygen, the hemoglobin is saturated. Hemoglobin that contains Oxygen is called oxyhemoglobin, which has a bright red color.

When the saturated red blood cells arrive at areas in the body that have a lower partial pressure of Oxygen, the Oxygen is released. Each cell in our body burns sugar using Oxygen to generate energy, producing CO₂ as a waste product.

This creates a lower pressure of Oxygen in the cell and a higher pressure of CO₂. Our bodies transport the waste product CO₂ in three ways:

  1. Since CO₂ is relatively soluble, a large portion resides in our blood plasma.
  2. Some CO₂ transforms into bicarbonate that also dissolves in the plasma.
  3. Our red blood cells can not only bind to Oxygen but also to CO₂ to transport excess CO₂.

The blood that is now containing higher levels of CO₂ moves to our lungs.

At the lungs, the partial pressure of CO₂ in the blood is now higher than in the lungs. Therefore, CO₂ moves from the bloodstream into the lungs, and the circle is complete. But what happens when the partial pressure of CO₂ is higher in the lungs? The higher partial pressure of CO₂ in the lungs will result in a higher concentration of CO₂ in our bloodstream. The result of a higher CO₂ concentration in the bloodstream is a lower partial pressure difference between a cell and the bloodstream. This smaller difference translates into a slower removal of the CO₂ from each cell, and thereby a lower efficiency.

Above is an Illustration of Oxygen and CO₂ exchange between the cells and the blood. (3)

Both temperature and the environment’s acidity surrounding the red blood cells impact oxygen affinity to the red blood cells. The locations in our body where the temperature is higher coincide with where we use more energy. For example, in muscles during exercise or in the brain while solving tasks requiring concentration. Similarly, acidity changes in areas of the body where we burn a lot of energy. The acidity changes because the concentration of CO₂ in these areas is higher, and Carbonic acid forms when CO₂ increases. As a result, the pH level decreases, and again the affinity of the red blood cells to Oxygen also decreases.

The relationship between how efficiently Oxygen can bind to red blood cells and the pH level of the blood was a discovery made by Christian Bohr in 1902. Christian Bohr discovered that blood holds more Oxygen when the pH level is higher and less when the pH level is lower. This phenomenon is called the Bohr effect. Coincidentally, Christian Bohr was the father of the famous Nobel laureate Niels Bohr, who was fundamental in understanding the structure of the atom.

At a local level, temperature and pH level help our bodies to optimize our inner workings and control where it is optimal to release Oxygen. In the situation where our bodies are in an environment with increased CO₂ levels, however, Henry’s law and the Bohr effect mean that our bodies work less efficiently.

Why do our bodies work less efficiently in an environment with increased CO₂ levels? In the respiration step, higher CO₂ levels are impacting levels inside the lungs linearly. In other words, nothing special is happening during this phase of our breathing cycle.

When Oxygen and CO₂ traverse the tissues that separate the blood from the air inside of our lungs, Henry’s law is in play. Because CO₂ is about 22 times as soluble as Oxygen at body temperature, the concentration of CO₂ in the blood also increases 22-fold, relative to Oxygen. That means that small changes in the concentration of CO₂ have a relatively strong impact on the CO₂ concentration in the blood.

The higher CO₂ concentration impacts the pH level of our blood, lowering it slightly. The Bohr effect states that the affinity of our red blood cells to Oxygen decreases with decreasing pH levels, which means that the efficiency of our Oxygen transport system also decreases. When the efficiency of our Oxygen transport system decreases, our bodies need to work harder to achieve the volume of Oxygen arriving at each cell. The heart needs to pump more, and we need to breathe more deeply or faster. These consequences are very similar to high altitude sickness and are indeed somewhat related.

At higher CO₂ concentrations in the air we breathe, the CO₂ created by each cell leaves our bloodstream more slowly due to a lower partial pressure difference. Again, this slower removal of the waste product CO₂ decreases the efficiency of our bodies.

The most determining feature that sets humans apart from other species in the animal kingdom is our intelligence. The brain makes up about 2% of our body weight, and at the same time, the brain uses about 20% of the total energy used by our bodies. Increasing CO₂ levels will reduce the amount of Oxygen that arrives in the brain. A reduced amount of Oxygen will slow down the process of burning sugars to generate the energy that our synapses use to fulfill their function — thinking. This slowness also translates into tiredness, an effect most of us have experienced when in poorly ventilated rooms.

Can our bodies adapt to higher levels of CO₂? The answer to this question is yes, within certain limits. Athletes already today use environments with elevated CO₂ levels to train, similar to high altitude training. The body will respond to such activity by producing additional red blood cells to counteract the decreased efficiency of the blood transport system. However, the pH level of the blood will remain at a reduced level compared to a more normal environment. Do we achieve record-breaking results at high altitudes when training? Not really.

We do not yet know the long-term impacts of increased atmospheric CO₂ levels on our bodies. Will we see increased numbers of patients with Osteoporosis, a disease that weakens the bones? How are unborn children affected while they develop in their mothers’ wombs, and what happens to children’s bodies as they grow up?


This is the sixth chapter of my book “Atmosphere, CO₂ on my mind”. You can find more information and references on my website.

Previous chapter

You can continue by reading the previous chapter (CO₂ & Observed impacts on humans) here.

Next chapter

You can continue by reading the next chapter (Insufficient CO₂ Reduction Strategies) here.


  1. The Mechanics of Human Breathing. (2020, August 15). https://bio.libretexts.org/@go/page/14029
  2. https://commons.wikimedia.org/wiki/File:Gas_exchange_in_theaveolus_simple(en).svg
  3. Anatomy & Physiology. Provided by: OpenStax CNX, https://courses.lumenlearning.com/suny-ap2/chapter/gas-exchange/ License: https://creativecommons.org/licenses/by/4.0/



Daniel Lux

What scares me about climate change is the effect that high CO2 levels have on our bodies and intelligence, yet very few are writing about this.