New Study Supports Testing Grass for Heavy Metals
In short: Legal cannabis is tested for heavy metals in most states. This is good because we know that weeds absorb metals from the soil. And now researchers have published a link between cannabis use and metal exposure in a group of smokers between 2005 and 2018. Read the details below.
What’s in McGraw’s 2023 Marijuana Metal Study?
Cannabis sativa is what scientists call a hyperaccumulator. Plants in this class, of which there are more than 700 (other members include sunflowers, barley and tobacco), accumulate metals from soil, water and fertilizers in quantities hundreds or thousands of times higher than average. However, a new study suggests that some of these metals may also be accumulating in the bodies of cannabis users – likely due to contamination in illicit markets.
Researchers at New York’s Columbia University searched a large database from the U.S. Centers for Disease Control’s long-running National Health and Nutrition Examination Survey to determine whether cannabis users had higher levels of 17 different metals in their blood or urine. They analyzed data from 2005 to 2018 from 7,254 participants who reported their diet, health, demographics and drug use, and provided individual blood and urine samples.
Researchers couldn’t say what type of grass was used, where it came from or where the participants lived. However, they considered other factors that influence metal exposure and excretion, including race/ethnicity, age, gender, education and seafood consumption.
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Why is it so important to test cannabis products?
In the study group, researchers found that cannabis-only users had, on average, blood lead levels 27% higher than non-users of cannabis and tobacco. Additionally, pure cannabis users had 21% more lead in their urine than those who abstained. Researchers also found elevated cadmium levels in cannabis users, even when controlling for tobacco use. Participants in the study who consumed only weed had 22% more cadmium in their blood than the average abstainer.
These results support why legal states require cannabis to be tested for metals. Failed batches must be destroyed or repaired, and states routinely issue recalls for any erroneously released products that fail subsequent safety tests. The streets don’t issue recalls. In June and July 2023, Oregon recalled flowers that tested hot for cadmium, mercury and arsenic.
Contamination with legal tobacco is worse than with illegal cannabis
The smallest amounts of lead or cadmium can harm human health. The U.S. Environmental Protection Agency considers all exposure to lead dangerous and has classified cadmium as a probable human carcinogen.
None of the other 15 elements examined in the study – including arsenic, cobalt, manganese and mercury – showed a clear causal link to cannabis use. And tobacco users fared significantly worse. For example, their urine cadmium levels were three times higher than those of exclusive cannabis users, while their blood lead levels were 26% higher.
Exclusive tobacco use was also associated with elevated levels of antimony, barium, tungsten and uranium. (Legal cannabis generally undergoes far more stringent testing than tobacco.)
Study “an enormous contribution”
The study appeared in late August in Environmental Health Perspectives, an open access journal published by the United States National Institutes of Health.
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“It stands out as one of the few to date that looks at real-world connections between cannabis use and exposure to environmental pollutants,” says Maxwell Leung, an assistant professor of pharmacology and toxicology at Arizona State University who has researched cannabis pollutants. but was not related to the new study.
“It is a tremendous contribution to our understanding of this public health problem.”
…2005 to 2018 represents a time when few Americans had access to cannabis that had been verified for safety by a certified testing laboratory.
Still, the relevance of the research to ongoing exposures is somewhat unclear. That’s because the study period from 2005 to 2018 represents a time when few Americans had access to cannabis that had been verified for safety by a certified testing laboratory.
Today, nearly half of Americans live in a state where adults can purchase legal weed that is tested for pesticides, solvents, microbes and metals. In many of these states — including California, the country’s largest legal marijuana market since sales began in January 2018 — cannabis flower can contain less than 0.5 parts per million (ppm) of lead and 0.2 ppm of cadmium.
But the fact is that most smokers still don’t buy from verified sources – and in some legal states, the illegal market reigns supreme. In the country’s largest market, California, an estimated two-thirds of cannabis dollars are spent in the illicit market, where growers do no testing.
Next steps: Test legal and illegal weed exposures
More research is needed to sort this all out, says Robert Thomas, a Maryland-based analytical chemist and cannabis testing consultant who was not involved in the research.
“Two of the most toxic elements known to mankind have appeared in the blood and urine of regular cannabis users. I think that tells us something,” he said. “But we need to look at more data, we need to look at different subsets, and we need to look at different states to see if there is a difference.”
Lead author Tiffany Sanchez, an assistant professor of environmental health sciences at Columbia University, said a follow-up study comparing metal levels among cannabis users by state is already in the works. Her group also plans to investigate metal contamination of untested CBD products.
“[Cannabidiol] is federally legal but is not regulated by the FDA or USDA,” says Sanchez. “So the big question I have is: How clean is it?”
Another new paper confirms that lead and cadmium are not the only contaminants that consumers of untested cannabis should be concerned about.
The report, published in August in the Journal of Cannabis Research, reports that of the 24 samples of illegal cannabis seized by Canadian authorities, 22 tested positive for pesticide residues. Police found 23 different chemicals in 22 samples, an average of 3.7 pesticides per sample – some of them in strikingly high concentrations.
…the 92% detection rate for illegal cannabis is shocking.
Maxwell Leung, assistant professor of pharmacology and toxicology, Arizona State University
However, of the 36 samples of licensed cannabis tested, only two contained chemical residues: a single hit each for the fungicide myclobutanil and the herbicide dichlobenil, both in very small quantities.
“This is another case of buyer awareness,” says Leung.
“I believe this is the only study in the literature to date that compares pesticide contaminants in legal and illegal cannabis. Although the sample size is small, the 92% detection rate for illicit cannabis is shocking.”
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