CO₂ & Observed impacts on humans





David Bowie’s seminal song “Space Oddity”, about Major Tom, was released only five days before the United States’ Apollo 11 mission and an additional five days before the first human walked the Moon. It was a song that underscored a new era in space travel and echoed through a time of social shifts and change.

Scott Kelly remembers being aware of the moon landing aged five, mindful of those famous words on humankind’s giant leap, spoken by his fellow American Neil Armstrong. What he, of course, was unaware of was that he too would be breaking new frontiers, more specifically to hold the record for the longest single space mission by an American, with 342 consecutive days spent away from Earth.

When asked during the beginning of the Corona pandemic to reflect on his year-long confinement at the International Space Station between March 2015 and March 2016, Scott remembers how nature was a central figure in his experience. His longing for the color green, for the smell of fresh dirt — proved one of the significant challenges to his year in orbit. Conversely, he also found solace and a sense of belonging in bird song recordings, rustling trees, and buzzing mosquitoes. In the weightlessness above the atmosphere, nature held a specific emotional gravity for Scott Kelly.

We live in a time when the world community faces significant challenges, environmentally, in terms of pandemics, and on many other fronts. At this time, it seems poignant to take in Kelly’s description of our planet. Seen through the window of the International Space Station; “Seen from space, Earth has no borders. […] All people are inescapably interconnected.” Perhaps this elementary and humanist view on Earth truly resonates, as it comes from someone who had left our planet and one of the less than 600 humans ever to have ventured above the Earth’s atmosphere.

Kelly’s journey to the International Space Station follows a different trajectory than what we might expect from an astronaut. A wayward and fearless child, today he would, in his own words, probably have been diagnosed with ADHD (Attention Deficit Hyperactivity Disorder). His father’s alcoholism colored his childhood, and he defines his teenage years as a time adrift and disengaged from studies. However, years of staring out of classroom windows were replaced by a view of the Earth from space. Kelly contributes this change to the very fearlessness of character that often got him into trouble as a youth. Two other vital influences were Tom Wolfe’s book “The Right Stuff” on the first Project Mercury astronauts for the N.A.S.A. Space Program; and his twin brother, the astronaut Mark Kelly.

According to Scott, Mark, a more academic and focused character, helped shape his brother’s slightly bumpy path to N.A.S.A. And when Scott took off on his nearly year-long journey into space in March 2015, Mark, in many respects, took on the role of his brother’s keeper again, acting as his genetic ground control back on Earth. As identical twins, Scott and Mark’s physical and genetic match was an invaluable source of insight into how the environmental changes experienced during spaceflight affect the body (1).

The results of a year in orbit, during which astronauts survive in atmospheric conditions maintained by a chemical machine converting Oxygen from CO₂, manifested themselves in Scott Kelly’s body. Effects were recorded on, for instance, his DNA, eyesight, muscle mass, and bone density.

All changes were occurring during his year in orbit, while the body of his identical Earth-bound twin displayed none of the same changes.

The physical impact of space travel was, as mentioned, manifold, and some effects reverted once Scott Kelly returned to Earth, while others appeared more permanent. Some of these effects came as expected. To Kelly, possibly part of the known and unknown side effects also spoke to his innately fearless and curious nature. The danger is the dance partner of the astronaut; it is an acknowledged, and all too many times proven fact, that space travel carries the risk of losing one’s life. But “lost in space” can also be understood physiologically. Bone loss, or spaceflight osteopenia, is one of the characteristic adverse effects of space travel. It refers to the phenomenon that astronauts commonly lose over 1% bone mass per month spent in space.

Scientists commonly explain spaceflight osteopenia as a consequence of long-term weightlessness. Since the 1960’s when we first observed these physiological consequences, exercise has become a part of the daily regime for astronauts, alongside increasing dietary calcium and vitamin D. These countermeasures have nevertheless proven somewhat unsuccessful in preventing bone loss (2).

