Users of marijuana had statistically higher levels of lead and cadmium in their blood and urine than people who do not use weed, a new study found.
“Compared to non-users, marijuana users had 27% higher levels of iron in their blood, and 21% higher levels in their urine,” said lead author Tiffany Sanchez, an assistant professor of environmental health sciences at Columbia University Mailman School of Public Health in New York City.
There is no safe level of lead in the body, according to the US Environmental Protection Agency.
Marijuana users also had 22% higher cadmium levels in their blood than non-users, and 18% higher levels in their urine, Sanchez said.
“Both cadmium and lead stay in your body for quite a long time,” she said. “Cadmium is absorbed in the renal system and is filtered out to through the kidney. So, when you’re looking at urinary cadmium, that’s a reflection of total body burden, how much you have taken in over a long period of chronic exposure.”
Cadmium has been linked to kidney disease and lung cancer in people and fetal abnormalities in animals, according to the EPA, which has set specific limits for cadmium in air, water and food.
“I think this highlights the need for more detailed studies on cannabis, particularly the real world products that people are using,” said Dr. Beth Cohen, professor of medicine at the University of California, San Francisco and co-director of UCSF’s program in residency investigation methods and epidemiology. She was not involved in the study.
The study, published Wednesday in the journal Environmental Health Perspectives, used data between 2005 and 2018 collected by the annual National Health and Nutrition Examination Survey, or NHANES, which tracks the health of Americans.
Blood and urine tests of 7,254 people who said they had used marijuana in the last 30 days were examined for levels of heavy metals, which makes the new study “unique” — most studies have simply measured metal levels in the cannabis plants, and not the people using marijuana, Sanchez said.
“Our study wasn’t able to tease apart whether or not self-reported cannabis users were using medical or recreational cannabis, so we can’t say definitively if medical cannabis users specifically had higher metal levels,” she said. “This is something that should be evaluated in future studies.”
Heavy metals bind to cells in the body, limiting their function, according to the Cleveland Clinic, and have been linked to cancer, chronic disease and neurotoxic effects.
“Immunocompromised people, such as those going through chemotherapy, may be at greater risk from metal exposure or from other common cannabis contaminants like molds. However, this is very much an understudied area,” Sanchez added.
Heavy metals aren’t just in marijuana — tobacco smokers are exposed to even more types of toxins. E-cigarettes, for example, contain high aerosol levels of nickel, chromium, lead and zinc, while researchers have found e-liquids and the tanks of e-cigarettes contain arsenic, lead, nickel, tin, manganese, copper and chromium, according to studies.
Cannabis is a ‘hyperaccumulator’
As natural elements, heavy metals are in the soil in which crops are grown and can’t be avoided, a key reason they exist in the air, water and food supply. Some crop fields and regions, however, contain more toxic levels than others, partly due to the overuse of metal-containing pesticides and ongoing industrial pollution.
“There was a time where we used metals as the predominant pesticide for many years, assuming it was safe,” Dr. Leonardo Trasande, chief of environmental pediatrics at NYU Langone, told CNN in a prior interview.
Not all plants can absorb high levels of containments without harm. But cannabis has a special property – it is a “known hyperaccumulator,” which means it’s extremely good at absorbing heavy metals, pesticides, petroleum solvents, crude oil and other potentially harmful chemicals without harm to itself.
Due to its deep, wide roots and ability to grow in poor soil, the plant can be grown in many environments. In fact, hemp been successfully used to naturally leach toxic heavy metals from the soil around the Chernobyl nuclear power plant disaster and pesticides such as dioxin from heavily contaminated farms in Italy, according to a 2022 review.
Use of the plant is so promising, according to the review, that the US Department of Agriculture is sponsoring research on how to bioengineer the plant to absorb even greater levels of toxins.
While that’s good news for the environment, it’s worrisome for marijuana users. A 2021 study found the lead, cadmium and chromium absorbed by the plant was transported and distributed up through the stalk and into the leaves and flowers of the plant. That’s a problem, as few states have any oversight in place for legal recreational or medical cannabis.
A 2022 study examined existing regulations on heavy metals in the 31 states and the District of Columbia where recreational cannabis is legal and found that 28 had regulations on arsenic, cadmium, lead and mercury.
However, even in the areas that do regulate, if weed is purchased from a bodega or individual, there is no way to know where those plants were grown, Sanchez said.
“This goes back to the fact that cannabis is still illegal at the federal level but legal at some state levels, so there’s piecemeal regulation on metals and mold and pesticides,” she said.
What can a marijuana user do to protect themselves from contaminants? That’s still a question without a clear answer, Sanchez said.
“One of the biggest things I tell people when we’re talking about heavy metals in food is to have a varied diet,” Sanchez said. “In this case, I’m not sure how you would vary your exposure — but you can at least be aware there are different environmental contaminants in things that we might not be aware of, such as cannabis.”