Why Can You Smell Fart Through the Mask?

As that anti-masker who had a tantrum in Panera pointed out, you can smell the fart even as it passes through the fabric of your pants. Her point of view seems to be that fabric cannot block air, and therefore masks cannot block things that are in the air. She’s wrong, but let’s see why.

The purpose of the cloth mask is to block the droplets containing the coronavirus . It works best to block large droplets that come from your own mouth and nose (so you don’t risk infecting others), but it can also provide some protection against droplets produced by other people.

To learn more about wearing masks, watch the video below:

How big are the holes in the mask?

The holes in the mask do not have to be the same size, and there are other factors that affect how much particle size can pass. (For example, multiple layers of fabric will protect better than one.) Therefore, the more important question to ask, rather than how big the holes in the mask are, is how well the mask filters different particle sizes.

A recent article on cloth masks tested this and found that different tissues block 5% to 80% of particles less than 300 nanometers in size. For larger particles – over 300 nanometers – some tissues performed better than others, blocking a range of 5% to 95% of particles.

However, once you combine multiple layers of fabric, the filtration efficiency will increase. They found several combinations that filtered out over 80% of the smaller particles and over 90% of the larger ones.

How big is a farts?

The smell of fart is caused by its sulfur-containing molecules. Chemistry professor Trevor Makal points out on Twitter that a typical methanethiol is around 0.4 nanometers . This is very, very little – the proverbial mosquito through the mesh of the fence.

As a result, you can smell the fart after it has gone through your pants and underwear. You can also smell it through the mask.

How big is the virus?

Viruses come in different sizes, but the coronavirus that causes COVID-19 was measured 60-140 nanometers , or 14,900-34,900% larger than a small, smelly molecule like methanethiol.

What about respiratory drops?

Aha! You paid attention. The virus does not necessarily travel by itself; it comes out of our mouth and nose in the form of drops. The droplets are much larger than the other things we talked about and are often measured in micrometers, but here I will convert them to nanometers to keep things consistent.

The droplets from sneezing are about 100,000 nanometers in size. Cough drops are smaller, about 1000 nanometers. (I got my number from this overview document, SARS-CoV-2 (COVID-19) by numbers , which will answer many of your numerical questions about coronavirus.)

What about aerosols ? The definitions of “aerosol” and “airborne” are somewhat controversial, so we will simply say that we are concerned about small droplets, which are thought to be droplets less than 5,000 nanometers in size. A good mask should stop most of them.

What else goes through the mask?

Air. Oxygen and carbon dioxide are less than 1 nanometer wide . They do not fall into droplets and can move freely through the mask when breathing. A quick way to check this is that the average exhaled air volume is about 500 milliliters , or about the same volume as a water bottle. The distance between the mask and the face is much less than the volume of a water bottle. Thus, the air you exhale is clearly coming out of the mask with each inhalation.

Some of the air passes through the pores of the mask, but if the mask is loose, some of the air may pass through the slits on the sides of your cheeks, the sides of your nose, or under your chin. This is why it is so important to wear the mask correctly so that the material of the mask does not let in as much air as possible.

One last thing: if you are going to argue with someone who tells you that masks don’t work, keep in mind that they may argue that masks cannot block viruses, while at the same time they argue that masks are harmful because they block the air. Both cannot be true at the same time, so you can ask them to clarify their position (is it being filtered too well or not well enough?) Before proceeding.