However, it may prove significant to focus not so much on the gravitational conditions aboard spaceships but rather to examine the atmospheric ones to understand whether they could play a part in the consistent bone density loss observed in astronauts.

As noted, the conversion of Oxygen from CO₂ indeed keeps these astronauts alive during space travel. Still, the conversion isn’t optimal, and they survive in atmospheric conditions where CO₂ levels fluctuate between 2,000 ppm and 5,000 ppm. To understand if CO₂, similarly, to reduced gravity, may have a part to play in skeletal deterioration, we must shift our geographic focus entirely. Let’s examine another frontier that we humans continuously attempt to conquer — redirecting our gaze from man in space to man under water.

Submarine atmosphere

Like Scott Kelly, Clifford Smith was aware of certain risks when he stepped aboard the diesel-powered U.S. submarine the Gudgeon in 1957. In August that same year, the Gudgeon and its crew were detected on Soviet territory while on a spy mission. The Soviet forced the submarine to go without fresh air for three days, consequently building up high CO₂ levels within the vessel, causing intense headaches and fatigue to the crew aboard. CO₂ levels finally rose to near-deadly conditions, with crew members gasping for breath, and the untenable situation ultimately forced the captain to resurface to save the lives of his men (3) (4).

The Gudgeon resurfaced and eventually returned to U.S. waters. It underwent two conversions and served in the Pacific and Hawaii before being leased to Turkey. Today it serves as a public museum in Izmit, Turkey. Clifford Smith survived to tell a story about this vessel, and report what it was like being trapped within its walls under such toxic CO₂ levels, as high as 5,000 ppm. He may also be the source of another important story. A story found inside his body, which may provide insight into the bone loss experienced by his fellow explorers in space.

Submarines and spacecraft may seem very different — indeed, they are separated by two very different elements and belong to two very different worlds. However, both types of vessels share many features. Both feature exercise rooms for their crew, and both keep their crews alive through a mechanized conversion of CO₂ to Oxygen. CO₂ levels are also similar on both spacecraft and submarine travel, and crucially, crew members on both types of vessels show a consistent bone loss after a prolonged time away from land (5). Scientists believe long-term weightlessness causes spaceflight osteopenia. However, weightlessness cannot be the reason for the same signs of skeletal damage in submariners, who live in normal gravitational conditions. They have access to exercise and have a similar increase in dietary calcium and vitamin D as astronauts.

A relatively overlooked common denominator for individuals exposed to long periods in submarines and spaceflights is the prolonged exposure to similar atmospheric conditions — namely high levels of CO₂. Crews on both types of vessels live under conditions where CO₂ levels fluctuate between 2,000 ppm and 5,000 ppm. Might there be more to uncover about the changes observed in the bones of these crews, and the atmospheric conditions under which they live for long periods?

If we return our thoughts to the simple ocean snail, we described in chapter 4. We may find clues to this question. Recall how its cloudy, ragged appearance was the sign of a damaged shell — the very embodiment of ocean acidification? We may also recall the reason for this. The increased CO₂ in the ocean causes a drop in pH levels and an increase in acidity. Remember how this increase of acidity caused more bicarbonate to form, leaving our shell-building organism with less shell-building calcium carbonate? And consequently, the damaged shell?

Is it possible that the same process is occurring in the human body when exposed to high levels of CO₂ over a prolonged period? Could it be the case that the pH levels in our bodies drop, and acidity increases, causing our bones — made of the same material as the shell of our simple ocean snail — also to erode? The bone loss observed in both astronauts and submariners seems to witness skeletal damage as a physiological consequence of prolonged exposure to increased CO₂ levels.

Our bodies find themselves in a constant state of bone remodeling. During childhood, bone formation exceeds bone resorption creating the strong bones we see in children. Bone resorption is the process by which osteoclasts, or bone cells, break down the tissue in bones. As we get older, bone resorption exceeds bone formation, and this shift in balance is what weakens our bones in old age.

