A report by U.S. and Canadian officials concludes only about half of prescription drugs and other “chemicals of emerging concern” are removed from sewage by treatment plants. The findings come from the International Joint Commission, a group of officials studying the Great Lakes.

Better water treatment is needed, the report concludes. “The compounds show up in low levels – parts per billion or parts per trillion – but aquatic life and humans aren’t exposed to just one at a time, but a whole mix,” said study lead author Antonette Arvai. “We need to find which of these chemicals might hurt us.”

The researchers analyzed 10 years of data from wastewater treatment plants around the world, to see how effective they are at removing 42 compounds increasingly being detected in the Great Lakes, Scientific American reports. Six of the compounds were detected frequently, and had a low rate of removal in treated sewage. Five of the six were drugs: an anti-seizure medication, two antibiotics, an antibacterial drug and an anti-inflammatory drug. The sixth compound was an herbicide.

Diana Aga, a chemistry professor and researcher at the University of Buffalo who studies emerging chemicals in the Great Lakes, told the magazine that even without knowing the impact of the drugs in treated sewage, it is concerning to see antibiotics showing up. “Even at low levels you don’t want to have people ingest antibiotics regularly because it will promote resistance,” she said.