With a lethal concentration for humans of about 10%, the current concentration of carbon dioxide in Earth’s atmosphere, 0.04%, would seem to pose only a minor threat to us. Carbon dioxide poisoning has only been reported from closed environments (submarines and mines) and from the explosive release of CO2 from the bottom waters of Lake Nyos in Africa, which killed nearly 1800 people in 1986. Because it dissolves in water, high carbon dioxide drops the pH of blood (acidosis), which normally increases the respiration rate in humans. The toxicity of high concentrations of CO2 is often more related to concurrent low levels of oxygen than to the CO2 itself.

CO2 is produced in our normal metabolism, reaching levels of about 4% in the air we exhale. Despite this high value, collectively, the humans on Earth have a trivial impact on the carbon dioxide content of the atmosphere or the rise in CO2 concentration during the past century (https://blogs.nicholas.duke.edu/citizenscientist/human-carbon/ ).

A recent report suggests that we should pay more attention to indoor CO2 levels. Increasingly, we spend more and more time indoors, and levels of CO2 in schools, office buildings, and perhaps the White House, can rise to 0.5%, at which point normal human cognition slows. Studies of carbon dioxide effects on humans, leading to death, often show that mental perception slowed to the point where obvious solutions—like opening the window—were not apparent. Even at levels of 0.14%, student performance on tests dropped more than 25% from normal levels. We could expect CO2 at that level globally by the end of the century.

Life on a hot planet, where everyone has lost a quarter of their mental capacity due to high CO2 is not a reassuring future to contemplate.

References

Allen, J.G., P. MacNaughton, U. Satish, S. Santanam, J. Vallarino, and J.D. Spengler. 2016. 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. Environmental Health Perspectives 124: doi: 10.1289/ehp.1510037

Karnauskas, K. B., S.L. Miller, and A.C. Shapiro. 2020. Fossil fuel combustion Is driving indoor CO2 toward levels harmful to human cognition. Geohealth 4: doi: 10.1029/2019GH000237

Kling, G. W., W. C. Evans, M. L. Tuttle, and G. Tanylleke. 1994. Degassing of Lake Nyos. Nature 368: 405–406.

Pementier, K., S. Vercammen, S. Soetaert, and C. Schellemans. 2017. Carbon dioxide poisoning: a literature review of an often forgotten cause of intoxication in the emergency department. International Journal of Emergency Medicine 2017: 10-14.

Piantadosi, C.A. 2003. The Biology of Human Survival. Oxford University Press.