Several studies on submarine crews point to a clear correlation between skeletal deterioration and exposure to high CO₂ levels over prolonged submersion periods.

A study on bone metabolism and strength, during a 30-day submersion with high CO₂ exposure to crew members, found a simultaneous decrease in bone building and increase in resorption markers in the blood, indicating a reduction in crew members’ bone metabolism. A second paper observed decreased bone density on submarine crews at the end of a prolonged submersion period. At the 6-month follow-up, this study also found that bone density had returned to its pre-embarkation normal level after crew members had returned to normal atmospheric conditions (6) (7).

Medical research shows that an increase of CO₂ in the bloodstream lowers the pH levels of our blood (8). The lower the pH levels, the higher the acidity. Excessive CO₂ causes acidosis — a high acidity level in the blood — which increases the activity of the bone-resorbing cells, the osteoclasts, and decreases the activity of the bone-forming cells, the osteoblasts — the result; bone loss (9).

A further indication of the physiological effects that CO₂ can have on our blood is that lower blood pH levels affect the kidney, the organ that filtrates our blood. Higher CO₂ levels in our blood will also decrease pH levels in urine. A lower urinary pH strongly influences the formation of various types of kidney stones — and one of the recurrent adverse effects of space flight is, precisely, kidney stones (10).

Increased CO₂ exposure, it therefore seems, has a similar effect on us humans as it does on our Earth cohabitant, the ocean snail. As creatures — man and ocean snail — we may understand ourselves as fundamentally different, as space is to the sea. But like the physiological impact observed in both astronauts and submariners, the damage recorded in humans and ocean snails alike, bear witness to the effects of elevated CO₂ levels. It may manifest itself in either a ragged and cloudy shell or bone loss and kidney stones. Either way they are all the physical manifestations of damage caused by prolonged exposure to high CO₂ levels.

Fitness center atmosphere

If high CO₂ levels make themselves known inside our bones, where else in our bodies may they also manifest themselves? A suitable place to examine this would be in fitness centers where the main activity and focus are physical exertion and performance. In a research paper discussing the CO₂ levels in fitness centers, the authors recorded levels above 7,000 ppm in specific venues. While the high-level exposure to the fitness center users is limited to a few hours and will not manifest itself as, for instance, skeletal damage, the researchers highlight the potential for such CO₂ levels to cause drowsiness and even pose a risk to users’ respiratory health (11).

The well-known and much-researched effects of high altitude on humans parallel many of the known effects that high-level CO₂ has on us. The paper’s authors are aware of and address the impact high-level CO₂ conditions can have on the physical well-being of fitness center users. They highlight potential adverse effects on their respiratory system. However, a surprisingly limited amount of research exists on the impact of high-level CO₂ on physical performance.

High altitude exposes us to atmospheric conditions where oxygen levels are lower than usual. The adverse effects to these atmospheric conditions count headaches, drowsiness, shortness of breath, and in extreme cases, death — much like the effects recorded on humans exposed to high levels of CO₂. Elite athletes, therefore, often train at high altitudes. Training in such conditions, with less Oxygen available to the athlete, is an initial disadvantage to their performance. However, the long-term advantage for the athlete is that the body generally performs better once it returns to a lower altitude. The body gets a natural boost when more Oxygen is available — and we may apply a similar equation to the impact on physical performance in high CO₂ conditions.

However, the key for both phenomenon is the athlete’s choice of being exposed to such atmospheric conditions. At the same time, the athlete has the option of returning to and performing under normal atmospheric conditions. Should high CO₂ levels become the norm, that choice may no longer be a choice, and our physical performance may suffer as a result. Indeed, iconic moments in elite human performance such as Roger Bannister’s sub-4-minute mile in 1954 and Eliud Kipchoge’s sub-two-hour marathon in 2019 may be a thing of the past if high CO₂ levels become the norm.

