NORTH EAST, Pa. — The newest environmental threat to the Great Lakes is very, very small.

Tiny plastic beads used in hundreds of toiletries like facial scrubs and toothpastes are slipping through water treatment plants and turning up by the tens of millions in the Great Lakes. There, fish and other aquatic life eat them along with the pollutants they carry — which scientists fear could be working their way back up the food chain to humans.

Scientists have worried about plastic debris in the oceans for decades, but focused on enormous accumulations of floating junk. More recently, the question of smaller bits has gained attention, because plastics degrade so slowly and become coated with poisons in the water like the cancer-causing chemicals known as PCBs.

“Unfortunately, they look like fish food,” said Marcus Eriksen, executive director of the 5 Gyres organization, speaking of the beads found in the oceans and, now, the lakes. His group works to eliminate plastic pollution.

Studies published in recent months have drawn attention to the Great Lakes, where there may be even greater concentrations of plastic particles than are found in oceans. The National Oceanic and Atmospheric Administration has also been looking at the impact of microplastics on marine life.