A pilot study by the San Francisco Estuary Institute, sampling east of Angel Island in Tiburon, detected microplastics as small as 20 microns — a fifth the size of prior Bay measurements — and may inform a California-wide monitoring expansion this summer.

The research vessel R/V Mike Riegle was anchored east of Angel Island last month while scientists from the San Francisco Estuary Institute calibrated the instruments they'd brought to find something they hadn't been able to measure before: plastic particles a fifth the size they'd previously tracked in the Bay.

The pilot study, published June 21 by SFEI as part of its Regional Monitoring Program for Water Quality in San Francisco Bay, pushed the lower detection limit from 0.1 millimeters down to 20 microns — roughly the width of a human hair. That's the smallest measurement of microplastics ever recorded in Bay monitoring. SFEI lead scientist Diana Lin said particles at that scale may account for the vast majority of the plastics present in Bay water.

"It's from the food that we eat, the air that we breathe," Lin told Bay City News. "It's in the dust from our carpets, from ripping plastic items where little particles get threaded." Tire-wear particles and clothing fibers are among the materials the new method captures.

The study's practical aim is standardization: SFEI science communicator Sierra Garcia said the work tests a reproducible way to sample sub-millimeter plastics in surface waters. California's Ocean Protection Council, a cabinet-level body, is set to launch a statewide plastics monitoring project this summer, and the new methodology could be folded into that effort.

What the Tiburon sampling found wasn't new pollution so much as a measurement gap — a reminder that the instruments we use to look at the Bay define what we think is there. Smaller particles have implications not just for human health but for coastal ecosystems and the Bay's capacity to pull carbon from the atmosphere.

The water east of Angel Island looked the same. It just isn't, quite.