On Carbon Dioxide Toxicity

The other evening I became engaged in a fascinating discussion (for me) with a climate skeptic on the topic of carbon dioxide, specifically whether it was “toxic” or not. After that discussion I committed to do a write-up on the toxicology of carbon dioxide so here we go.

One of the first things my skeptic colleague did was to try and convince me that carbon dioxide was not “toxic”. He did so by citing the Wikipedia article on the topic. Now I know you are going to laugh, but Wikipedia is actually a very good place to go to for basic information about chemical compounds. I often use it to get basic information because it almost always gets the basics right. In this case it got the basics right but fell down in explaining the details [which explains why we have blogs 🙂 ]. Specifically, Wikipedia had this to say about carbon dioxide toxicity:

CO2 is an asphyxiant gas and not classified as toxic or harmful in accordance with Globally Harmonized System of Classification and Labelling of Chemicals standards of United Nations Economic Commission for Europe by using the OECD Guidelines for the Testing of Chemicals. In concentrations up to 1% (10,000 ppm), it will make some people feel drowsy and give the lungs a stuffy feeling.

A person reading this paragraph may, incorrectly, believe that carbon dioxide was not toxic because that is how the text reads; but that is not what the text actually says. The text correctly points out that carbon dioxide is an asphyxiant gas. That, in and of itself, does not preclude it from being toxic (more on that later). It also points out that the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) does not classify it as toxic. Admittedly, it incorrectly includes the term “harmful” which is absolutely wrong. Unfortunately the GHS isn’t in the business of assessing chemical toxicity per se and says nothing about whether a compound is harmful.

You may ask: what is the GHS? Well, most of you out there were probably forced to take a Workplace Hazardous Materials Information System (WHMIS) course in your day. GHS is the new global standard replacing WHMIS. The GHS has a mandate for labeling chemicals during transportation and for use in non-household environments. The target audiences for GHS include workers, transport workers, emergency responders and consumers.

GHS has a relatively complicated method for establishing whether a chemical is “toxic” for the purposes of labeling. Here is a link to the document saying how to do it and here is a link to a GHS-compliant SDS for carbon dioxide. Scroll down the sheet to the “Acute Toxicity” section and you will see a note:

Based on a LC50 value of 470000 ppm/0.5h (167857 ppm/4h) (PATTY (5th, 2001)), this substance was classified as “Not classified”

For those of you who don’t read SDS what this says is that carbon dioxide kills half of the test subjects who encounter a concentration of 470,000 parts per million for a half hour. So while, it is not considered toxic for the purposes of the labeling system, it certainly kills people (and is absolutely “harmful”). The thing to understand is that since it doesn’t do so in the course of normal industrial and commercial use it is not necessary to label it as “toxic” for the purposes of the labeling system. What this might also say is that, yes carbon dioxide is toxic, just not toxic in a standard work environment, so don’t worry about it too much.

Remember the first argument my colleague noted above was that carbon dioxide was not toxic because it was an asphyxiant. This is a common misconception based on a misunderstanding of the difference between the two terms since a chemical can be both toxic and an asphyxiant. As an asphyxiant, it will displace oxygen and at high enough concentrations it will kill. Specifically the Bureau of Land Management Health Risk Evaluation for Carbon Dioxide points out:

A value of 40,000 ppm is considered immediately dangerous to life and health based on the fact that a 30-minute exposure to 50,000 ppm produces intoxication, and concentrations greater than that (7-10%) produce unconsciousness (NIOSH 1996; Tox. Review 2005). Additionally, acute toxicity data show the lethal concentration low (LCLo) for CO2 is 90,000 ppm (9%) over 5 minutes (NIOSH 1996).

But carbon dioxide is not just an asphyxiant, as described above, it is also toxic. Now remember from my earlier post on the subject, the toxicity of a chemical is based on the dose. A compound that is harmless, or even beneficial, at low concentrations can be toxic at higher concentrations. Consider that water is toxic at high concentrations; the effect is called hyponatremia. So technically water can be called “toxic” as well.

