Carbon Dioxide

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Marine biology | Deep Dives; Seals keep from drowning by sensing oxygen in their blood. The Economist, Mar 22, 2025, at page 68.
https://www.economist.com/scienc ... hy-dont-seals-drown

paragraph 2: "When a mammal [any mammal except seals (according to this Science report)] holds its breath, the amount of oxygen in the body begins to decrease, while the amount of carbon dioxide steadily climbs. In humans low levels of oxygen mostly do not ring any alarm bells on their own -- it is high levels of carbon dioxide that eventually produce the unpleasant urge to breathe.  

Note:
(a) This article is based on
McKnight JC et al, Cognitive Perception of Circulating Oxygen in Seals Is the Reason They Don't Drown. Science, 387: 1276 (Mar 20, 2025)
https://www.science.org/doi/10.1126/science.adq4921
(Abstract: "Generally, oxygen is cognitively imperceptible to [all] mammals that instead sense rising carbon dioxide as a proxy for low oxygen. Not perceiving oxygen, however, is risky for [all] diving mammals. * * * We exposed diving seals to inhaled gas mixes that were experimentally altered to affect circulating levels of oxygen and carbon dioxide. Dive duration [of seals] was positively correlated with circulating oxygen levels but unaffected by carbon dioxide levels and pH")
(b) In the air, there is 20% of oxygen. Physiology textbooks tell us that a person usually does not die when oxygen in a closed room runs out, for long prior to that carbon dioxide builds up to 5% and the person sinks into unconsciousness (and died consequently). This is called carbon dioxide narcosis. The same happens in a fire scene, where toxic fumes as well as intense heat add dangerousness (and people there should drop to the floor and climb out, as carbon dioxide and toxic fumes move up).