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.