A detailed discussion of the toxicity of carbon dioxide is presented in the article Toxicity of Carbon Dioxide: A Review by Guais et al. A review study by MIT and NASA (prepared for among other things the space program) also presents a very good overview of the effects of carbon dioxide, especially at lower doses. As described in the two reports, the initial physiology of carbon dioxide toxicity is based on the perturbation of the acid/base balance in your blood resulting in acidosis. The human body is able to handle this sort of thing. Our initial response is cellular buffering that occurs within minutes-to-hours. In the continued presence of elevated carbon dioxide renal compensation occurs over around 35 days. What this means is that your body will adapt to higher concentrations, over time. Carbon dioxide is also a potent vasodilator and as anyone who has migraines knows, vasodilators are also associated with headaches. Reading the literature, the following is clear:

  • At high concentrations carbon dioxide is an asphyxiant that displaces oxygen and kills affected individuals.
  • At moderately high concentrations it has both short-term and long-term toxic effects.
  • At moderate concentrations it is tolerable as your body will adapt to the exposure.

What we have not discussed is the effect at lower concentrations. We all know that at atmospheric concentrations it is essentially harmless, but what about concentrations between atmospheric (400 ppm) and the problematic doses (over 10,000 ppm). Well the answer is that it varies. The literature is clear that your body will adapt to those concentrations and will quickly adapt back once the exposure is reversed. As for what happens in the meantime, well there is an entire academic field on “sick building syndrome” that gives a pretty clear indication of what happens. The following references all discuss how elevated carbon dioxide concentrations (Apte et al, 2000, Wargocki et al, 2000, Seppanen, Fisk and Mendall, 1999) effect human health. All point out that a proportion of the human population reacts poorly to daily variations in carbon dioxide concentrations. This is understandable since, as a vasodilator, it would be expected to have particular effects on people prone to migraines or headaches.

By far the most interesting new study comes from Satish (et al, 2012) and suggests that even moderately elevated carbon dioxide concentrations have the potential to affect decision-making. This study has not yet been replicated but certainly opens a new level of discussion.

I will end this post on that thought, but since I have been asked to write them down, please find below a quick break-out of some of the numbers I found while doing my research.

Critical Concentrations for Carbon Dioxide:

300,000 ppm (30%) – Immediate coma, convulsions and death (BLM)

100,000 ppm (10%) – Unconscious in one minute and unless prompt action is taken, further exposure to these high levels will eventually result in death (BLM and CDC)

40,000 ppm (4%) – NIOSH Immediately Dangerous to Life or Health Concentration – CDC

30,000 ppm (3%) – OSHA short-term exposure limit (15 – minute STEL) to protect against metabolic and respiratory changes CDC

10,000 ppm – 30,000 ppm (1.5% – 3%) – electrolyte imbalances and other metabolic changes have been associated with prolonged exposures (CDC)

10,000 ppm (1%) – US OSHA 8-hour permissible exposure limit (PEL) to protect against metabolic and respiratory changes CDC

5,000 ppm (0.5%) – British Columbia Occupational Health and Safety Regulation, 8-hour Time Weighted Average exposure limit (OH&S Reg)

2,000 ppm – 5,000 ppm – level associated with headaches, sleepiness, and stagnant, stale, stuffy air.  Poor concentration, loss of attention, increased heart rate and slight nausea may also be present. (WDHS)

2,500 ppm (0.25%) Satish et al 2012 observed large and statistically significant reductions occurred in seven scales of decision-making performance in office workers

1,000 ppm – 2,000  Levels associated with complaints of drowsiness and poor air (WDHS)

1,000 – 1,100 ppm – The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62-2007 Ventilation for Acceptable Indoor Air Quality provides standards for ventilation based on surface area and occupancy, “a surrogate for human comfort (odour)” but “not considered a health risk.” (National Collaborating Centre for Environmental Health)

1,000 ppm (0.1%) Satish et al 2012 observed moderate and statistically significant reductions occurred in seven scales of decision-making performance in office workers

400 ppm – 1,000 ppm – Typical levels found in occupied spaces with good ventilation (WDHS)

