Science & tech

Researchers Ask: Is Cannabis Contributing to Denver’s Smog?

Published on July 31, 2019 · Last updated July 28, 2020
Denver is one of America's smoggiest cities. Now researchers want to know if cannabis grows are unwittingly contributing to the problem. (IlexImage, pleshko74/iStock)

Could Colorado’s legal cannabis be contributing to Denver’s air pollution problem, even before it’s smoked?

Colorado’s capital city has the nation’s 12th worst air quality, according to the American Lung Association’s latest State of the Air report. The cities were ranked based on ozone pollution, aka smog, which can lead to breathing problems, cardiovascular harms, and premature death.

Cannabis terpenes aren't pollution. But when they evaporate they free up an ingredient in the formation of toxic ozone.

Here’s where cannabis may come in. Cannabis plants produce terpenes, the aromatic oils that give cannabis its smell and differentiate flavors across strains. Those terpenes are classified as volatile organic compounds, or VOCs, because they evaporate easily and release molecules into the atmosphere.

Plant-based VOCs are released by an enormous variety of vegetative species. They’re not pollution in and of themselves. But the molecules they release can provide an ingredient necessary for the production of ozone smog. Ozone forms when nitrogen oxides (NOx) from sources like power plants and gas-powered vehicles mix with VOCs and react with the heat of the sun.

What are cannabis terpenes and what do they do?

One of Many Sources

Cannabis plants are but one of many, many sources of VOCs. But it stands to reason that the more than 600 grow facilities within the Denver city limits may be playing some role in the city’s ozone problem. The city has lots of nitrogen oxide from cars and industrial sources, which means that any additional VOCs make for more ozone.

The fact that lots of cannabis facilities are near highways—because of where light industrial districts are zoned—potentially adds to their impact. Plenty of other agricultural crops produce terpenes, but not many are sitting right next to concentrated sources of tailpipe exhaust.

11 of the 15 smoggiest cities in America are located in legal adult-use states. Most are in California.

Nobody knows just how many and what kind of VOCs cannabis farms are putting out, which means it’s impossible to know what role the industry plays. But Colorado state officials are interested in finding out. So they’ve mounted a study of four commercial cannabis grow facilities to get the first robust data on the question.

Researchers believe they’ll offer state officials the first glimpse of what kind of emissions the industry creates—and what growers can do to limit their impact.

The study could have wide-reaching impact. Of the 15 most polluted cities in America, 11 are in legal adult-use states. (Nine are in California; Las Vegas is 13th on the list, right behind Denver.) So if Denver were to take regulatory action based on the study’s findings, other cities and states might follow its lead.

A Fixable Situation

The good news, says Kaitlin Urso, is that if cannabis-produced VOCs are found to be a problem, an easy solution should be within the industry’s reach. Urso is the project lead within the Colorado Department of Public Health and Environment’s small business assistance program, which helps cannabis companies reach sustainability goals. The same carbon filters that control odor, she says, can also trap VOCs before they form ozone.

“The industry understands odor control very well at this point,” says Urso. “What we can do is bridge the gap to say the odor isn’t just a nuisance, it’s contributing to pollution. So we’re not trying to control the odor just for the sake of the odor.”

Testing a Greeley Grow

Smokey’s 420, a mid-size medical and adult use company with locations in Ft. Collins and Garden City, is one of four participants in the study. Management at Smokey’s allowed Leafly to observe testing at their grow facility near Greeley.

Smokey's 420 in Ft. Collins volunteered to have its grow rooms tested. 'We want to know what's going on, good or bad,' said master grower Scott Brady.

Urso and partners from the Desert Research Institute (DRI) work with their industry partners to take live samples throughout the facility, covering plants before and in harvest and during the drying and cutting stages. The idea is to be able to account for terpenes throughout the full growing process. A facility with thousands of plants at harvest will have a different terpene production profile than one where new growth is starting. The profile can also differ across strains.

The researchers use tubes with a filter media that can trap the VOCs like flypaper. They take 10-minute exposures in different rooms, and will analyze them later using equipment back at the DRI lab. The full results won’t be available until the fall, but early testing has already shown some interesting results. Drying and cutting rooms, for example, have shown higher terpene levels, the theory being that agitating the leaves releases more VOCs.

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A Supportive Industry

Urso said the industry has been largely supportive and participatory in the study, a necessity to get the most accurate data possible.

Scott Brady, master grower and grow operations manager at Smokey’s 420, said that opening the operation’s doors to the state fit in with the company’s commitment to social responsibility. The company already has adopted a living soil technique to minimize waste and seeks out organic materials wherever possible. Participating in the state study followed that mission.

“Whether it’s good or bad news, it’s important for us to know what’s going out there,” says Brady. “If we’re doing something that’s harmful, we want to take an honest look at it.”

Previous Studies

A handful of other researchers have sought to measure VOCs, but the Colorado study will have by far the most robust data.

One previous project, a partnership between the University of North Carolina and the University of Colorado, saw researchers growing cannabis plants and taking air samples directly off them to estimate per-plant VOC rates. But those researchers acknowledged that their study, which involved plants grown in a garage under less-than-ideal conditions, wasn’t capturing the terpene volume that commercial-size grows would produce.

Likewise, samples taken outside facilities can pick up VOCs from cars, smokestacks and even outdoor plants, making it hard to determine the industry’s impact.

No Federal Regs

There are no federal regulations on the books dealing with VOCs from cannabis, because growing the plant remains federally illegal under the Controlled Substances Act. Also, most legal states consider cannabis farming an agricultural activity, and agriculture is exempt from federal air rules. So there’s not much the state can do to force compliance even if the VOC levels are off the chart.

Instead, Urso plans to use the findings as an educational tool, potentially using the data to find agreement on voluntary guidelines about how best to manage carbon filters or adjusting operations on high-ozone days.

The state has already worked on voluntary sustainability programs to help reduce energy use and waste in the cannabis industry. Urso says that even if cannabis is helping fuel more ozone air pollution, the industry should be able to help limit it on its own.

“What we’ve found is that if you teach companies and give them the best management strategies, they will take advantage of it, and they’re willing to participate,” she says. “We get so much further with a voluntary program than we ever would have with a regulatory mandate.”

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Jason Plautz
Jason Plautz
Jason Plautz is an award-winning journalist based in Denver, covering the environment, politics, and the intersection of the two.
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