Twenty-second anniversary of an environmental catastrophe

Yesterday was the 12th 22nd anniversary of an environmental catastrophe in Cameroon. On August 21, 1986 Lake Nyos in that West African country belched a huge load of carbon dioxide and suffocated 1700 people as they slept. Like its monoxygenated cousin carbon monoxide, carbon dioxide is "a colorless, odorless gas that can kill you in your sleep," as the marketing gurus of monoxide detector like to say, but it was true in spades on that fateful day 22 years ago. Carbon dioxide doesn't kill the same way as carbon monoxide, which binds tightly to the homoglobin in your blood, shutting out the oxygen. But in large enough quantities it is just as effective in killing you. When CO₂levels reach 10% you suffocate from lack of oxygen.

Lake Nyos is a volcanic lake, meaning it sits in the crater of a volcano. A brief note in Wired tells what happened:

Magma deep underneath the lake releases carbon dioxide into its depths. Lake Nyos is 690-feet deep, enough for the water pressure to keep the CO2 dissolved in the lake water, rather than letting it bubble up and escape to the surface. And the crater rim towers above the lake, blocking winds which could otherwise stir the surface and create convection currents that would circulate the deep, CO2-saturated water upward to areas of lower pressure. The lack of seasonal variation less than seven degrees north of the equator also contributes to the lake's placidity.

Volcanic rumbling or other seismic activity could have triggered the sudden release of the gas that deadly night, but there's no record of any tremors and no evidence that anything shook off the shelves of homes in nearby villages. It's possible the gas at the lake's bottom just got so concentrated that even under pressure it came out of solution and formed bubbles. Once the bubbles started rising, a "chimney effect" would have rapidly siphoned huge amounts of gas to the surface.

The gas burst through the surface with a rumble, generating a giant wave that scoured vegetation from the shores. The CO₂ cloud was at least 300-feet high, because it suffocated cattle on hillsides that far above lake level. Iron from the deep water oxidized and stained the lake waters with rust.
Then the gas crept down the mountain valleys, invading homes. It extinguished oil lamps and suffocated people in their sleep. Some who were awakened by the loud gas bubble stood up and lived, because their heads were above the invisible gas near the ground. But many who went outside paid with their lives.

Few survived. Those from neighboring villages who discovered the devastation recalled with terror the legends about evil demons living in mountain lakes.(Wired)

Lake Nyos

The Wired link has a slideshow of ten other "Death by Volcano" catastrophes of one kind or another (not mainly CO₂ events). But you don't need a volcano to die from carbon dioxide suffocation. It's long been known as a workplace hazard, not just because it won't sustain animal life but because it is heavier than air and can settle in trenches, pits, valleys and other work spaces. The EPA surveyed CO₂ workplace fatalities between 1975 and 2000 and found 51 incidents and 72 deaths from accicental discharge of fire extinguishing systems (see here). Beside displacing necessary oxygen, there are probably some toxic effects on the central nervous system, but in mass events like Lake Nyos, suffocation was probably the main mechanism.

Twenty-two Twelve years ago the Lake Nyos catastrophe was front page news. But life goes on and most of us had forgotten about it. CO₂ is much in the news again, but not at the parts per hundred levels that killed people in Lake Nyos but in the parts per million levels that contribute to the Greenhouse effect (see our post here, here, here). Lake Nyos is a reminder that even relatively non-toxic gases can kill quickly rather than over a century if the circumstances are just right.

22 years ago yesterday they were just right. Or perhaps more appropriately, horribly wrong.