Low CO2 is toxic?

Blair King of A Chemist in Langley made some claims about CO2 in a conversation I got into. Specifically, he said

that 1000 ppm of CO2 can cause people to have complaints:

that an afternoon in 2000 ppm of CO2 would cause crushing headaches:

He was quite assured as he spoke from experience:

I expressed skepticism at the stated effects of CO2 at such low concentrations. Blair ended up writing a post summarizing the human effects of ambient CO2 at different concentrations.

Here’s the passage on low CO2 concentrations from his blog post. Basically, he repeats his claims:

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.

What does he say?

  1. The literature on sick building syndrome says what happens when humans are exposed to CO2
  2. The listed papers discuss the effects on elevated CO2 on human health
  3. CO2 is a vasodilator and causes headaches so the above is understandable, i.e, there is a biologic mechanism to underpin the epidemiologic and experimental observations

The problem is, many of these things are wrong.

[1] Sick building syndrome is a constellation of symptoms and findings occurring in people in indoor environments that is thought to be related to air quality, among other things.

The exact cause of sick building syndrome (SBS) is not known. But ventilation and renewal of indoor air ameliorate the syndrome in many instances. Because the entities present in indoor air thought to cause SBS are numerous, and unknown in specific cases, researchers have used ambient air CO2 concentrations as a proxy for air quality.

Indoor CO2 increases in buildings where people live and work. Ventilation refreshes indoor air volumes, removes any causative factor present and removes CO2 that builds up in the same indoor environment. Which means CO2 becomes an effective proxy for air quality and adequate ventilation and does away with the need to identify and measure the offending substance in each instance.

Blair appears to have confused the role played by CO2 in SBS research to its causative agent.

Look at the abstract of Apte et al 2000, one of the papers cited:

Higher indoor concentrations of air pollutants due, in part, to lower ventilation rates are a potential cause of sick building syndrome (SBS) symptoms in office workers. The indoor carbon dioxide (CO2) concentration is an approximate surrogate for indoor concentrations of other occupant-generated pollutants and for ventilation rate per occupant …

[2] You can examine the literature on sick building syndrome. They are the result of work examining different causative agents – bioaerosols, mold, bacteria, volatile organic compounds (VOCs), temperature, humidity, particulate matter, building dust, paper, printer ink, and so forth.

Studies that examine ventilation efficacy, using CO2 as a proxy, cannot be used to infer the health effects of elevated CO2. The studies are observational and the several potential causative factors – as listed above – simultaneously co-vary with CO2.  The human effects of elevated CO2 – whatever they might be – are confounded in these studies.

[3] CO2 has central nervous system vasoactive actions in a linear fashion. Vasodilation-mediated headache is reliably seen starting between 2-5% inhaled CO2 whereas 2000 ppm corresponds to 0.2%, a value 10 times lower. Reports of headaches occurring in concentrations lower than 2% certainly exist but are anecdotal.

In fact, literature on the effects of low CO2 concentrations of the 0.2-2% range (not SBS studies) is sparse. A recent review by NASA (Stanovic et al 2016) examined 76 studies and reported headache to be a common physiologic symptom. But many, if not most of them looked at concentrations much higher than 1000-2000 ppm CO2 – typically in the 3-7.5% range.

What about effects other than headache?

One outlier set of studies that demonstrated dramatic effects for low indoor CO2 on mental performance come from Usha Satish and co-authors.

You can incur brain damage by reading Thinkprogress hype on one of the studies from the group:

Romm Hype CO2 Cognition

The Thinkprogress article makes the same  error as Blair but goes well beyond in typical Romm fashion pinning 20 years worth of low ‘productivity, learning, and test scores’ in SBS studies on CO2:

All of this new research is consistent with — and actually helps explain — literally dozens of studies in the past two decades that find low to moderate levels of CO2 have a negative impact on productivity, learning, and test scores. See here for a research note and bibliography of some 20 studies and review articles.

On the other hand if one actually reads the research note Romm links to, it simply reviews the SBS literature with studies examining every known indoor pollutant – from new office furnishings emanating VOCs and old carpets giving off mold.

As noted, no categorical statement/s about effects of CO2 can be made from such studies.

A similar rush to judgment and loose methodological  language blaming indoor CO2 is seen in literature examining classroom performance of children in schools. Many are observational in design with indoor CO2 co-varying with other ambient constituents. Nevertheless, the measurement proxy is turned into the offender.