400 ppm (0.04%) Approximate current atmospheric concentration

Author’s note:

I’ve had numerous people note that submariners etc.. live in conditions with elevated carbon dioxide concentrations. That is both true and doesn’t take away from what I have written. As I noted, the body adapts to higher carbon dioxide concentrations and so if you are put in a cylinder with 8,000 ppm carbon dioxide for a month you will adapt admirably. What causes the human health complaints is rapid shifts from areas of high to low concentrations which leave your body trying to adapt. The results can be, as described, uncomfortable but not life-threatening. Most sick building reports identify occupants as showing effects after being exposed for hours at a time, just long enough for your body to respond but not long enough to reach a stable condition.

This entry was posted in Chemistry and Toxicology, Risk, Uncategorized. Bookmark the permalink.

18 Responses to On Carbon Dioxide Toxicity

  1. Neodymigo says:

    Blair, I think you should have tabulated the ppm’s of CO2 and general effects, there seems to be enough meat in the reference articles to do that. You would probably be famous or at least your tabulation would be. Sort of like they do with sound levels, i.e. whisper, normal conversation, up to jet engine. Apte et al show effects at about double ambient CO2. A good discussion point in how much CO2 is too much for your circumstance. There should also be studies available on the effects on submariners and astronauts, all of whom experience long term exposure.


  2. Morley Sutter says:

    “Poison is in everything, and no thing is without poison. The dosage makes it either a poison or a remedy.” – Paracelsus 1493-1541.

    Liked by 1 person

  3. DHR says:

    Submarines routinely have abt 4,000 ppm and sometimes higher. No ill effects I am aware of.


    • Blair says:

      As described, people can adapt to the higher concentrations if exposed consistently over a prolonged period. Where people suffer is when they yo-yo in and out of elevated concentrations as their bodies do not get the opportunity to adapt.


  4. mseverijnen says:

    Prolonged close contact with other humans should be avoided, since we exhale 40.000-50.000 ppm CO2 …….And if the other human makes you breathless for a minute, your CO2 emission might reach a dangerous 60.000-70.000 ppm….


  5. Pingback: Low CO2 is toxic? | Shub Niggurath Climate

  6. Neodymigo says:

    ??? Typo …. 1,000 – 1,1000 ppm In table


  7. Whither says:

    Having been suffering a sick building, it’s not the CO2, but rather bad ventilation combined with various glues that were commonly used in 1970’s, possibly added with mouldy structures through which the replacement air comes. Being sick has nothing to do with CO2, but of course CO2 can be used to measure ventilation efficiency per inhabitant.

    One of the poisons included is formaldehyde, which made me replace all inner walls in my apartment. You can smell it, so it is pretty easy to track.


  8. howb says:

    Thanks for taking the time to write these great posts. I found your website yesterday and have read through a few of your posts starting with the crybully article.

    I think that your comments are right on the mark, and I really enjoyed learning about CO2 toxicity. I would never have thought about it because it seems so unlikely that I’d encounter such conditions.
    Having said that, I think that you’re a little imprecise in using the term “climate skeptic”. I think that I’m probably in that group that you call climate skeptics, but I have to say that:
    – I believe that “climate” exists,
    – I believe that we’re adding more CO2 (and more importantly, water vapour) to the atmosphere,
    – I believe that these are greenhouse gasses,
    – I believe that this must have an effect,
    – but I do not believe that this is having a significant effect, it’s likely not measureable let alone detectable (the CO2 portion)
    regard belief see: https://judithcurry.com/2016/01/09/on-distinguishing-disbelief-and-nonbelief/

    I read on another blog (still looking through Climate Etc., BishopHill, WUWT, and Climate Audit) a post that discusses the effectiveness of CO2 as a greenhouse gas. The blog stated that at some point adding more CO2 to the atmosphere does not cause more heat to be trapped, and these CO2 concentrations are well below present values. It kind of parallels changing the CO2 concentrations from 30% to 35% won’t change the toxicity outcome. If concentrations are already at that level then the changes have no effect. Or to put it a way that west coasters might appreciate – wearing a second gore text coat won’t keep you twice as dry.