CO₂ influence on mental capabilities

Let’s move our attention to another place where performance and competitive levels are equally high.

The room is full yet almost silent. The only noises making themselves known are sighs, deeply drawn breaths, the click of buttons atop the doubled clocks, the sound of both defeat and victory as wood meets wood when the crucial piece, the king, finally falls. A chess tournament is a tour de force of the mind, intense strategic decision making under time pressure. A complex and cognitively demanding activity. Chess tournaments form the hierarchy of the chess community; this is where participants will either advance or lose the prestigious positioning they have battled for, sometimes over many years. A tournament edition comprises several rounds and can stretch over months as players move through despair and hope, defeats and victories. All of the battles are fought in focused silence, mind fighting mind.

Therefore, a chess tournament served as a fascinating subject for researchers to understand how CO₂ can affect our cognitive abilities. Over several tournaments held in 2017, 2018, and 2019, researchers conducted a study focused on 596 chess games, 121 players, and 30,000 moves that examined the quality of moves made by individual players. Each player’s activity was evaluated by a chess engine and compared to actions deemed optimal according to its algorithm. The scientists then merged this data with the atmospheric levels recorded inside the tournament venues to estimate the effect of indoor air quality on the chess players’ quality of decisions. CO₂ levels ranged between 1,000 and 2,500ppm, and researchers discovered that the increase in CO₂ coincided with an increase in errors made by individual chess players (12).

The research and data point towards a deterioration in human cognitive performance with increased CO₂ levels. These findings lead us to other, more detailed studies of the impact of atmospheric conditions on the human brain.

Above the atmosphere, within the confines of the International Space Station, Scott Kelly, and his colleagues kept their sense of connection to the Earth through recordings of bird songs and other sounds from nature. It served as their emotional gravity out there in the vast and weightless space in which they lived for nearly a year. However, our daily connection with nature back here on Earth has shifted; The industrial revolution, followed by the technical revolution, has irreversibly changed the way we live and work. We now spend 90% of our time indoors, and office environments play a critical role in our well-being. Therefore, the indoor environment has received much attention from scientists, which resulted in several studies on CO₂ levels in office environments.

And this research on work environments, and the cognitive impact of CO₂, shows a clear line of decreased performance of the brain function relative to concentrations of CO₂.

In a study on green office buildings, twenty-two participants, divided into six groups, were exposed to three different office-like chambers where the scientists recorded atmospheric CO₂ conditions at respectively at 600, 1,000, and 2,500 ppm. In each chamber and their respective atmospheric environment, the participants completed computer-based tests of decision-making performance. Compared to their performance at 600 ppm, the participants all displayed moderate but statistically significant decrements in six of nine scales of decision-making when performing the tests at 1,000 ppm. When performing a similar test in CO₂ levels of 2,500 ppm, results showed large and significant reductions in their decision-making performance in seven out of the nine scales.

Impact of CO₂ on human decision-making performance. Error bars indicate 1 SD. (13)

A second study, conducted with 24 participants over six days, took place in three different controlled atmospheric settings that simulated indoor environmental air quality as it exists in three separate buildings — defined as respectively Green+, Green, and Conventional buildings. Scientists exposed the participants to CO₂ levels of respectively 550 ppm (Green+), 945 ppm (Green), and 1,400 (Conventional). The participants had to forego cognitive assessment daily via a validated computer-based test. The employed test originally is used to assess the effectiveness of management-level employees and their higher-order decision-making.

At the study’s conclusion, the researchers found that measured against their performance in CO₂ conditions of 1,400 ppm; the participants scored 61% higher when working in CO₂ conditions of 945 ppm and 101% higher in CO₂ conditions of 550 ppm.

