COVID-19 can spread through particles the size of dust mite poop. Here's how

Photo by Shawn Ang on Unsplash

When I think of aerosols, I think of a brick wall getting soaked with spray paint. So when I first started hearing the term “aerosol transmission” getting thrown around in relation to the COVID-19 pandemic, I was more than a little confused. There’s been no shortage of information (and misinformation) surrounding the virus this past summer, so I decided to see for myself exactly what aerosols were, how they spread, and how important they really are.

Spoiler alert: the answers I found have nothing to do with street graffiti.

Tiny, but deadly

The most common advice to protect ourselves from catching COVID-19 is to wear a mask and stay six feet apart from the people we’re not quarantining with. Why do we do this? To prevent the spread of droplets, which are icky globules of mucus or saliva that come out of an infected person’s mouth when they cough or speak. If these droplets touch a healthy person’s eyes, nose or mouth, there’s a possibility of them catching the virus.

Here’s their weakness: droplets are heavy. Think of them as miniature bowling balls — an infected person can hurl them out of their mouth, but they can only travel so far before plummeting to the ground. Staying six feet apart from someone limits the risk of a droplet making the trek all the way from someone else’s mouth to yours.

Aerosol particles can come out of an infected person’s mouth, too, but they’re much smaller than droplets. In fact, they’re smaller than 5 micrometers. That’s on the level of pollen, hairspray particles, and dust mite poop. Not only are they smaller than droplets, they’re also lighter: light enough to float in the air like a paper airplane and get carried around with the air currents until someone inhales them.

Initially, scientists thought that COVID-19 spread only through droplets. But, researchers are now gathering evidence suggesting that the virus’ particles can surf the air as aerosols, too.

Okay, so aerosols are small. What does that mean for COVID-19?

Aerosols are a big part of the reason indoor spaces can be more dangerous than outdoor ones when it comes to catching COVID-19. When we’re outside, there’s plenty of air for aerosol particles to travel on, so they don’t get stuck in one place. But in an indoor space—especially a crowded indoor space—there’s only so much air for everyone to breathe. Add a bad ventilation system that doesn’t replace mucky old air with clean air from outside, and aerosol particles from an infected person’s mouth can circulate around a closed area with the potential to infect many more people.

Inhaling one aerosol particle isn’t going to give someone the virus, but spending 10-15 minutes in an enclosed space with an infected person might. The chances are higher in an indoor space with lots of talking, cheering, or singing, like a choir practice or fitness class. So, knowing about aerosol particles doesn’t necessarily mean that nowhere is safe. It just means we have more of a reason to choose a socially distant outdoor picnic over dinner at an indoor restaurant.

How common is aerosol transmission?

According to the Center for Disease Control and the World Health Organization, aerosol transmission is a possible, but minor way that COVID-19 spreads (UPDATE 10/5 from the CDC: COVID-19 spreads "sometimes" through aerosol transmission). According to a growing body of scientists, though, it’s a dangerous method of spread. The jury is still out on exactly how common aerosol transmission is. With a pandemic like this, there’s no shortage of theory, but it takes time to make definitive statements about a virus that we still don’t fully understand. Until we know for sure, it’s worth doing exactly what we’ve been doing: masking up, staying socially distant, and avoiding crowded spaces: especially if they’re indoors. Whether you get it from a particle that’s like a bowling ball or a paper airplane, COVID-19 is not worth catching.

[Want to know more? This chemistry professor made an entire Google doc about aerosol transmission.]