How a Booming Legal Marijuana Industry Could Harm Air Quality

marijuana farms, air quality
Recreational cannabis grow rooms, outfitted with 24-hour LED grow lights, are becoming common in U.S. cities where marijuana farming is legal. Ben McLeod/Getty Images

Not a lot of legitimate scientific research involves growing marijuana plants in a garage, but that's what it took for William Vizuete, associate professor of environmental sciences and engineering at the Gillings School of Global Public Health at the University of North Carolina, to begin to untangle the problem of whether the booming cannabis industry in Colorado is affecting air quality in Denver.

As more states legalize marijuana for medicinal and recreational use (as of 2018, 33 states and Washington D.C. have decriminalized the drug in some form or fashion), studies suggest marijuana farming impacts the environment in a variety of ways, from wildlife habitat fragmentation to its extreme energy and water requirements. But until Vizuete and a Ph.D. student went to Denver and bought and monitored 12 marijuana plants — three specimens of four popular cannabis varieties — in a garage, nobody had ever figured out the toll cannabis farming could be having on air quality.


"I was collaborating with the National Center for Atmospheric Research, but I couldn't grow plants in North Carolina where I live — I couldn't even bring them into a federal facility in Colorado," says Vizuete. "In Colorado, you can legally buy the plants and bring them home to your private residence and grow them, so we did that. We set up shop in the garage and took measurements using the techniques and methods we use for every other plant."

Gas Emissions

Vizuete's research, published October 30, 2018 in the journal Atmospheric Environment, set out to discover the composition and the rate at which marijuana plants emit gases called terpenes. There are many different kinds of terpenes — an oak tree makes a lot of one called isoprene, and when you're walking through a pine forest, it's a terpene called alpha-pinene that you're smelling in the air. Terpenes like eucalyptol are what give marijuana plants their distinctive aroma, but these compounds also affect the atmosphere:

"The Appalachian Mountains are called the Smoky Mountains because of that white haze you see there," says Vizuete. "This happens because particles are formed when these terpenes react in the atmosphere, forming aerosol and causing that mist."


Vizuete and his team found marijuana plants to be pretty average emitters of terpenes, and that the composition of and rate at which the terpene gases are emitted by the plants depend on the strain of the marijuana plant.

But what does all this have to do with air quality?


Terpenes and VOCs

Terpenes are a type of chemical called volatile organic compounds (VOCs), which are released into the atmosphere and contribute to the chemistry that produces air pollution. Scientists realized this about 40 years ago, and the rest of us discovered it in 1981, when President Ronald Reagan famously claimed that "trees cause more pollution than automobiles do." He was partly right — in a city like Atlanta, Georgia, it's hard to get the air quality models right without taking into consideration the colossal numbers of oak trees in the southeast United States.

It works like this: In the presence of sunlight, VOCs like terpenes react with another type of compound called nitrogen oxides (NOx), which come from combustion sources: power plants, automobiles, any industry that makes a flame. When you have the right amount of NOx and VOC in the presence of sunlight, you get particulate matter and ozone — both air pollutants.


"It's a very complicated chemistry, which is why scientists have to run air quality models to figure it out," says Vizuete. "Depending on the ratio of VOC to NOx emitted in any one place, different kinds or amounts of those pollutants can be made. For example, the southeastern U.S. has a lot of VOCs due to all the trees, but because most of the southeast is rural and there aren't a lot of sources of NOx, the chemistry isn't right for making air pollution. However, if you go to Atlanta, there is plenty of NOx from all the combustion sources — mainly cars. Now there's the right amount of NOx and VOC to make air pollution."

Denver, Colorado, on the other hand, is basically high desert, so there aren't a lot of biogenic VOCs. Unlike Atlanta, Denver has a lot of NOx but not a lot of trees. If you wanted to make air pollution — ozone and particulate matter, you'd add VOCs to that equation.


Enter Marijuana

Which is where all the marijuana plants come in.

In Denver, the cannabis industry has created an indoor marijuana forest in warehouses, converted into growing facilities with hundreds of thousands of plants. There are over 600 of these outfits in Denver alone, most of them right at the intersection of two interstate highways.


"If I wanted to do an experiment in which I made a bunch of ozone and particulate matter, I'd put a bunch of trees in the middle of downtown Denver, and that's exactly what's happening with this industry," says Vizuete.

Vizuete and his team are starting to gather some basic data on marijuana as a crop. Since it's a federally listed substance, all the methods and technologies that have been developed over the years to understand the emission factors of crops like tomatoes, corn and pine trees have not been applied to pot plants. As it is, Denver is nearly at nonattainment for air quality (meaning it's an area with persistent air quality problems that has violated federal health-based standards for outdoor air pollution), so a forest of marijuana in the middle of downtown it not likely to help matters, but the data has to be collected before they can say one way or the other.

The other issue is that, since marijuana is unregulated, it's not controlled.

"There are all these regulations for other industries," says Vizuete. "If I ran a gas station and I was supposed to be capturing VOCs so they didn't go into the atmosphere, I could be fined or put out of business if I wasn't doing that. It's not an expensive or unconventional thing to do, but nobody's ever done it for marijuana because no federal regulations, oversight or research exists."