The graphs display cognitive function scores by domain and participant and the corresponding carbon dioxide concentration in their cubicles. Each line represents the change in an individual’s CO₂ exposure and cognitive scores from one condition to the next, normalized to the average CO₂ exposure across all participants during the Green+ conditions. (14)

Both papers concluded that high levels of CO₂ have a direct adverse effect on human cognitive performance. The first study also highlighted how these findings are of economic importance, as we may not be able to perform and be as productive in our work environments when exposed to increased levels of CO₂. They make an essential and valid argument. At the same time, their research also points us towards a potential future. A future where, in case CO₂ levels continue to increase, as examined in chapter 4, we could face atmospheric conditions that impact us cognitively and decrease our brain’s performance functions (15).

This scenario seems almost unimaginable to us now. We often leave an indoor environment to clear and refresh our minds outside. The Corona pandemic has undoubtedly highlighted how many of us momentarily escape the world’s pressures through seeking solace in nature. Listening to birdsongs or smell fresh dirt as Scott Kelly did in space. Yet, suppose CO₂ indeed has such a profound impact on our bodies. In that case, we must entertain the real possibility that future atmospheric conditions could impact our ability to function as we have become used to, both cognitive and physical. Remember our giant birds and the effect atmospheric composition change had on their evolutionary path?

CO₂ compared to alcohol

Unimaginable as it may seem, most of us most likely have experiences to refer to that could make this scenario more imaginable. If we compare the exposure to CO₂ with alcohol intake, we can begin to form a picture of how we may respond if we were to live under higher CO₂ levels.

Many adults have experienced the effects of alcohol, but we may not immediately reflect that many of us have also experienced the impact that CO₂ has on the body. Think only of time spent in poorly ventilated meeting rooms. At first, we become less and less able to focus, and subsequently, we experience tiredness. In some rare cases, the ventilation in a room is so insufficient that we experience headaches. All of these effects are related to elevated levels of CO₂.

We measure alcohol levels in the blood (blood alcohol content or BAC) in tenth, hundredth or even thousandth of a percent. There is ample documentation of the effects on behavioral change of alcohol in the blood. Even though hangovers are not a pleasant thing, luckily, the influence of alcohol is gone again within about 48 hours once we stop drinking. The same is true for the effects of CO₂ once we leave the environment containing higher levels.

There are, however, also long-term effects in the cases where people are consuming alcohol daily. The higher the daily dosage is, the tougher the toll on the body, as we know. We can also describe alcohol dosage in units of alcohol equivalents. One unit is, for example, one beer of 0.33 cl with an alcohol percentage of 5%.

CO₂ levels are measured similarly to alcohol levels, and by now, we are pretty familiar with the ppm measure, or parts per million. To put a picture on it imagine 1.000.000 balls, which represent the composition of our atmosphere. Approximately 780.800 of these balls would be Nitrogen, and 209.400 would be Oxygen. About 9.300 would be Argon. Only about 400 would be CO₂.

The remaining balls would represent other gases like Methane, Nitrogen dioxide, and different gases in very low quantities.

As we have seen, in many confined spaces such as classrooms, meeting rooms, or even insufficiently ventilated bedrooms, CO₂ levels can rise to levels as high as 3.000 ppm or 0.3%.

As with alcohol, the time of exposure to increased levels of CO₂ matters; unfortunately, we haven’t documented the long-term effects of CO₂ exposure at higher levels as well as those for alcohol. There is also very little data available on the impact of increased CO₂ levels on the development of children.

To visualize the short-term and long-term effects, of both alcohol and CO₂, on the human body, I have divided the effects into the following seven categories.

The illustrations are somewhat individual and not absolute, and before we examine them, we should clarify the definition of long-term effects (weeks, months, and years). We are thinking of an individual consuming the mentioned dosages of alcohol every day during the specified period. For CO₂, the individual is exposed to the specified levels of CO₂ without a break during the specified periods (16) (17) (18) (19).

First, let us consider a healthy lifestyle where we breathe fresh air and consume next to no alcohol.

