If a mask blocks carbon dioxide, it will be fantastic against COVID-19. And a mask doesn’t have to be able to block individual viruses to reduce transmission.

I keep coming across people who claim that masks make it hard for them to breathe. OK, maybe that’s true. But there’s a subset of these people who claim that wearing a mask b oth a) doesn’t work on COVID viruses and b) is unsafe because it increases the concentration of carbon dioxide (CO₂) behind the mask.

I call bullshit on that. Masks literally can’t do both of those things.

Allow me to explain why.

Virologists who have looked at the SARS-CoV-2 (COVID-19) virus under microscopes have measured it to be between 80 and 120 nanometers (nm) in size. For reference, human hair is between 17 and 181 micrometers (μm) in diameter and a single red blood cell is between 6.2 and 8.2 μm in diameter.

So the virus is really, really small, at least as compared to things we’re used to thinking about like hair.

But what about compared to CO₂ or oxygen (O₂) molecules? A CO₂ molecule is 232 picometers (pm) long, and an O₂molecule is 152 pm. If you’re not familiar with the term “picometer,” it’s 1/1000 of a nm, which is 1/1000 of a μm, which is 1/1000 of a mm.

To put these scales in perspective, it may be more useful to scale them up to sizes we’re more used to on a day-to-day basis. So let’s say that an O₂ molecule was 1 inch across. That would make CO₂ molecule almost exactly 1.5 inches long. On this scale, the smallest COVID-19 virus would be 14.5 yards across, and the larger ones would be about 22 yards across. A red blood cell would be between 0.6 and 0.8 miles across, while the human hair would be between 1.8 miles and 18.6 miles across.

The point of all this math is that any mask that can prevent the flow of a gas like CO₂ will be fantastic at better at preventing COVID-19 from passing through the mask. And a mask that can block COVID-19 is probably not going to significantly affect the exchange of oxygen or CO₂ around your face.

So why do people claim this? Well, some videos purport to show that masks increase CO₂ concentrations behind the mask. That’s possible, but none of the videos I’ve seen show how the measurements were calibrated, or compared the CO₂ levels with and without a mask (for example putting the probe in someone’s mouth while they’re exhaling to measure the CO₂ concentration in standard exhaled breath) or the like. It’s also possible that the mask they checked was really that good, but if it was that good against CO₂, it would be amazing at blocking COVID-19. And it’s also possible that the video makers were simply lying.

But let’s take this one step further and ask a related question about masks, namely “could a mask not work on the virus itself but still be effective.” Well, since we exhale a lot of water vapor, spittle, and snot, let’s look at how big those can be.

Sneeze droplets range in size between 500 nm and 12 μm, smaller than a hair but still significantly larger than the COVID-19 virus itself. Cough droplets range between 100 nm and 900 μm in size (we’ll ignore the wads of phlegm that can be expelled). So the smallest cough droplets are about the size of a single COVID-19 virus, but the largest ones are nearly a mm in size – easily captured in a single layer of cloth, to say nothing of a proper mask.

Let’s put this into perspective on the same scale as above. If COVID-19 can be between 14.5 and 22 yards across, then a sneeze droplet could be as small as 91 yards or as large as 1.25 miles across. Cough droplets could be as small as 18 yards or as large as 93 miles across.

So it’s entirely possible to have a mask that can’t block a single virus but that can still block the bulk of droplets from sneezes, coughs, and just basic breathing.

And how will most virus transmission happen? By inhaling droplets that someone else exhaled, not by inhaling individual viruses.

Masks aren’t perfect. Nothing is. But they do help reduce the spread of COVID-19. So keep wearing a mask if you already are and start wearing one if you aren’t.