That's the takeaway from a new report by the Clean Label Project, which was released on January 9.
"The study's findings highlight that despite the growing health-conscious market, many products may contain elevated levels of contaminants not reflected on traditional nutrition labels," the report says.
So, which heavy metals were found -- and which protein powders are safe? Here's what a food safety expert wants you to know.
Meet the expert: Felicia Wu, PhD, is a professor of food safety, toxicology, and risk assessment at Michigan State University.
Lead and cadmium were found in a majority of the protein powders that were tested. There is no safe level of lead, which has been linked to developmental disorders and high blood pressure.
And chronic cadmium exposure can damage the kidneys, bones, and lungs, as well as raise your risk for certain types of cancer, according to the Occupational Safety and Health Administration (OSHA).
"Lead is a known neurotoxin, and cadmium has been shown to cause harmful effects to bones and kidneys," says Felicia Wu, PhD, a professor of food safety, toxicology, and risk assessment at Michigan State University.
The January 9 report only shared results for lead, cadmium, BPA, and BPS testing. Data on other contaminants will be released in a later update, Clean Label Project executive director Jaclyn Bowen told CNN.
The good news? BPA and BPS were only found in three of the 160 protein powders that were tested.
Unfortunately, the report did not include the brands or product names. "We do not disclose the names of companies we test in order to maintain fairness and consistency and to avoid potential conflicts of interest," Bowen told CNN.
According to the report, 77 percent of plant-based protein powders, 79 percent of organic protein powders, and 65 percent of chocolate-flavored protein powders tested over the California Proposition 65 safety thresholds for toxic metals.
Plant-based protein powders -- made from rice, peas or soy -- were found to contain triple the amount of lead compared to whey protein powders, which are made from the liquid byproduct of cheese, per the report. Meanwhile, chocolate-flavored protein powders contained four times more lead and up to 110 times more cadmium than vanilla-flavored powders.
Researchers tested 160 products from 70 top protein powder brands, which represented 83 percent of the market, the report says. For the study, an independent certified laboratory ran nearly 36,000 tests on 258 different contaminants.
The Clean Label Project has also released a list of "the best" protein powders "based on our rigorous product testing." It includes brands like WICKED Protein, Ritual, and Needed., and can be found here.
It's tough to say, Wu says. The California Proposition 65 safety recommendations are for amounts per day, but it's unclear how much protein powder the Clean Label Project assumed people were consuming each day, she points out.
"Without this more detailed information, we cannot say what the real risk to human health is," Wu says.
However if you're concerned, the data suggests that whey- or egg-based and vanilla-flavored protein powders contain have the least amount of heavy metals. If you follow a plant-based diet, vanilla-flavored protein powders made from peas appear to have the least amount of heavy metals.