ANTHONY RICCIARDI, a scientist at McGill University, was looking for evidence that an invasive Asian clam had colonised a warm spot in the St Lawrence river when a member of his team made a more headline-worthy discovery. Peering through a microscope at sand scooped up from the riverbed, the student saw hundreds of tiny plastic spheres that stood out for their perfect roundness and unnatural colours.

“Ugelstad spheres”, named for the Norwegian scientist who invented them in 1976, are used in cancer research, HIV treatments and the manufacture of flat-panel televisions. Only in the past decade has the cosmetics industry discovered how useful they are for scrubbing teeth and faces. New Yorkers now rinse 19 tonnes of microbeads down drains each year. Too tiny to be caught by municipal water filters, they easily flow into the world’s oceans, lakes and rivers. In water they can break down, releasing toxins, or become coated with other poisons, such as PCBs.

Scientists had assumed that they floated in fresh water and were flushed downriver to the sea. Yet Mr Ricciardi has shown that some sink to the bottom of lakes and rivers, where they are eaten by bottom-feeding fish, which then develop diseases. Some of these fish, such as yellow perch, end up on dinner plates. Less is known about what happens to people who eat them. Dentists report finding the tiny orbs in patients’ gums.

This finding adds to alarm caused by new evidence of plastic pollution in the Great Lakes, which contain a fifth of the world’s fresh surface water. Sherri Mason, a chemist at the State University of New York in Fredonia, found 1m plastic particles per square kilometre in Lake Ontario. The Great Lakes have as much plastic debris as ocean gyres (currents), a team from the University of Waterloo found. Industry and governments are beginning to tackle it.

On July 30th Canada’s labour minister declared by the shores of Lake Ontario that microbeads will be considered a toxic substance. The government now plans to prohibit the manufacture, import and sale of “personal-care” products that contain them. Eight American states have already enacted bans, starting with Illinois in 2014. The Illinois ban does not apply to biodegradable microbeads, though their safety is unproven (it is unclear whether or not Canada’s will). California and New York are contemplating tougher restrictions. Four European countries, led by the Netherlands, are pressing the EU to prohibit their use in cosmetics.

Public pressure has forced some manufacturers to take action on their own. Unilever and Colgate-Palmolive have both stopped using microbeads; Proctor & Gamble and Johnson & Johnson say they will follow in 2017. Loblaws, Canada’s largest retailer, will remove them from their in-house brand a year later.

Unfortunately microbeads are not the only man-made materials wreaking havoc on underwater ecosystems. Microfibres from synthetic textiles—which come off in the wash and are also too small to be caught by water-treatment plants—are appearing in the digestive tracts of fish, too, says Mr Ricciardi. So brushing your teeth is bad for the environment, but doing your laundry could prove worse.

Correction: This article was changed slightly on August 5th, 2015, to reflect the fact that Colgate-Palmolive had already removed microbeads from its products.