What are Microplastics?

Microplastics are very small (generally less than 5 millimeters in size) plastic particles that can originate from a variety of sources, such as ingredients in cigarette filters, textile fibers, cleaning or personal care products, and dust from car and truck tires.


You may hear the terms “primary” and “secondary” microplastics. Primary microplastics are manufactured to be tiny in order to serve a specific function. Secondary microplastics come from the breakdown of larger plastic items.

A team of researchers, led by the San Francisco Estuary Institute and the 5 Gyres Institute, a nonprofit research group focused on reducing plastic pollution, set off to create an inventory to identify all the ways these different microplastics were getting into San Francisco Bay. They analyzed hundreds of samples from fish, sediment, surface water, wastewater and stormwater runoff and tried to trace the origins of all these particles.

One of many new findings, from the most comprehensive study to date on microplastics in California is that 

rainfall washes more than 7 trillion pieces of microplastics, 

much of it tire particles left behind on streets, into San Francisco Bay each year — an amount 300 times greater than what comes from microfibers washing off polyester clothes, microbeads from beauty products and the many other plastics washing down our sinks and sewers.

Driving is not just an air pollution and climate change problem — turns out, it just might be the largest contributor of microplastics in California coastal waters.