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At a glance – Is CO2 a pollutant


At a glance – Is CO2 a pollutant

Posted on 5 September 2023 by John Mason, BaerbelW

On February 14, 2023 we announced our Rebuttal Update Project. This included an ask for feedback about the added “At a glance” section in the updated basic rebuttal versions. This weekly blog post series highlights this new section of one of the updated basic rebuttal versions and serves as a “bump” for our ask. This week features “Is CO2 a pollutant?“. More will follow in the upcoming weeks. Please follow the Further Reading link at the bottom to read the full rebuttal and to join the discussion in the comment thread there.

At a glance

If you look up the definition of pollution in a dictionary, you will soon realise it’s rather subjective. There are many substances out there that are harmless at certain levels but harmful at others.

Carbon dioxide is well-mixed in our atmosphere. That’s because when it is emitted, by any mechanism from a vehicle exhaust to a volcanic eruption, it stays in the air for many years. Unlike water, it does not condense and fall back out as rain. Turbulence does a splendid job of mixing it evenly into the air. But there are places on – and in – Earth where much higher concentrations of CO2 may be encountered.

The trouble with CO2 is that it cannot be seen and neither can it be smelt. In other words we cannot detect it from a safe distance.

In caves and mines, high concentrations of CO2 are a well-known hazard. They can result from things like rotting timber, oxidising coal and particularly by poor ventilation, where that mixing into the air fails to occur. Because CO2 is heavier than air, in poorly ventilated areas underground it may collect into pockets waiting for the unwary.

Miners or underground explorers breathing a higher than normal concentration of CO2 will experience gradually increasing ill effects. It depends on the concentration of the gas. For example the UK Health and Safety Executive has defined safe CO2 limits for the workplace. The limit for long-term exposure is 0.5% (5,000 ppm) but for shorter encounters it is 2%. Anything over that figure is regarded as a risk to human health. There have been many accidents and fatalities over the years caused by high concentrations of CO2 in underground workings and to a lesser extent in caves. Coal-miners refer to CO2 as black- or choke-damp in recognition of the hazard.

Possibly the worst CO2-related disaster was that of 21 August 1986 at Lake Nyos, in northwestern Cameroon in western Central Africa. The lake, only some 2 x 1 km in size but more than 200 m deep, is one of a number of flooded volcanic vents in a sporadically-active volcanic belt. Carbon dioxide-bearing springs are common in this area and some are present in the lake-bed.

Lake Nyos is typically stratified, meaning that normally its waters occur in distinct layers with different chemistry that do not normally mix. In something of a loaded gun scenario, the bottom layer used to become saturated with CO2 from those lake-bed springs. On 21st August 1986, something caused an overturning of the lake, meaning the deep CO2-saturated water headed for the surface. Like taking the top off a shaken-up pop bottle, a vast cloud of CO2 was instantly released and travelled out from the lake along the ground. At least 1,746 people and 3,500 livestock died instantly from asphyxiation.

Modern technology and international cooperation have since been successful in controlling the build-up of CO2 in lakes like Nyos. But clearly, in specific circumstances, CO2 is as deadly a pollutant as any other.

Please use this form to provide feedback about this new “At a glance” section. Read a more technical version below or dig deeper via the tabs above


In case you’d like to explore more of our recently updated rebuttals, here are the links to all of them:



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