The next time pot smokers think about lighting up, they may want to consider the findings of a new study from Columbia University's Mailman School of Public Health. It found significant levels of harmful metals in blood and urine samples among marijuana users.

The research, published in the journal Environmental Health Perspectives, analyzed data from samples of more than 7,200 adult participants. The samples were collected between 2005 and 2018 as part of the National Health and Nutrition Examination Survey, a program conducted by the National Center for Health Statistics.

Exposure to heavy metals can lead to several serious medical issues ranging from cancer to brain damage.

The study's volunteers were broken up into four categories. Those who:

  • Exclusively used marijuana
  • Exclusively used tobacco
  • Used both substances
  • Used neither substance

The results? Compared to those who said they didn't use either marijuana or tobacco, the 358 people in the study who reported using marijuana within 30 days of the sample collection were found to have blood levels of lead that were 27 percent higher than non-users and cadmium levels that were 22 percent higher.

Dual users of both tobacco and marijuana, as well as those participants who only used tobacco, also had high levels of cadmium and lead in their blood and urine. This finding was less surprising, since it's been known for decades that these toxic metals are part of the dangers of smoking cigarettes.

The toxic effects of heavy metals are long-lasting.

Exposure to heavy metals can lead to several serious medical issues ranging from cancer to brain damage. For instance, breathing high levels of cadmium damages people's lungs, and exposure over time can also build up in the kidneys causing kidney disease and fragile bones. In fact, cadmium is so toxic that it's considered a human carcinogen by the World Health Organization.

High levels of lead can cause brain and central nervous system damage. Kids exposed to lead can develop intellectual disabilities and behavioral issues.

Unfortunately, the cannabis plant is particularly skilled at absorbing these toxic elements from its surrounding environment — from the air, water and earth. On top of it, the toxic effects of heavy metals are long-lasting.

“For both cadmium and lead, these metals are likely to stay in the body for years, long after exposure ends,” said Tiffany Sanchez, an author of the study and an assistant professor of environmental health sciences at Columbia's Mailman School of Public Health.

The data didn't distinguish between how the marijuana was consumed, either by edibles or smoking, but Sanchez pointed out that, generally speaking, inhaling lead is worse than eating it in food because, “The absorption rate from inhalation is 100%.”

Marijuana is the third most commonly used drug in the world behind tobacco and alcohol. Right now, marijuana is legal for recreational use in 21 states and for medical use in 38 states. Since laws vary from state-to-state, there are no standard rules for contaminants.

So how can you tell if the weed you're smoking contains heavy metals?

The best route is to buy your pot products from a state's legal dispensary. Health department websites are likely to offer a list of legal dispensaries. Once you're in the shop, you can ask one of its staff for a “Certificate of Analysis.” This should show if there are any heavy metals in the product you're buying. The rest is up to you.