A: TL;DR. Masks work to prevent disease spread because they keep out the bigger particles and still let you breathe.
Even though mask policies and behaviors are relaxing in many places right now (Summer 2021), we want to remind you of the benefits of mask wearing with this fun question from Summer 2020. If you are indoors with unvaccinated people, it’s a good idea to wear your mask.
Why can I still smell things through a mask? Well, masks aren’t actually supposed to block all air flow (which actually gets at another misunderstanding about masks — that they are dangerous because they interfere with breathing).
Masks instead block droplets containing COVID — mostly from exiting your mouth/nose and exposing others, but they do also offer the mask wearer some protection from others as well. It turns out that the smell from a fart is caused by methanthiol and other similar sulfur containing molecules, and these molecules are TINY. Much, much smaller than coronavirus.
Check out the lifehacker piece for some numbers, but coronavirus is up to 34,000% larger than fart molecules! Think about the approximate comparison between a basketball and a poppy seed!
This means that the virus particles have a much harder time getting through a mask than odors in the air. That is not to say that masks protect 100% — unfortunately almost nothing in life is 100% — but they do offer a lot of protection. As far as oxygen and CO2, they are also both much smaller than the virus, so that is why they can pass through (and it is safe to wear a mask all day).
BOTTOM LINE: Masks block things that are bigger, and allow things that are smaller to pass through. Luckily for us, air and CO2 are small (unfortunately, fart molecules are also small) and coronavirus is big. Many of the conspiracy theories about masks are contradictory — masks are both useless because the virus can pass through AND dangerous because oxygen and CO2 cannot. It’s actually the opposite — they work because they block virus while also, thankfully, allowing airflow.