by Robin Meadows
In 2019, the San Francisco Estuary Institute (SFEI) published a three-year study of microplastics in the San Francisco Bay that was―and still is―among the most thorough assessments of these tiny contaminants globally. The study was also groundbreaking. Much to the researchers’ surprise, microplastics that entered the Bay in runoff from storms dwarfed those from wastewater treatment plants.
“This was a paradigm shift for us,” says Ezra Miller, an SFEI environmental toxicologist who is part of a team working on microplastics in the Bay. “Most people’s gut reaction is that wastewater effluent is the main source.”
The team’s next steps include figuring out how microplastics get into stormwater, which will help identify strategies for keeping them out of aquatic ecosystems. Miller presented SFEI’s latest microplastics work at a recent symposium called Microplastic Pollution: Impact on the SF Bay Delta and Remediation Strategies.
Microplastics range from 5 millimeters―about the size of a popcorn kernel―to far too small to see with the naked eye. These minuscule particles are found just about everywhere, including in the oceans, polar ice, soil, the air we breathe, the water we drink, and the food we eat. Sources of microplastics in the environment include the tire wear particles that are generated by driving on asphalt; fibers from polyester and other synthetic textiles; and fragments of larger plastic items that break down when exposed to sun, wind and waves.
Wildlife can eat microplastics by mistake, displacing food. Microplastics can also carry chemical contaminants and pathogens, and harm aquatic organisms by, for example, blocking feeding structures, clogging gills, causing lacerations, and damaging cells.
Animal studies suggest that microplastics may harm people, posing a hazard to our digestive, reproductive, and respiratory systems, according to a 2023 report to the California State Legislature.
However, the thresholds for microplastics harm to aquatic ecosystems and human health are unknown. Understanding these risks is critical, especially as global plastics production is projected to triple by 2060.
To learn more, Robin Meadows spoke with Dr. Miller about SFEI’s findings on microplastics in the Bay Area, how to keep these contaminants out of the Bay, and the need to avoid solutions that fix one problem only to cause another. This conversation has been edited for conciseness and clarity.
How do microplastics get into the San Francisco Bay?
One of the key takeaways of our 2019 monitoring study is that stormwater runoff is the biggest contributor of microplastics in the Bay. People hadn’t really thought of that―it’s a huge gap in our understanding. Most stormwater flows into the Bay untreated and it’s a much larger concern than wastewater treatment plant effluent―we estimate that stormwater discharges 300 times as many microplastics as wastewater (7 trillion versus 17 billion per year, respectively).
Where do microplastics in the Bay come from?
Most microplastics in the Bay are either tire wear particles or fibers, which can come from cigarette butts and synthetic clothing. While rubber is a naturally-occurring substance, tire wear particles are considered to be microplastics because tires also contain many man-made compounds.
With the exception of tire wear particles, it’s very difficult to determine where microplastics came from. They’re hard to analyze because they’re so small. It’s a lot of manual labor to assess each piece under a microscope and then do chemical analysis to identify what it’s made of.
Size is not super indicative of the source because microplastics keep breaking down into smaller pieces. All we have to go on is the shape, primary polymer (like polyethylene or polystyrene), and color―and there are so many plastic products in our world that it’s very challenging to take this tiny particle and trace it back to its source. For many microplastics we just don’t know.
But there are some clues. If you go to where your clothes dryer vents outside, there’s probably a lot of lint there. There’s a lot of plastic in our lives, including in our clothes, and we think clothes dryers are one of the main sources of all those microplastic fibers in the landscape.
We’re doing a study of volunteer Bay Area households who installed rodent guards with fine mesh over their dryer vents to catch fibers. My family participated and the trapped fibers look just like what you clean out of the lint trap inside your dryer. Preliminary results suggest that about 14% of dryer lint being vented outside is plastic and that the average 3-person household vents about 3 grams of plastic fibers into the environment per year. That doesn’t sound like very much until you think about just how many households are in the Bay Area, each one doing laundry and venting plastic fibers into the environment.
What can we do to help keep microplastics out of the Bay?
We’ve found that rain gardens, which allow runoff to soak into the ground, are very, very good at removing microplastics. But you can’t install them everywhere. There isn’t always space, and they’re very expensive to build and maintain.
Microplastics remediation is the least effective and most costly approach because you’re still putting them into the environment. It’s more important to look further upstream at what is being emitted in the first place―source elimination is the most effective way of reducing microplastic pollution.
I’ve tried to reduce plastic in my own life and it’s hard. You can wear clothes made of natural fiber like cotton and wool, stop using takeout containers, and drive less. But it’s not possible to avoid plastic entirely. It’s a wicked problem.
What would you study next if you had unlimited resources?
While our 2016-2019 study has some of the most comprehensive data on microplastics in the world, it only captured the larger microplastics. I’d love to redo the study, capturing smaller particles. These are harder to monitor but they’re much more concerning―they can move more easily into cells and penetrate more deeply into organs.
Why should we tackle microplastics now even though the environmental and human health impacts are not fully understood?
Microplastics are really complicated. We think of them as one class of contaminants but really they’re infinite because they come in so many shapes, sizes and materials. There’s still a lot we don’t understand but we do know that they’re persistent and that we keep putting more into the environment. Even if we don’t yet understand the threshold for risk, we know we’re moving toward it.
What else would you like people to know about microplastics?
We should avoid regrettable substitution, which solves one problem but creates another. Cars were touted as a solution to the pollution problem of horse poop but we now know they cause other problems. Paper takeout containers sound good but they’re often coated with PFAS―toxic “forever chemicals”―for grease resistance. Is that better than plastic? No, it’s just a different problem. Now we’re inventing bioplastics, which may sound good but we don’t know how their microplastics release and toxicity compares to that of conventional plastics.