    I’m a geologist who uses precision like +/- a million years, and I admire your ability to measure and detect in moles and micrograms. I have to say that “climate skeptic” is a pretty broad brush in the “climate advocate” – “lukewarmer” – “skeptic” spectrum. I’d appreciate it if you’d refer to me as “someone who doesn’t believe that we’re heating up the atmosphere by adding trace amounts of a trace gas”.

    Being the imprecise geologist that I am, I think that if CO2 increases were actually making a difference as the models suggest, then the measured temperatures would be increasing as the models suggest. But this isn’t the case. As Feynman said – “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.” Temperatures have stayed flat (within the error bars) over the last 17 years as CO2 increased.

    Also, if AGW/CAGW was real then we’d be breaking temperature records from last year, not from the 1930’s or earlier (i.e. temperature records that existed from before CAGW).

    Anyway, I love your blog, I’m jealous of your writing skills, and I really appreciate your efforts to help the scientifically illiterate. I’ve forwarded “crybullys” and “BPA” to pals of mine, and I’m looking forward to reading the rest of your blog.



  9. Chris Manuell says:

    On that basis oxygen should also be treated as toxic.
    The result of breathing increased partial pressures of oxygen is hyperoxia, an excess of oxygen in body tissues. The body is affected in different ways depending on the type of exposure. Central nervous system toxicity is caused by short exposure to high partial pressures of oxygen at greater than atmospheric pressure. Pulmonary and ocular toxicity result from longer exposure to increased oxygen levels at normal pressure. Symptoms may include disorientation, breathing problems, and vision changes such as myopia. Prolonged exposure to above-normal oxygen partial pressures, or shorter exposures to very high partial pressures, can cause oxidative damage to cell membranes, collapse of the alveoli in the lungs, retinal detachment, and seizures.
    It just gets silly doesn’t it.


  10. Minarchist says:

    Interesting but ultimately irrelevant to the climate change debate.

    We have a saying in medicine that all drugs are poisons and all poisons are potentially drugs. The same principle applies: Yes, CO2 is toxic at high enough concentration, but we are talking orders of magnitude here. This is also true for O2 and a whole lot of otherwise benign substances.

    The toxicity issue arose because of the nonsense of the EPA Endangerment Finding, which was based on very bad science and allowed Obama to side-step Kyoto-like accords and regulate CO2 as a “pollutant” by regulatory fiat. It’s not a pollutant — CO2 is life: 6CO2 + 6H2O —> C6H12O6 + 6O2. Show that formula to the average liberal who probably doesn’t know the difference between CO and CO2 and their eyes glaze over. They also won’t understand the log dependence of CO2 IR absorption.

    But here are a few more benchmark levels, just for fun:

    560 ppm – the concentration around 2080-2110 (the “doubled CO2” relatively to the pre-industrial values) relevant for the calculations of climate sensitivity); a concentration routinely found outdoors today
    700 ppm – the concentration in an average living room
    900 ppm – concentration in an average kitchen
    1,270 ppm – the concentration used to double the growth of Cowpea in that famous video

    1,700 ppm – the average concentration in the Cretaceous 145-65 million years ago (early mammals came, plus figs, magnolias, birds, modern sharks)
    4,500 ppm – the concentration 444-416 million years ago (the Silurian dominated by corals and mosses); see other values in geological epochs
    10,000 ppm – sensitive people start to feel weaker
    40,000 ppm – the concentration of CO2 in the air we breath out
    50,000 ppm – toxic levels at which the animals like us get weaker in hours; the value is 5 percent of the volume
    180,000 ppm – the concentration of CO2 in exhausts of a healthy motor; that’s 18 percent
    1,000,000 ppm – pure CO2


  11. tom says:

    given that human sensitivity to abnormal co2 levels is most likely a spectrum, at what levels could, say 1% of the population experience health problems or breathing difficulties?


  12. Pingback: No Rising Carbon Dioxide Concentrations will not make us Stupider – Confusing Acute versus Chronic effects of Elevated Carbon Dioxide Concentrations on Human Cognition | A Chemist in Langley

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