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The sudden, catastrophic release of gas from Lake Nyos on 21 August 1986 caused the deaths of at least 1700 people in the northwest area of Cameroon, West Africa. Chemical, isotopic, geologic, and medical evidence support the hypotheses that (i) the bulk of gas released was carbon dioxide that had been stored in the lake's hypolimnion, (ii) the victims exposed to the gas cloud died of carbon dioxide asphyxiation, (iii) the carbon dioxide was derived from magmatic sources, and (iv) there was no significant, direct volcanic activity involved.
Once an eruption occurs, a large CO2 cloud forms above the lake and expands to the neighbouring region. Because CO2 is denser than air, it has a tendency to sink to the ground while pushing breathable air up. As a result, life forms that need to breathe oxygen suffocate once the CO2 cloud reaches them, as there is very little oxygen in the cloud. The CO2 can make human bodily fluids very acidic, potentially causing CO2 poisoning. As victims gasp for air they actually hurt themselves more by inhaling the CO2 gas.
At Lake Nyos, the gas cloud descended from the lake into a nearby village where it settled, killing nearly everyone. In this eruption, some people as far as 25 km (16 mi) from the lake died. A change in skin color on some bodies led scientists to think that the gas cloud may have contained a dissolved acid such as hydrogen chloride as well, but that hypothesis is disputed.[4] Many victims were found with blisters on their skin. This is believed to have been caused by pressure ulcers, which are likely to have formed from the low levels of oxygen present in the blood of those asphyxiated by the carbon dioxide.[5] Thousands of cattle and wild animals were also asphyxiated, but no official counts were made.
The carbon dioxide released from the lake is at a very low temperature believed to cause frost bite, which is consistent with blisters found on the skin of survivors of the incident at Lake Minoun. The survivors also reported a smell of rotten eggs and feeling warm before passing out; this is explained by the fact that at high concentrations, carbon dioxide acts as a sensory hallucinogenic.
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