The following illustration shows the effects of moderate alcohol consumption and CO₂ levels as we find them indoors. When examining the long-term effects (Months, Years), most people with a healthy lifestyle will not fall into these categories. We do take a break from drinking, and we do go out into the fresh air.

When increasing both alcohol consumption and CO₂ levels further, the harmful effects increase, particularly when considering the long-term effects. While two glasses of beer or wine are not harmful when consumed occasionally, they will be more damaging if they become a habit. For the increased CO₂ levels, as seen in submarines or space stations, are reported to cause headaches and other side effects.

As we further increase alcohol concentrations, the effects also increase. We classify people with alcohol consumption at these levels over a more extended period as alcoholics. CO₂ levels as high as described in the following illustration are relatively rare, and only a few people risk exposure to such high levels.

A person drinking an amount of alcohol described in the following illustration is a heavy drinker, if this happens daily. When exposure to CO₂ levels this high at work is unavoidable, we should keep the exposure time below 15 minutes.

As we further increase the levels, the effects become more and more dangerous, even after short exposure times.

Go high enough with the dosage of something harmful, and it will become deadly.

This comparison might call for a deep intake of air and a strong cup of coffee.

We need to broaden our view on the impact of CO₂. We need to see its direct influence on life on Earth, not simply consider it a marker for climate change.

The impact of rising CO₂ levels is complex and potentially far-reaching, with significant impact on equally complex creatures, including the human in the many guises we come: The office worker, the chess player, the sportsman, the submariner or the astronaut like Scott Kelly — or Major Tom….

“Planet Earth is blue.”


This is the fourth 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 here.


  2. Peter R. Cavanagh; Angelo A. Licata & Andrea J. Rice (June 2005), “Exercise and pharmacological countermeasures for bone loss during long-duration space flight”
  5. Amit Saad, Carmel Kala, Sharon Ohayon, Lior Feldman, Eran Galil, Yoav Yanir, Dan Nemet, Itamar Netze, “Assessment of the Risk of Fractures Because of Service on Diesel Submarines: A Retrospective Cohort Study“
  6. Gasier HG, Hughes LM, Young CR, Richardson AM., “The assessment of bone mineral content and density of the lumbar spine and proximal femur in US submariners”
  7. Luria T, Matsliah Y, Adir Y, Josephy N, Moran DS, Evans RK, Abramovich A, Eliakim A, Nemet D., “Effects of a prolonged submersion on bone strength and metabolism in young healthy submariners”
  8. Shivani Patel; Julia H. Miao; Ekrem Yetiskul; Anya Anokhin; Sapan H. Majmundar., “Physiology, Carbon Dioxide Retention”
  9. Nancy S Krieger, Kevin K Frick, David A Bushinsky, “Mechanism of acid-induced bone resorption”
  11. Alexandro Andrade, Fábio Hech Dominski, Marcelo Luiz Pereira, Carla Maria de Liz and Giorgio Buonanno, “Fitness centers demonstrate CO2 concentration levels above recommended standards”
  12. Steffen Künn, Juan Palacios, Nico Pestel, “Indoor Air Quality and Cognitive Performance”
  13. Reproduced with permission from Environmental Health Perspectives, Usha Satish,Mark J. Mendell,corresponding author Krishnamurthy Shekhar, Toshifumi Hotchi, Douglas Sullivan, Siegfried Streufert, and William J. Fisk, “Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderate CO2 Concentrations on Human Decision-Making Performance”
  14. Reproduced with permission from Environmental Health Perspectives, Joseph G. Allen , Piers MacNaughton, Usha Satish, Suresh Santanam, Jose Vallarino, and John D. Spengler, “Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments”
  15. Kristopher B. Karnauskas, Shelly L. Miller, Anna C. Schapiro, “Fossil Fuel Combustion Is Driving Indoor CO2 Toward Levels Harmful to Human Cognition”
  17. Phil Bierwirth, Australian National University, “Carbon dioxide toxicity and climate change: a major unapprehended risk for human health”



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.

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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.