Carbon dioxide: Downregulating the human species

With carbon dioxide (CO2) levels rising in the air at unprecedented rates questions arise. What does CO2 do to the human physiology as well as to other living species at levels we may be seeing into the next century? What CO2 level in the atmosphere should be considered ‘acceptable’? These are the questions explored by the discussion paper, Carbon Dioxide’s Direct Impact on Down-regulating the Human Species, published in the journal Science of the Total Environment (September 19, 2023.)

A carbon crisis

Carbon dioxide (CO2 ) is a fundamental part of our world. It is all around us, and also within us. CO2 is a by-product of both fossil fuel combustion and biological activity. We breath in air that contains oxygen, and exhale air laden with CO2. Some people claim that CO2 is benign. However, much like how a rising tide can back up a city sewer system, so too can rising CO2 levels in the air inundate the respiratory process that removes CO2 from living beings – our whole respiratory system including lungs, airways, and blood vessels involved in removing waste gases from our bodies.

Many building ventilation engineers go by a rule of thumb measurement to discern whether air is ‘bad’. The general rule is that a concentration of one thousand parts per million (1000ppm) CO2 concentration in the air will cause humans to feel uncomfortable. The discomfort we feel at 1000 ppm is often attributed to the odour and particulates that come with the increased levels of CO2 in the air, rather than the CO2 itself. This assumption was little challenged until recently. 

Carbon in the atmosphere

Palaeontologists (people whose job is to study fossils to understand past life on earth) have observed that mass extinctions tend to occur when the level of atmospheric CO2 rises rapidly above 1000ppm. An ‘uncanniness’ appears at around the value of 1000ppm for atmospheric CO2, giving rise to the question: Could it be that 1000ppm of atmospheric CO2 holds more significance than being a mere rule of thumb for ventilation engineers? 

Although the atmospheric CO2 content has risen some 140% from 75 years ago, the rise of CO2 in the human bloodstream is only around 0.2%. This is because the pressure of CO2 gas dissolved in the bloodstream is significantly higher than the pressure of CO2 gas in the atmosphere. So while a unit change of gas pressure can make a large percentage change in the atmosphere, that same unit change in the bloodstream would be small percentage-wise. Today’s atmospheric CO2 concentrations on human physiology should be of little concern.

Concern about carbon?

Humans may not be able to maintain their current indifference to atmospheric CO2 should trends in anthropogenic activity not abate soon. With a level of 1000ppm CO2 concentration in the atmosphere projected by the end of the century, impact on human physiology will become clearly noticeable. While humans can handle acute exposure to high CO2 levels well, it is when CO2 levels remain chronically elevated that our ability to sustain a healthy metabolism becomes questionable.

And the issue isn’t limited only to humans – other species share our planet and are affected by our actions. Animal species like fish have much lower blood CO2 gas pressures than humans, and as a result will be impacted much sooner.  

Adding to climate change, the impact of elevated levels of CO2 in the atmosphere directly on humans and other living beings will make the future even more daunting. Perhaps you can run from rising sea levels, or shelter from extreme heat, but when you are immersed in ever-increasing levels of CO2, there is no escaping the direct effects.

Robert Stumm is a Professional Engineer in the United States now retired having served 38 years in the shipbuilding industry whose work included the development of ventilation systems on ships.


Stumm, RE (2023) Carbon dioxide’s direct impact on down-regulating the human species. Science of the Total Environment. doi: 10.1016/j.scitotenv.2023.167198. Epub 2023 Sep 19. PMID: 37734619.

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