Supplements that promise healthier skin and other benefits are very popular right now, but a new report says some of them contain heavy metals that could be harmful to users.

The study by the Organic Consumers Association and Clean Label Project looked at nearly 30 of the top-selling collagen supplements sold on Amazon.com. It found the products contained a wide range of levels of heavy metals, including arsenic, lead and mercury.

The Organic Consumer Association’s associate director, Katherine Paul, said the research raises concerns about where these supplements are coming from.  

“These collagen products essentially are sourced from industrial factory farms,” she said, “and that’s the source of the heavy metals.”

In some cases, only trace amounts of metals were detected, but the report authors say there could be harmful side effects for people who use these products on a daily basis for long periods of time. In some, the amounts of cadmium exceeded safety levels for products sold in California. The groups said the findings should prompt other states to adopt health standards for heavy metals.