When a car rolls down a freeway, a fine spray of microplastics spews out from its tires. When you wash your clothes, millions of tiny synthetic microfibers spill into waterways.

And those tiny pieces of plastic may be harming our health, a new study shows.

In a paper published Wednesday in the journal Environmental Science and Technology, researchers at the University of California at San Francisco evaluated dozens of studies in mice and humans to learn how microplastics may be harming digestive, respiratory and reproductive health. They found that these shards — which are now virtually everywhere in the air we breathe, the water we drink and the food we eat — are suspected of links to colon cancer and lung cancer.

"This systematic review is one of the most up-to-date assessments available right now," said Luís Fernando Amato-Lourenço, a postdoctoral researcher at the Free University of Berlin who studies microplastics in the body and was not involved in the study.

Scientists are racing to understand the health impacts of microplastics, which have been found in the testicles, placenta, lungs, and numerous other organs in the last few years. Production of plastics is also rapidly increasing — supply of the materials, which are mostly made from fossil fuels, has doubled since the early 2000s and is expected to triple by 2060.

Still, there are few studies that look at how microplastics have harmed people's health. Research this year found that patients with more microplastics in a key artery were more likely to suffer heart attack, stroke or death from any cause.

To assess the risks to humans, the researchers examined thousands of studies, largely in mice, that evaluated microplastic exposure and its impacts on three bodily systems. They looked for evidence of changes in the colon and lungs, as well as signs that the microplastics were having carcinogenic, or cancerous, effects. Those included chronic inflammation and oxidative stress, which is an imbalance of antioxidants in the body.

"These are basically biological mechanisms that have already been shown to be part of the link between chemical exposure and cancer," said Tracey Woodruff, professor and director of the UCSF Center for Reproductive Health and the Environment and one of the paper's authors.

The scientists concluded that there is evidence linking microplastics to lung cancer and colon cancer, as well as some reproductive problems.

The study is just one step in a long process to establish what, exactly, microplastics do to human health. It's a knotty problem — there are dozens of types of plastic, which can break into different shapes when exposed to the sun and wind. At the same time, those plastics can include thousands of chemical additives — each of which could also have an effect on health.

Still, scientists suspect microplastics may be linked to some diseases that have been increasing in recent decades. Double the number of adults under age 55 are being diagnosed with colon cancer today compared with a decade ago. The rate of lung cancer among nonsmokers is also rising.

"It's important to focus on health effects that we see increasing in the population," Woodruff said.

There are limitations to the current research. Most of the studies included in the review looked at a particular type of microplastic — spherical beads that are easier for researchers to test and use in the lab. But experts know that most microplastics in the environment are shards or fragments that have sloughed off of plastic items over time. Studies in mice also tend to use higher doses than humans may be exposed to, although it is difficult to create an exact 1-to-1 ratio.

Some researchers not involved in the new study praised it as a significant advance. "The conclusion about microplastic exposure being a 'suspected' hazard to human digestive, reproductive, and respiratory systems represents a stronger stance than many previous reviews in my opinion," said Amato-Lourenço.

But others worried that its findings were premature. "We are still learning about the nature of the plastics in the human body and there is a lot of published research based on shaky assumptions," Matthew Campen, a professor and toxicologist at the University of New Mexico, said in an email. "I am not really critical of those papers, because we have to start somewhere, but using data from those studies to assess human health is just too soon."

"Scientists agree microplastics are a complex issue and work to establish protocols to appropriately characterize the microplastics they are testing," said Kimberly Wise White, vice president of regulatory and scientific affairs at the American Chemistry Council, a plastics industry group. "Unfortunately, this rapid review does not appear to include these considerations, calling into question the validity and quality of the conclusions."

There are few laws around the world that protect consumers from microplastics in food, water and air. A growing number of scientists have been pushing for action from policymakers, urging governments to treat plastics as potentially harmful.

But progress has stalled; last month, negotiations for a global plastics treaty that might have curbed production collapsed in Busan, South Korea.

"The government waits," Woodruff said. "And while we're waiting, exposures are not only continuing but are projected to increase."