There is no federal oversight when it comes to testing cannabis. Decisions about testing thresholds are often left up to states, which lean heavily on professional organizations and nonprofits to set standards. 

Cannabis testing standards are derived from a handful of groups such as the Association of Official Agricultural Chemists (AOAC), the US Pharmacopeia, and the American Herbal Pharmacopoeia. 

“A lot of regulators tend to rely on them because some of the regulations around herbal supplements are about as close as we have for cannabis right now,” said Greg Newland, founder and chief science officer of lab Nova Analytics in Maine. “Though the guidelines for herbals do provide some reference to the closest thing that we really have, it is just really just a base and a starting point for a lot of these discussions”

Despite what one would expect from a patchwork of standards, not much varies state to state. But that’s not true for the threshold of lead, one of many cancer-causing metals, which are absorbed by cannabis plants through water and the environment. 

Newland said that in some cases, the levels for metal in cannabis plants are higher than what’s allowed in drinking water.

“Part of the reason that I think the levels are a little bit higher is simply because cannabis is a great absorber of those contaminants from the soil. And as you use even somewhat clean water to feed your plants, the plant will accumulate those heavy metals in its growth cycle. Then levels will actually increase over the time of its life to the point where the levels would be higher than what you would see in the drinking water,” Newland said. 

“Many, many cultivators would have a hard time passing a lot of the state's requirements for cannabis testing for heavy metals if they don't have extremely clean water, and this is why a R.O. [reverse osmosis] system is often used for most water systems and large cultivation facilities to ensure that all those heavy metals are scrubbed from the water before being fed to the plants.”

The cannabis industry mirrored a lot of the standards from the food industry as it got started, said Dr. Michele Glinn, chief science officer for Viridis Labs in Michigan. 

“They needed to have somewhere to start because it's so new. So 0.5 and 1 [parts per million] are the typical thresholds that states have started with and that's what Michigan is using as well. That could change in the next few years. But right now, that's where we're going with,” Glinn said.

Some health advocates say any amount of lead is unsafe, especially when it comes to water consumption. The Environmental Protection Agency (EPA) has set the “maximum contaminant level goal” for lead in drinking water at zero.

“[L]ead is a toxic metal that can be harmful to human health even at low exposure levels. Lead is persistent, and it can bioaccumulate in the body over time,” according to the EPA.

Regulated cannabis testing standards vary in the Midwest, mid-Atlantic, and New England regions, but most states’ lead limit is around 0.5-1 ppm.

There are exceptions, though. Connecticut has a lead limit of 0.00029 ppm, while New Hampshire’s limit is 8.7ppm – 30,000 times greater. 

The New Hampshire Department of Health and Human Services, which oversees the state’s medical cannabis program, did not respond to inquiries by publication. 

This issue of testing metals is longstanding. And there are some proposed solutions. 

The National Organization for the Reform of Marijuana Laws (NORML) wrote in a 2022 report titled Marijuana Policies in Legal States, “An important feature of regulated cannabis programs is quality assurance of products… [F]ederal officials may wish to consider recommending or imposing minimum national requirements for cannabis testing labs via a nationally recognized accreditation board. These nationally imposed standards would benefit consumers by reducing the chances that tainted or adulterated products make it onto shelves.”

Glinn said, “The country as a whole is trying to move to some kind of consensus as to what those [standards] should be. Michigan is one of those states and as more information comes in the standards are somewhat evolving, but that's to be expected in a new industry.”

Correction: A previous version of this article reported Ohio’s testing threshold was 0.00029 ppm, as written in the administrative code. However, state regulators have told Grown In it is 0.87 ppm.

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Cynthia Fernandez is a data reporter for Grown In. Previously, she was a politics reporter for Spotlight PA, a nonpartisan newsroom based in Harrisburg and reported at the Boston Globe. In 2019 she graduated...