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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Is CO2 a pollutant?

What the science says...

Select a level... Basic Intermediate Advanced

A single substance can be both a pollutant and a non-pollutant. It all depends on context.

Climate Myth...

CO2 is not a pollutant

'To suddenly label CO2 as a "pollutant" is a disservice to a gas that has played an enormous role in the development and sustainability of all life on this wonderful Earth. Mother Earth has clearly ruled that CO2 is not a pollutant.' (Robert Balling, as quoted by Popular Technology)

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!

Further details

We commonly think of pollutants as contaminants that make the environment dirty or hazardous to Nature and humans. A vivid example is sulphur dioxide, a common by-product of industrial activity. High levels of sulphur dioxide cause breathing problems. Too much SO2 causes acid rain, because it is so highly water-soluble. Sulphur dioxide has a direct effect on health and the environment. Fortunately, it reeks and so can be detected by us quickly, at concentrations as low as 1 ppm.

Carbon dioxide, on the other hand, is a naturally occurring gas that existed in the atmosphere long before humans. Plants need it to survive. The CO2 greenhouse effect keeps our climate from freezing over. These are all popular talking-points that any climate change denier will rattle out in its defence. How on Earth, they say, can CO2 be considered a pollutant?

Well, CO2 is very definitely a hazardous substance when it collects due to being denser than air or suddenly invades an environment in large quantities. Under either scenario it is capable of instant asphyxiation of any living thing with the bad luck of being in that place. The Lake Nyos disaster of 21st August 1986 (Tanyileke et al. 2019) is probably the most notorious example. A stratified lake - where the waters usually do not mix - underwent sudden overturn. The overturn brought pressurised CO2-saturated deep water up to the surface in an explosive release. It sent a cloud of CO2 - estimates vary but it could have been as much as a billion cubic metres - hurtling over the lake's rim. The gas cloud swept outwards at an estimated 72 kilometres per hour. In its path were several villages; the most distant to be affected, Mashi, was some 20 kilometres from the lake. At least 1,746 people and 3,500 livestock were asphyxiated.

If you explore mines or caves, or work in them, you will be well aware of the lethality of isolated high concentrations of CO2. These typically occur in badly or non-ventilated areas. Here, sources of CO2 such as an underground mineral-spring or rotting timbers, are not buffered by mixing-out of the gas in air. Instead, the gas can lurk in pockets and is a lethal hazard to the unwary. The experienced carry portable meters that constantly read the concentration of oxygen and other gases. Thus they can provide immediate warning that a pocket of 'bad air', as it's known, has been entered.

So under certain circumstances, CO2 is very definitely noxious to say the least.

What about the effect of raising atmospheric CO2 levels? Over the past 10,000 years, the concentration of carbon dioxide in the atmosphere has remained at relatively stable levels of around 280 ppm. However, human CO2 emissions over the past few centuries have upset this balance. Since industrial times started, CO2 levels have risen to over 420 ppm - a 50% increase (fig. 1).

The increase in CO2 due to human emissions has direct effects on the environment. For example, as the oceans absorb increased CO2 from the atmosphere, that leads to acidification. Acidification affects marine ecosystems. It can lead directly to mass-mortality of calcifying organisms. In other words it can effectively destroy oceanic food-chains. Whole fisheries may disappear as a consequence.

CO<sub>2</sub> levels over the past 10,000 years

Figure 1: CO2 levels (parts per million) over the past 10,000 years. Source: Berkeley Earth.

Last but by no means least is the impact from rising CO2 in the form of warmer temperatures. Rising CO2 levels cause an enhanced greenhouse effect. This leads to warmer temperatures which have many consequences. Some effects are beneficial such as improved agriculture at high latitudes and increased vegetation growth in some circumstances. However, the negatives far outweigh the positives. Coast-bound communities are threatened by rising sea levels. Melting glaciers threaten the water supplies of hundreds of millions. Species are becoming extinct at the fastest rate in history.

How we choose to define the word 'pollutant' is a play in semantics. To focus on a few positive effects of carbon dioxide is to ignore the broader picture of its full impacts. The net result from increasing CO2 are severe negative impacts on our environment and the living conditions of future humanity.

Last updated on 27 August 2023 by John Mason. View Archives

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Further reading

For a good overview of CO2 acidification, read Ken Caldeira's What Corals are Dying to Tell Us About CO2 and Ocean Acidification.

Denial101x video

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


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Comments 26 to 36 out of 36:

  1. I think the question hangs on whether you believe global warming is on net harmful at the levels predicted. If you don't believe that the net effects of a 2 degree temperature rise are harmful then it's hard to get to the point of calling CO2 a pollutant. While there have been many dire predictions based on the 2 degree theory I have yet to see a well reasoned quantification of the relative benefits versus harms. Where can I find that?
  2. I need a link to EPA's listing of CO2 as an air pollutant. This blog states "...the US Environmental Protection Agency (EPA) Administrator must publish a list of certain air pollutants....Greenhouse gases including CO2 unquestionably fit the Clean Air Act's broad definition of air pollutants and must be listed and regulated by the EPA if it can be determined that they endanger public heath and/or welfare." However, I cannot find an EPA list of air pollutants that includes CO2, and the links in this blog don't help. They lead to an EPA Endangerment Finding but not a statement that CO2 is an air pollutant. Or is that the same thing? I appreciate it.

