Credit: Anton Eye | Air quality expert Alex Guenther measures volatile organic compounds emitted from cannabis plants using leaf enclosures that he optimized for cannabis.
As the cannabis industry grows, so does its environmental footprint. Cannabis production uses a lot of energy, particularly when plants are grown indoors. It also consumes large amounts of water, emits stinky gases, and generates waste. Federal prohibition of cannabis in the US has prevented nearly all research on the cannabis industry’s environmental impact. It has also hindered researchers from developing solutions to minimize those effects. State regulators and companies are stepping in to fill some of the data gaps, but more research is needed.
Cannabis growers in the US are under increased scrutiny for straining the electric grid. They have been blamed for water shortages in California and Oregon. They face complaints about the plant’s strong skunk-like odor and potential to lower air quality. The industry also generates a growing amount of waste, including paper and plastic consumer packaging and electronic waste from vaping devices.
But data on energy and water use, air emissions, and waste from the cannabis industry are limited. A small fraction of the US industry is embracing the opportunity to fill those data gaps and address problem areas. A few state regulators are also stepping in to address concerns about air quality and energy use. And academics are relying on bootstrapped funding to conduct energy-efficiency research.
Cannabis is federally illegal in the US, but 19 states and Washington, DC, have legalized it for adult use. Dozens of states have also legalized the medical use of cannabis.
The federal prohibition means that the Department of Energy and other federal agencies can’t fund research to determine how to improve energy efficiency and minimize carbon dioxide emissions at cannabis cultivation facilities. The Environmental Protection Agency is barred from researching cannabis production to determine air-emission factors, which companies use to estimate how much air pollution they produce.
Also, cannabis growers can’t get federal tax breaks for installing new technologies to reduce their energy use, as other industries can. They also don’t have decades of agricultural research on how to optimize cannabis growing conditions in greenhouses and indoor facilities.
The US Department of Agriculture (USDA) has funded research for other agricultural commodities since the 1970s, says Shawn Cooney, cofounder of the Sustainable Cannabis Coalition, a group of North American cannabis cultivation and manufacturing experts. Founded in early 2021, the organization promotes best practices related to the environmental sustainability of cannabis cultivation and manufacturing.
Cooney grows lettuce and greens in shipping containers in the Boston area. When he began looking to add cannabis to his business, the lack of research on growing cannabis was “like night and day” compared with other crops, he says.
The USDA can fund research on hemp, defined as cannabis with no more than 0.3% ∆9-tetrahydrocannabinol (THC) on a dry-weight basis, thanks to legislation that Congress passed in 2018 to legalize hemp in the US. But the USDA still lacks the authority to fund research on cannabis that does not meet the hemp definition.
When Cooney first started talking with industry peers and vendors about sustainability, most had little interest, he recalls. “People were always afraid of it” because of the industry’s intense energy use, he says.
The coalition hopes to change that mindset by supporting academic research, working with international standard-setting bodies, and promoting software solutions that make it easy for the cannabis industry to monitor energy use, waste, and other environmental effects.
The group persuaded the standard-setting organization ASTM International to establish a subcommittee on cannabis sustainability. Cooney chairs that panel, which formed in March. Its goal is to determine “what to measure, how to measure it, and what to report in a consistent way across global cannabis industries—focusing mainly on energy, water, waste, and social responsibility,” Cooney says. “We’re not in the business of establishing standards of what the end goal should be,” he adds. The subcommittee hopes to vote on the standards by early next year, he says.
Energy use is one of the biggest environmental impacts of the cannabis industry, particularly when plants are grown indoors. In cities, cannabis is often grown in retrofitted industrial warehouses that were not designed for agriculture. In many other places, cannabis is grown in large-scale greenhouses. Regulations in some municipalities, such as Denver, make growing cannabis indoors the only practical option.
Most states where cannabis is legal, including Colorado, do not require cultivation facilities to meet specific energy standards, though Illinois and Massachusetts do.
Installing light-emitting diode (LED) bulbs can help cannabis growers lower their energy use and meet state regulatory standards. But it’s not always feasible, and it doesn’t always make a big difference in electric bills.
“LED lights have come a long way,” says Melinda Kadinger, chief operating officer and chief financial officer at Smokey’s Cannabis, a cannabis cultivation and retail company in Colorado known for its sustainability efforts. “We tried LED lights when they first came out, and it was a bomb. But now we have converted an entire room to LED lights, and it is our best-performing room” in terms of cannabis quality and amount, she says.
“Our energy impacts are large, and we’re well aware that growing indoors is not the most sustainable way to do it,” says Jason MacDonald, director of cultivation operations at Native Roots, one of the largest cannabis producers in Colorado. “But it is the way to get final, quality product that people are looking for. So we have tried to make changes wherever we possibly can save power,” he says.
Native Roots has two facilities in Denver where it grows cannabis. One facility produces about 45 kg of dried cannabis flowers every 10 days. The other produces that amount in a day, MacDonald says.
