Your post-workout protein shake may contain some hidden dangerous ingredients. A new report found that a large proportion of over-the-counter protein powders contain potentially unsafe levels of lead and other toxic metals.

Researchers at the non-profit organization Clean Labels Project conducted the studywhich tests several brands of consumer protein powder. Of the 160 products tested, 47% had levels of lead, cadmium, and other toxic metals that exceeded the safety threshold established by Proposition 65, a 1986 law passed in California that regulates exposure to over 900 substances within the state. Organic, plant-based, and chocolate-containing protein powders are more likely to contain these metals than other brands, according to the report.

Although lead exposure in general has decreased significantly since the 1970s (thanks in large part to the phasing out of lead-infused gasoline), there are still some consumer products that are known to have higher risk of contamination. A study last summerfor example, found that nearly half of all cocoa products, especially dark chocolate, may contain unsafe levels of lead. The Clean Labels Project researchers decided to test several different protein supplement products for lead, cadmium, and other metals.

The researchers purchased and tested 160 protein powder products from 70 different brands, which reportedly accounted for 83% of the market. In addition to the 47% of products that exceeded Prop 65 levels of lead and other metals, 21% were found to have lead levels twice as high as the Prop 65 limit. About 65% of chocolate-containing powders have high levels of lead; 77% of plant-based powders contain high levels of lead; and 79% of organic powders contain high levels of lead, including 41% of products with lead levels twice as high as the Prop 65 limit. These types of products are more likely to contain cadmium than other brands.

“The study’s findings highlight that despite the growing health-conscious market, many products may contain high levels of contaminants that do not appear on traditional nutrition labels,” the authors wrote in the report. The report declined to list specific brands found to contain lead and other metals, although it noted that whey and collagen-based powders are less likely to be contaminated with them.

The safety standards for lead and other substances established by Prop 65 are stricter than those used by other organizations, including the Food and Drug Administration. Some research has is also suggested that the average exposure to heavy metals from the consumption of protein powder supplements is unlikely to cause people adverse health effects, such as a higher risk of cancer. At the same time, no one true safe level of lead exposure, and lead exposure early in life is known to increase the risk of developmental issues. And the report’s authors say their report is intended to serve as a “wake-up call for consumers, manufacturers, retailers, and regulators.”

There is at least one silver lining to the findings. Levels of bisphenols—hormone-disrupting chemicals believed to negatively affect people’s health—in protein powders have dropped dramatically over the years. Only about 2% of products (3 out of 160) tested positive for these chemicals in this latest report, compared to 55% of products the organization tested as part of the same report last year. 2018.