  3. fake progress @27...  I believe they would be the same thing.

  4. Earthling,

    CO2 is poisoness to animals and (especially) marine life at low concentrations.  Past mass extinctions have been caused by high CO2 levels.  Calling anything that causes mass extinctions a fertilizer is ludicrous.

  5. Traditionally CO2 has not been considered a pollutant because it does not directly cause human harm (from inhaling it, like particulates), nor does it promote harm to human health indirectly by contributing to smog creation (as NOx leads to the creation of ozone, which in turn may trigger athsma), nor does it lead to property or wildlife damage (as SO2 causes acid rain).  CO2, like H2O, is an odorless, harmless emission that is beneficial to humans and wildlife.  However, since about 2010, the EPA has tried to intentionally conflate CO2 with other pollutants using the new "carbon pollution" moniker.  There is a deliberate intent to confuse CO2--the odorless gas--with black carbon soot (particulates).  CO2 itself does not cause any direct or indirect human harm or property damage.  There is a theoretical link of global warming to more inense adverse weather events, and a theoretical link of increased CO2 to sea level rise, which may necessitate waterfront modification, depending on the magnitude of the sea level rise.  However, in practice, CO2 could double or triple in the atmosphere without any temperature change because there is so much H2O in the system that has about the same greenhouse gas effect.  So if you want to classify CO2 as a pollutant, then H2O would also fit the definition.  Historically, CO2 has not caused warming.  Rather, global warming was caused by some other factor (volcanoes, meteors, sun, etc.), and the warming has caused CO2 to rise as tundra melts.


    [TD] Provide evidence for your claim of deliberate intent to confuse CO2 with carbon soot. Else do not make such baseless, conspiratorial claims here.

    Your claim of CO2's influence being trivial because of the H2O in the atmosphere is completely, factually wrong, because water vapor is a feedback rather than a forcing. Read the post "Explaining How the Water Vapor Greenhouse Effect Works." Read the Basic tabbed pane there, and then the Intermediate tabbed pane. If you want to comment on that topic, do so there rather than here; comments on that topic here will be deleted for being off topic.

    Your claim that historically CO2 has not caused warming also is completely, factually incorrect. Read the post "CO2 Is Not the Only Driver of Climate"--the Basic tabbed pane and then the Intermediate one. If you want to comment on that topic, do so there, not here. Then watch climatologist Richard Alley's talk "The Biggest Control Knob: Carbon Dioxide in Earth's Climate History."

    [RH] Extended warning snip. It would be worthy to note that the EPA initially rejected the idea of making a ruling on CO2. It was a court case "Massachusetts v. EPA" that went to the Supreme Court where the court ruled that the EPA was required to make a ruling on greenhouse gases under the Clean Air Act (Wiki). The commenter here clearly lacks sufficient understanding of the issue s/he's opining on and would be well advised to fully inform her/himself before continuing commentary.

  6. Sorry, actually, the "carbon pollution" moniker was introduced by the Obama administration in 2013.


    [RH] Searching Google Trends for the usage of the phrase "carbon pollution" shows that you are incorrect. Try it out.

  7. Actually aed939, that CO2 was defined as a pollutant dates back to Massachusetts vs EPA in the US Supreme Court in April2007 when SCOTUS ruled that the greenhouse gases are 'air pollutants' as defined under the Clean AIr Act.

    President Bush issued an executive order in May 2007, authorising the EPA to regulate GHGs for mobile sources.

  8. If we can define pollution as the contamination of air, water, or soil by substances that are harmful to living organisms, most substances can fill-the-bill, especially when we consider high/low concentrations and temperatures.  Plants that grow in areas that we don't want them will be considered as weeds.  Carbon Dioxide that becomes too concentrated for our environment certainly should be considered a pollutant, but the question, and I think it is still unanswered scientifically, is what is that level?  The dynamic complexity of the Earth's ecosystem suggests that when we begin tinkering with nature we may find ourselves creating additional problems for mankind that are currently unforseen.

  9. I think carbon dioxide is not a pollution. Let talk about definition ,the definition of pollution in Webster’s dictionary is "to make physically impure or unclean: Befoul, dirty." By that definition, carbon dioxide is not pollution. However, Webster's also has the definition: "to contaminate (an environment) esp. with man-made waste." Carbon dioxide is a waste gas produced by fossil fuel combustion, so can be classified as man-made waste.

  10. Aomsin@34,

    You provided two definitions of pollution then provided the connection between human created CO2 from burning ancient buried hydrocarbons to the second definition.

    Think seriously about why you opened your comment by declaring "I think carbon dioxide is not a pollution."

    The interpretation of the definition is a game played by legal-minded people trying to argue against government regulations to limit harmful impacts of human activity. The EPA written legal mandate is to limit 'pollution'. That is the cause of the attempts to legally argue that CO2 from burning fossil fuels is not 'pollution' (an argument has been lost in the courts). Continuing to argue about the definition distracts from the undeniable harmful consequences of the activity. And regulatory bodies like the EPA should act to restrict the creation of harmful consequences, no matter what term is used to describe them.

  11. Please note: a new basic version of this rebuttal was published on August 27 which includes an "at a glance“ section at the top. To learn more about these updates and how you can help with evaluating their effectiveness, please check out the accompanying blog post @

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