The transcript from the video above has been edited for length and clarity

A sprig of moss is growing on a tree in the Duwamish Valley, the most polluted area in Seattle. 

While that moss might seem ordinary, it has a secret superpower: It’s keeping a record of the pollution in the air — from the trucks, trains, planes, cargo ships, and hundreds of industrial facilities that surround it. Now, a group of community scientists are using this overlooked plant to take control of the air they breathe.

Moss is a type of plant. But it’s not like most plants. Mosses evolved hundreds of millions of years ago, long before the advent of trees or flowers. And, for the most part, they’re still pretty pretty low-tech plants. They don’t have roots, or any major system to circulate water and nutrients. Instead, they get everything they need by absorbing it from the air, directly into their spongy cells. 

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When moss is absorbing nutrients, it’s also absorbing everything else that’s in the air. And by sampling moss on a tree, and testing it in the lab, you get a fingerprint of what’s in the air in that specific place.

If you’re interested in what the air quality has been like, you can go out there and collect the moss and you can get an idea of what that looks like,said Sarah Jovan, a research ecologist for the U.S. Forest Service who studies mosses and lichens — which have similar pollution-detecting superpowers.

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Her research started in the wilderness: using mosses and lichens to learn about air pollution in our national parks and forests. After several years, she had an idea that no one in the U.S. had ever tried on a large scale: to use moss to study air pollution in cities.

At the time, her home city of Portland was facing a common problem: They only had one main air monitor for the whole city. That stationary monitor could tell you exact levels of pollution in the air, but it was really expensive. That meant Portland couldn’t afford enough monitors to track air pollution on a neighborhood level. Portland did, however, have a lot of moss.

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“There’s moss all over the place in this city, like we could collect almost anywhere we want to,” Jovan said. “From a statistician’s point of view, we could have a really bad ass design!

Sampling moss would be cheap, too. A study with hundreds of samples across Portland would cost about as much as a study with one official stationary monitor. While the moss couldn’t tell you exactly how much pollution was in the air, it could be a perfect screening tool. It could help identify which neighborhoods have the worst air pollution, and then an official monitor could follow up to measure just how bad that air actually is. 

In 2013, Jovan and her team sampled moss from 346 trees around Portland and tested them for metal pollutants like lead, arsenic, and chromium. And when they put all those results on a map, they found a pollution hotspot.

Their moss samples revealed a residential neighborhood in Portland that had much higher levels of cadmium and arsenic than the rest of the city. An official followup confirmed the source: a local stained glass factory that used these metals to make glass colorful and clear. 

“You’re glad you revealed it, because now we have a chance to address and change it,” Jovan said. “But it was a little horrifying. We were pretty darn stressed out about it.”

Their study created a scandal. But the findings caught people’s attention. It led to new environmental laws and more air monitors in the city. And it also caught the attention of another community, 150 miles to the north. 

The Duwamish Valley has been home to Seattle’s heavy industry for more than 100 years. And in the middle of that industrial valley are two residential neighborhoods: Georgetown and South Park. 

The Duwamish Valley, it’s an environmental injustice community,said Paulina López, the executive director of the Duwamish River Cleanup Coalition. I moved to this neighborhood because I wanted to be for sure surrounded by people that look like me. I’m from South America and I have always prioritized spaces that look like this neighborhood – families, river water access, and the diversity that exists within it.”

Residents know pollution in the valley is a problem. They face a lot of environmental health concerns, like asthma or heart disease.

“We know who are the polluters. But we haven’t been able to prove it,” López said. 

To clean up the valley, they needed a much more detailed picture of all the different sources of pollution. The Cleanup Coalition saw moss as a way to do that. 

The Portland moss study proved that scientists could use a simple plant to reveal big truths.

In Seattle, they wanted to take it a step further. They wanted to see if the community could do it themselves.

“I see the moss study like, oh, actually, we took matters into our own hands,” said Adrienne Hampton, Climate Policy and Engagement Manager for the Cleanup Coalition. “This is what we learned. Take it or leave it.”

Jovan — along with partners from other agencies, universities, and nonprofits — helped train a group of local youth scientists, who then went around south Seattle finding trees and collecting the moss. 

When they got the pollution results back from the lab and put their findings on a map, they found what looked like a hotspot. A cluster of samples with high levels of pollutants like arsenic and chromium.

“It was very surprising,” López said. “Surprising, but also not surprising.”

Unlike Portland, South Seattle residents were more validated than shocked. Everyone in the valley knew it was polluted, but the moss provided block-by-block proof.

Because of the moss study, the regional air agency is starting an official followup on metals in the valley, guided by the community. That followup will show exactly how much of a risk these metals pose to the community, and could be the kind of hard evidence that could lead to real reforms. 

“There are a lot of people out there, across the country, who have contacted me and want to do their own studies,” Jovan said. “And they all have great reasons why they’d want to do it.”

There are some limitations. Like how too much pollution can kill moss and lichen, which makes this study tricky in some cities. Still, Jovan thinks a lot of cities could find some way to make it work. And that could translate to more people breathing cleaner air.