The heights of low low-CO2 junk science hysteria are unsurpassed in a self-published review by  Phil Bierwirth of the Australian National University, in whose hands everything from kidney stones, cancers, neurological diseases, viral infections to mental incapacitation to extinction of the human race is brought about by low CO2

If allowed to persist, problems such as kidney calcification could lead to renal
failure. In the extreme case lifespans could become shorter than the time required to reach reproductive age. This would result in extinction of the species

One doesn’t have look too far to decode this  CO2-demonizing madness:

The main aim of this paper was to explore the question of toxicity for human breathing at levels of CO2 that could be attained with the continued unabated rise in atmospheric CO2 associated with climate change.

Back in the real world, what the results of the Satish-associated studies mean is not clear as their low CO2 findings are not yet reproducibly observed.

As Stanovic et al note:

On the other hand, several investigations have contradicted these observations entirely, finding no consistent relationships between CO2 exposure and either cognitive or motor functioning, in terms of speed, accuracy, or throughput.

The authors conclude:

While many studies have thus far addressed the impact of CO2 concentration on cognition, the inconsistent and contradictory nature of current findings limits the ability to draw firm conclusions about the impact of elevated CO2 exposure on sleep, cognition, and psychomotor performance.

In conclusion, the effects of low CO2 – in the 1000 ppm range – are mild, inconsistent, and mostly nonexistent in adapted individuals as almost all people working indoors are bound to be.

More importantly, they are not enough to label CO2 as being toxic at such concentrations.

Lastly it shouldn’t require to be pointed out causal inferences cannot be drawn from observational studies when confounders are present, especially when the studies themselves make this clear.


1. Apte MG, Fisk WJ, and Daisey JM. Associations Between Indoor CO 2 Concentrations and Sick Building Syndrome Symptoms in US Office Buildings: An Analysis of the 1994-1996 BASE Study Data. Indoor air 2000.
2. Bierwirth P. Effects of rising carbon dioxide levels on human health via breathing toxicity – A critical issue that remains unapprehended. 2014.
3. Brian JE. Carbon dioxide and the cerebral circulation. Anesthesiology 1998.
4. Burge PS. Sick building syndrome. Occupational and Environmental Medicine 61: 185-190, 2004.
5. Daisey JM, Angell WJ, and Apte MG. Indoor air quality, ventilation and health symptoms in schools: an analysis of existing information. Indoor Air 13: 53-64, 2003.
6. Law J, Watkins S, and Alexander D. In-flight carbon dioxide exposures and related symptoms: association, susceptibility, and operational implications. NASA Technical Paper 2010.
7. Ramalho O, Wyart G, Mandin C, Blondeau P, Cabanes P-A, Leclerc N, Mullot J-U, Boulanger G, and Redaelli M. Association of carbon dioxide with indoor air pollutants and exceedance of health guideline values. Building and Environment 93: 115-124, 2015.
8. Satish U, Mendell MJ, Shekhar K, Hotchi T, Sullivan D, Streufert S, and Fisk WJ. Is CO2 an indoor pollutant? Direct effects of low-to-moderate CO2 concentrations on human decision-making performance. Environmental health perspectives 120: 1671-1677, 2012.
9. Stankovic A, Alexander D, Oman CM, and Schneiderman J. A Review of Cognitive and Behavioral Effects of Increased Carbon Dioxide Exposure in Humans. NASA Technical Paper 2016.
10. Wargocki P, Wyon DP, Sundell J, and Clausen G. The effects of outdoor air supply rate in an office on perceived air quality, sick building syndrome (SBS) symptoms and productivity. Indoor Air 2000.




  1. Shub Niggurath

    Man in a barrel, Blair’s post was set off by his exchange with Patrick Moore, a conversation in which I jumped in. That’s how it started.

    Blair thinks the post above is an attempt to support CO2 the molecule. I guess he hasn’t encountered skeptics who care more for methodologic rigour, and don’t give a damn about CO2 politics.

  2. willard (@nevaudit)

    > I guess he hasn’t encountered skeptics who care more for methodologic rigour, and don’t give a damn about CO2 politics.

    I haven’t either.

    Got any names?

  3. Brad Keyes

    I pretty much ONLY care about method. Carbon dioxide politics bores me, TBH.

    Willard, I don’t think you’ll believe the following, and it’s hard to imagine any way I could convince you of it, if only because it’s largely a hypothetical, but I am very confident in this:

    Most “skeptics” would have no problem with “acting on climate” or paying a “price on carbon” IF the case for doing so had been expounded with methodological rigor. It hasn’t.