SAN FRANCISCO (KRON) -- Some types of cancer may be caused by microplastics drifting through the air we breathe, San Francisco researchers said. A major source of plastic in the air is from vehicles driving on roadways. Friction wears down tires that shed tiny pieces of plastic into the air, creating a type of pollution that may cause serious illnesses, according to a study published by UC San Francisco researchers on Wednesday. Microplastic particles can contribute to health problems including colon cancer, infertility, and chronic pulmonary inflammation that increases the risk of lung cancer, the study states. "These microplastics are basically particulate matter air pollution, and we know this type of air pollution is harmful," said Tracey Woodruff, a professor of obstetrics, gynecology and reproductive sciences. Woodruff is the senior author of the study, which appeared in Environmental Science & Technology Journal. Microplastics are less than 5 millimeters – smaller than a grain of rice – and they are ubiquitous in the environment. Each year, companies around the world produce nearly 460 million metric tons of plastic. “We urge regulatory agencies and policy leaders to consider the growing evidence of health harms from microplastics, including colon and lung cancer,” said researcher Nicholas Chartres.