A new report by the Clean Label Project has found that protein powders may contain something other than muscle-building nutrients: lead and cadmium, both of which are toxic.

The national nonprofit, which tests for various contaminants in products from wine and baby food to pet food and cleaning products, this time (like in 2018) focused on protein supplements—a $9.69 billion market in the U.S. as of 2023 that’s expected to more than double by 2033.

The report tested 160 products from 70 of the top-selling protein powder brands representing 83% of the market. Of the products tested, 47% exceeded one state or federal regulatory rule for safety, including California’s strict Proposition 65 safety thresholds for toxic metals.

Among those with the highest levels of heavy metal contamination on average, interestingly, were the organic products—which showed three times more lead and twice the amount of cadmium compared to non-organic products.

Plant-based protein powders also did not fare well, showing three times more lead than whey-based alternatives—while chocolate-flavored powders contained four times more lead than vanilla, aligning with other recent findings of heavy metals being present in cocoa-containing products.

“This study serves as a wake-up call for consumers, manufacturers, retailers, and regulators alike,” the report notes. “With the lack of comprehensive federal regulations specifically addressing heavy metals in dietary supplements, it is critical that the industry independently takes proactive measures.”

For the report, the Clean Label Project conducted 35,862 individual tests on contaminants including heavy metals (lead, cadmium, arsenic, mercury) and bisphenols (BPA, BPS), all of which can enter the products through environmental exposure, agricultural practices, and packaging materials. (Bisphenol levels were down significantly from its 2018 study results.)

The Clean Label Project did not respond to Fortune’s requests for more details about its methodology.

And because it does not offer many specifics in the report about its findings, some are urging caution.

Felicia Wu, Michigan State University distinguished professor of food science and human nutrition, notes that, while the heavy metal concentrations exceed California’s Proposition 65 recommended limits, “this report did not reveal what the actual numbers are, and the Prop 65 recommendations are for amounts per day—so it is unclear how much CLP assumed people were consuming protein powder every day.” She adds, “Without this more detailed information, we cannot say what the real risk to human health is.”

Further, the Council for Responsible Nutrition—which, notably, is the leading trade association representing dietary supplement manufacturers—has taken issue with what it calls the Clean Label Project’s lack of “critical context” and its risk of “misleading consumers rather than empowering them.”

In a rebuttal on its website, Andrea Wong, senior vice president of scientific and regulatory affairs, notes, “Modern analytical techniques can detect even trace levels of naturally occurring elements, such as heavy metals, which are present in soil, air, and water. These trace levels are often well below established safety thresholds set by federal agencies like the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA).”

Wong adds, “California Proposition 65, frequently referenced in CLP’s findings, imposes limits that are uniquely stringent and not aligned with federal guidelines… and it requires labeling of products over that amount, not a ban on them.”

Independent testing organizations, including both the Clean Label Project and Consumer Reports, have been raising concerns about heavy metals found in protein powders since at least 2010, according to a 2020 article in the journal Toxicology Report. But, that article urges, it’s important not to instantly panic, noting that “health risks of heavy metals in protein powder supplements should be conducted within the context of relevant background exposures and established health based standards instead of the presence of hazardous substances alone.”

What to know about lead and cadmium ingestion

According to the U.S. Food and Drug Administration, there is no known safe level of lead exposure. The very young (and in utero) are especially vulnerable to negative effects, which can include learning disabilities, behavior problems, and lowered IQ. Adults are prone to kidney dysfunction, hypertension, and neurocognitive issues from chronic exposure.

For exposure to cadmium, which is toxic to the heart and many other organs, there is a low threshold (0.21-0.36 mcg per kilogram, or per 2.2 pounds, of body weight per day), according to the FDA. Ingestion may cause stomach upset, nausea, and vomiting.

So where do the heavy metals come from? While they enter the environment “through natural processes like volcanic eruptions, weathering of rocks, and soil erosion,” accumulating in air, water, and soil over time before making their way into human food sources, the report explains, they can also become more concentrated from human industrial and agricultural activities.

“Lead may be present in food from the environment where foods are grown, raised, or processed,” explains the FDA. Levels of lead in the environment can vary depending on natural geographical makeup and proximity to current or past use or manufacturing of products made with lead.” For example, the FDA says, lead has entered the environment from the past widespread use of lead in products including paint, gasoline, and plumbing materials—and many have been phased out, but not all.

Cadmium, like lead, may be present in food from the environment where foods are grown, raised, or processed and can vary depending on past use of manufacturing with the metal and its effect on soil. It can be found at higher levels, for example, in areas where phosphate fertilizers are used and where processes such as smelting, mining, and burning of fossil fuels takes place.

The FDA monitors and regulates levels of lead in foods. But while supplements, including protein powders, are also regulated by the FDA, that’s typically done after they’re on the market.

So how to play it safest?

To conduct its tests, the Clean Label Project tested “160 of the top-selling protein powders, sourced from Nielsen and Amazon’s best-seller lists.” The report, however, does not namecheck any products tested and the top best-selling such products on Amazon changes frequently.

The report does list Clean Label–certified options, some of which are vegan, from the following companies: Genetic Nutrition (from the U.K.); Ritual; Oziva; Puori; Wellbeing Nutrition; Wicked Protein; Nutrabox; Gorilla.

You could also abstain altogether from protein powders, which most people do not need, experts recently told Fortune, preferring for protein to come from whole foods. And you’re likely getting enough protein, anyway, from food sources.