The most comprehensive study to date of microplastics in California has turned up a mind-boggling amount of plastic particles in the San Francisco bay.

An estimated 7tn pieces of microplastics flow into the San Francisco bay via stormwater drains alone, researchers discovered. Nearly half of the microscopic particles found in stormwater looked suspiciously like tiny fragments of car tires, which rainfall washes off the streets and into the ocean.

Treated wastewater contributed an additional 17bn particles of plastic, according to the study. Researchers also found plastic in sediment collected from the bay and its many tributaries and inside the digestive tracts of fish.

“It was basically everywhere we looked,” said Rebecca Sutton, an environmental scientist at the San Francisco Estuary Institute, a local institution that led the three-year, $1.1m research effort.

The findings fit with other recent studies that detected microplastics within the deepest reaches of the ocean, flowing through UK lakes and rivers and permeating US groundwater. Scientists have also found plastic in remote regions like the Pyrenees and the Rocky Mountains, suggesting these particles can travel with the wind for hundreds, if not thousands, of kilometers.

What surprised Sutton was the fact that her study of the Bay Area watershed turned up higher concentrations of microparticles than similar studies done in Europe, Japan and China.

That may be because the San Francisco bay – which has only one narrow opening into the ocean – keeps much of the plastic that flows into it trapped, Sutton noted. “Though it’s difficult to directly compare”, she said, as different studies used different methodologies to collect and analyze samples.

Study of the Bay Area watershed turned up higher concentrations of microparticles than similar studies done in Europe, Japan and China. Photograph: Courtesy of 5 Gyres

In this case, researchers accounted for two types of debris: microplastics, which are particles identified by a laser as fragments of plastic, and microparticles, which are particles the researchers suspect to be plastic, but couldn’t identify as such with the laser.

The fragments and fibers they analyzed included the remnants of plastic packaging and bottles, microscopic shreds of cigarette butts and fibers from clothing. Nearly half the particles found in stormwater were “these squishy black particles that we think might be from tires”, Sutton said. “But it’s really hard to get a definitive sense of where exactly it’s all coming from because there are so many sources of plastic pollution.”

The study is “extremely comprehensive”, said Stefan Krause, a microplastics researcher at the University of Birmingham. Even as scientists and engineers strive to clean up the hundreds of thousands of metric tonnes of the plastic debris floating on the ocean’s surface, it is becoming increasingly clear that “we have thousands and thousands and thousands of times more plastics that we can’t see”, Krause said.

“It can be overwhelming at times to know how to address the issue, but we have to focus on what we can change,” said Carolynn Box of 5 Gyres, a not-for-profit group that worked with researchers in the Bay Area to develop policies to stem the flood of plastic into the ocean.

Recommendations included phasing out single-use plastics completely – which the city of Berkeley, California, just east of San Francisco, has already committed to doing by next year. 5 Gyres also recommended building more rain gardens – which are patches of soil near roadways that collect and filter rainwater before it flushes into the sea.

“As scientists learn more about plastic pollution, I think that’s going to help us come up with more, better solutions,” Box said.

A big unknown is how exactly breathing and eating microplastics affect living beings. In August, the World Health Organization released a report that found – above all else – that scientists desperately need to better understand the health effects of plastic. “We urgently need to know more about the health impact of microplastics,” said Maria Neira, the WHO’s director of public health, in a statement. “Because they are everywhere.”