If you’re in the vicinity of a commercial cannabis grow you’ll smell it long before you see it. From a car, from the street, even from blocks away, the dank and potent aroma of flowering cannabis is sharp, unmistakable and unignorable—especially when it’s being grown at a near-industrial scale.
What your nose is picking up are calledterpenes. These chemical compounds are naturally occurring in plants and are responsible for their specific aromas, flavors and, sometimes, their colors. Limonene is found in lemons, pinene is found in pine trees and caryophyllene is in rosemary, cloves and hops. Cannabis has 400 known terpenes, and throughout nature scientists have discovered more than 20,000 of them.
But terpenes aren’t just fragrant organic compounds, they’re also volatile organic compounds (VOCs). Meaning they react with nitrogen oxide in the air to create ozone—a greenhouse gas that absorbs both ultraviolet and infrared radiation, significantly contributing to rising temperatures. VOCs like isoprene (another terpene) create more ozone than others and could be detrimental to our atmosphere if generated on a large enough scale.
Leaving many to wonder: How much VOC pollution does Colorado’s commercial cannabis industry generate? And is that contributing to ozone production in our atmosphere?
Kaitlin Urso, the small business consultant for the Colorado Department of Health and Environment (CDPHE), says the CDPHE gets odor complaints about cultivation facilities all the time. And if the pungency of Colorado’s grow ops is any indicator of their ozone contributions, the state could be dealing with a serious pollution problem—one that Urso points out wouldn’t be subject to the same air-quality permitting requirements as other businesses.
“They’re agriculturally exempt,” Urso says, meaning that, like farmers and ranchers, cannabis cultivators don’t need VOC emissions permits. “So it was definitely an unanswered question: What is the impact that cannabis cultivation facilities have on air quality?”
So Urso, alongside a team of researchers from the CDPHE and the Colorado Air Quality Control Division (AQCD), set out to answer that question. They designed a study, sampling VOC exhaust from three medium- and large-scale cannabis cultivation facilities in Denver—two producing less than 100,000 harvested plants per year, and one producing less than 20,000. Using a technique called “photochemical air quality monitoring,” the researchers assessed how impactful pot’s VOCs are on ozone production in this state.
And the conclusions they came to were totally unexpected, according to Urso.
They found that, despite the dank odor that hangs heavy on the air surrounding these facilities, and despite the massive terpene content of their crops, the VOC emissions rate for the sampled cultivation facilities was extremely low. The dominant terpenes they detected were caryophyllene, limonene, terpinolene, pinene, and myrcene, “respectively, by concentration,” the study notes. The emissions exhaust also appeared to lack isoprene, a very reactive terpene VOC emitted from many other plants, that has a particular knack for generating ozone at high rates.
In total, the cultivation of 2,000 pounds of cannabis only resulted in 11 pounds of VOCs emitted from the sample. To put that into perspective, within the Denver metro area, the threshold past which a business needs an air quality permit is 2,000 pounds of VOC emissions annually. That would require a cultivator to produce 726,000 pounds of cannabis in a single year—over a third of Colorado’s entire state cannabis crop yield for 2021.
The three facilities’ VOC emissions sampled in this study by CDPHE were so low that the resulting increase in ozone was just .005-.009%.
“I didn’t expect the sky to be falling emissions-wise, but I just didn’t expect them to be so low,” Urso says. She expected a full percentage point of change in ozone, at least. “The fact that there’s only 11 pounds of VOCs associated with that much cannabis is shocking.”
Small businesses like auto body paint shops and dry cleaners often exceed that 2,000 pound threshold for VOC emissions, Urso points out. It’s almost unheard of for production facilities of this size to emit such small quantities of VOCs across almost any industry.
“So we know that [cannabis cultivation facilities] are very odorous, but are they actually emitting pollutants?” Urso asks. “The answer that we found is essentially no. I mean, it’s a very, very low VOC concentration for a very high odor profile.”
What that tells us, she says, is that our noses are biologically tuned-in to sniffing out terpenes. Maybe that’s because homosapiens have evolved across eons to detect the dankest dank from blocks away. Or, perhaps more likely, it’s because terpenes are often associated with food and our ancestors were hungry to survive.
Either way, the odor that surrounds cultivation facilities isn’t pollution. It’s just your brain telling you there’s something good growing over there.
“What we found is that [the odor] is a nuisance for some people. It’s not a threat to public health or the environment,” Urso says. “It was great news. I mean, I couldn’t ask for a better surprise.”
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