When Colorado and other states began legalizing marijuana more than a decade ago, they faced a problem: how to guarantee a safe product, with little data about the long-term risks of pesticides and other contaminants.

“‘Safe’ is a funny word,” both because what is harmful to one person may not be for another, and because people vary in their risk tolerance, said Jeff Raber, CEO of marijuana consulting company The Werc Shop and an instructor in the University of Denver’s cannabis program.

The same amount of a chemical in a batch of cannabis could be benign or harmful, depending on the size of the person using it, how often they smoke and how much they take at one time, among other factors, he said.

Colorado requires growers to test their harvest for yeast, mold, aspergillus (a type of fungus), E. coli and some other bacteria, pesticides and heavy metals, such as lead. Manufacturers also have to test for residual solvents and chemicals used to create their products.

Regulators had to work with limited existing information on the possible risks of pesticides and contaminants in marijuana, because few, if any, studies have examined what levels might be safe in a smokeable or vapeable product, Raber said. Rules for tobacco aren’t a particularly helpful starting place, since growers use different pesticides, he said.

“With edibles, we at least can fall back on food safety standards,” he said.

A 2013 study that Raber co-authored found that significant amounts of pesticides could pass through water pipes or glass pipes to the user. Filtration reduced the amount the user could have inhaled, though some residues still made it through. The study predates states’ current pesticide limits, though, so the risk to people using regulated cannabis now could be lower, Raber said.

Colorado based its updated 2023 pesticide regulations on rules in place in Canada, after a group of researchers and stakeholders considered various sets of standards used in other places with legal marijuana.

Colorado’s Marijuana Enforcement Division referred questions about the standards used for different contaminants to the Colorado Department of Public Health and Environment, which said it couldn’t comment.

Ideally, more studies would settle what chemicals are risky in smokeable marijuana and whether any cannabinoids offset some of that risk, but most research is still trying to sort out how the plant itself affects people who use it, Raber said. And, of course, federal law limits researchers’ ability to grow and study cannabis, though Colorado scientists have ongoing projects looking at pesticide residues and heavy metal contamination.

The lack of direct data matters because not everything present on the bud makes it into the user’s body, said Mark Lefsrud, an associate professor who studies medical cannabis at McGill University in Canada.

For example, contamination with heavy metals would cause a problem in a concentrated or edible product, but lead and cadmium don’t turn into particles that users can easily inhale when smoking, he said.

“As a recreational consumer, I’d say it’s very low odds” that heavy metals in a smoked product would be dangerous, Lefsrud said.

The same goes for E. coli, which doesn’t fair well when set on fire, Lefsrud said. E. coli outbreaks periodically sicken and occasionally kill consumers — most recently, when contaminated onions showed up in McDonald’s Quarter Pounder hamburgers — but the state of Colorado hasn’t had any marijuana recalls attributed to bacteria since at least 2020. (Two of 61 recalls in those years mentioned unspecified microbial contamination, which could refer to bacteria or fungi.)

The biggest risk for an average marijuana consumer is from fungi, Lefsrud said. People with lung diseases or compromised immune systems can become seriously ill or die from inhaling mold spores, but even generally healthy people are at risk from toxins that aspergillus and other types of mold generate, he said.

Colorado allows growers to kill excess fungus on their product, but that process doesn’t destroy any toxins that the mold has already produced. Just because a bud looks and smells normal doesn’t mean it couldn’t contain toxins, Lefsrud said.

“In most cases, it’s the things you don’t see” that are dangerous, he said.

The state does require testing if a batch fails mold testing and the grower wants to remediate it for use in products such as concentrates, according to the Marijuana Enforcement Division. The division hasn’t issued any recalls or taken any actions against marijuana businesses over products that failed toxin testing.

States differ in what kind of contaminants they regulate and the limits they set, but overall, they erred on the side of caution, Raber said. Ultimately, they had to make judgment calls based on imperfect evidence, like American and European food regulators who came to different conclusions about whether certain dyes are OK to eat, he said.

“It’s an evolving picture, but it’s evolving to get better,” he said.

States are relatively well-positioned to catch if a batch makes consumers sick in the short term, Raber said. Effects from long-term use will be tough or impossible to sort out, though, because most people use multiple products, and other differences between groups of people make it hard to pin down how much to attribute to cannabis, let alone to pesticides used on it, he said.

Raber works in the cannabis industry and isn’t interested in telling adults not to smoke, but said people should understand the uncertainties around safety.

Consumers can somewhat reduce their risk by switching up the products they use — so they don’t consistently expose themselves to a contaminant that happens to be high on one type of marijuana — and by not overdoing their use, he said.

“I think that’s the best you can do today,” he said.

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