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.

“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.