On Sunday the MailOnline published an alarmist article on what they say are the risks of wearing a face mask, due to elevated carbon dioxide (CO2). Masks are an obstacle to breathing and so although when you wear a mask you just breathe a little bit harder and so get all the oxygen you need, they do retain a bit of the carbon dioxide you breathe out. When you breathe in, you tend to breathe in more carbon dioxide then if you are not wearing a mask. This is a well-known problem and the FFP2 standard for masks specifies that the carbon dioxide measured should be no more than 1%, which is 10,000 ppm* (parts per million). If you look at an FFP2 standard certificate you see they measure the carbon dioxide (via a method I don’t quite follow but I assume people have worked out a good way). For that case they measured carbon dioxide at 0.6% or 6,000 ppm.
It is true that 6,000 ppm of CO2 is quite high. To put it in context, atmospheric CO2 is around 400 ppm, and indoors in a reasonably well ventilated room it should be around 1,000 ppm or less (in crowded nightclubs it can hit 10,000 ppm). So wearing a mask does raise the CO2 in the air your breathe, by retaining some of the CO2 you breathe out. To a level equivalent to a very badly ventilated room.
The question is: Is breathing in CO2 at this level bad for you? Either at the time or in the long run. I am not sure we know the answer, but toxicology is far from my field.
The MailOnline article is based on a paper by Kisielinski and coworkers, in a journal I am not familiar with, called Helyion. I am not impressed by the paper, I do not think it is good careful work. This work is far from my field of expertise, so hard for me to judge but it does look sloppy. Another paper that looks rather more careful (but may also have flaws) at the effect of CO2, is by Guais and coworkers. Guais and coworkers cite papers that show some evidence of harm for CO2 levels over 10,000 ppm, but the results are for experiments done on animals not humans. And the studies were also I think quite short, so would not pick up longer term effects (years). The American CDC recommends that during a working day the average CO2 concentration should not exceed 5,000 ppm, so wearing an FFP2 mask is at the limit of what they recommend. I would guess that the CDC’s figure is probably itself a bit of a guess, in the absence of any definitive data.
I think there are a couple of take home messages. The first is that sensationalist journalism and a peer review system that lets through poor quality work in areas of direct public interest, is a bad combination. There are thousands of scientific journals, some of which publish all sorts of stuff. Random stuff gets through peer review all the time, and if it fits the narrative of a journalist who wants an exciting story but who is not interested in finding out the scientific consensus, then you get the MailOnline article, which has all sorts of scary speculation that I think is unhelpful. The second is that breathing in 1% or more CO2 for long periods of time may not be that great for our health, I am not sure we know enough to tell. If so then masks are not the only problem, badly ventilated rooms are, including homes. I got halfway to 5,000 ppm overnight in a decent sized bedroom, just by closing all windows and the door.
* The standard for surgical masks does not have a CO2 test, another reason why it is a poor standard. Having said that, typical surgical masks fit quite badly so CO2 levels are presumably mostly lower than for FFP2s. Most cloth masks aren’t made to any standard so some thick, well fitting ones may cause a bigger buildup of CO2. Maybe, nobody checks. Sensible thing to do is stick to FFP2 or a similar standard.
Chance & Necessity 