The company started out using about 2,000 high-pressure sodium bulbs, 1,000 metal halide bulbs, and 4,000 T5 fluorescent bulbs, MacDonald says. Replacing the T5 bulbs with LED lights that last five times as long halved the company’s power use, he says. Native Roots is now replacing all the metal halide bulbs with LEDs, he adds.
Unlike most grow facilities, which must counter the heat generated by non-LED grow lights, Native Roots needs that warmth at its largest facility.
“We count on high-pressure sodium lights for part of our heating,” MacDonald says. “If we were to switch from those bulbs to an LED solution, we would lose that heat. We don’t know if our system can handle that much heat loss,” he says.
To replace all its grow lights with LEDs, Native Roots would have to invest about $10 million to improve the heating and ventilation system at its largest facility, MacDonald says. It’s hard to justify that investment, he says. In addition, because cannabis is federally illegal, the company would not be able to claim any tax credits or deductions for the upgrade, unlike other businesses.
To better understand how cannabis cultivation facilities can reduce their energy use beyond switching to LED lights, the Sustainable Cannabis Coalition sponsored a project led by researchers at Dartmouth College. Over the past year, the team metered energy use from grow lights, dehumidifiers, and cooling systems at two indoor cannabis cultivation facilities—one in Massachusetts and one in Maryland. Preliminary results of that study, which have yet to be published, suggest that these facilities use two to three times the energy needed for optimum production, says Stephen Doig, senior research and strategy adviser at Dartmouth’s Arthur L. Irving Institute for Energy and Society and one of the lead researchers of the study.
The project is one of the first independent analyses of the potential for the cannabis industry to reduce costs while improving metrics, such as kilowatt-hours per gram of product and grams of CO2 emissions per gram of product, Doig says.
Doig and colleagues found their own funding for the study without help from federal or state government agencies. “We used our own time and money to get the initial results,” Doig says. Industry provided time, labor, and expert advice and allowed the researchers to access data and facilities, he adds.
The work has important implications for decreasing cannabis’s burden on the US electric grid, Doig says. The cannabis industry is responsible for about 1% of US energy demand, and that percentage is growing, he notes.
Federal legalization of cannabis is critical so that the federal government can fund this type of research, he says.
Some science on the energy efficiency of buildings conducted for other industries could be used as a starting point for the cannabis industry. So if federal funding becomes available, researchers would not have to start from scratch, Doig says. Federal money for research could lead to “a common way of measuring and monitoring and metering these facilities that would allow comparisons to be made,” he says. Such standardization would be a great boon to state regulators, the cannabis industry, and companies that provide equipment to the industry, he adds.
“Energy is the one area where cannabis is and continues to be phenomenally resource intensive,” says John Kagia, chief knowledge officer at New Frontier Data, a market research firm focused on the global cannabis industry. Outside California, most cannabis in the US is grown indoors. But “we’re also seeing a lot of interest in transitioning out of fully enclosed indoor environments to greenhouses,” Kagia says. In those mixed-light environments, growers can use the sun and supplement with artificial light as needed, he says.
“Our goal in the future, as long as rules and regulations allow, would be to go toward a greenhouse type of hybrid situation,” Smokey’s Kadinger says. “The greenhouse is, in my opinion, a better way to grow as opposed to doing it indoors,” for both the environment and product quality, she says.
Moving cannabis growing completely outdoors would decrease energy use. But it’s not always feasible. Cannabis is grown outdoors primarily in the western US, which is facing drought conditions and dwindling water supplies.
“For several years, the cannabis industry has been perceived as the culprit for some of the water-related challenges that California’s very large agricultural economy is facing,” Kagia says.
But research published in 2021 by New Frontier Data suggests that cannabis is not to blame. The data, collected in partnership with the Oregon-based nonprofit Resource Innovation Institute and the University of California, Berkeley’s Cannabis Research Center, show that California’s cannabis industry uses only a fraction of the water that is used to grow grapes, rice, wheat, and other agricultural commodities in the state. The data include water use by both the legal and illegal cannabis markets.
Even so, there is room for the cannabis industry to improve its water efficiency, Kagia says. In its 2021 report, New Frontier Data predicts that by 2025, the legal cannabis industry’s total water use will increase by 86% compared with use in 2020. Most water used by the cannabis industry over the next 5 years, however, will continue to be driven by the illicit market, according to the report. The data suggest that the illegal cannabis market accounted for 83% of the industry’s water use in 2020.
Water and energy use are the two biggest environmental impacts faced by Native Roots, MacDonald says. And like with energy, the company is doing what it can to conserve water. When the cannabis industry was getting started in Colorado, it was dogma in the industry that growers treat water by reverse osmosis (RO) to remove heavy metals and sodium before using it on their crops, MacDonald says. About half the water that is put through the RO system ends up as wastewater.
“We did a lot of testing and experiments to see if we could just use regular tap water instead of RO. And we found that we can,” he says. “We’re in the process of moving away from RO water,” he says.
Watering practices vary across the cannabis industry. A growing number of facilities are moving away from hand watering to drip irrigation, in some cases with sensors to monitor moisture conditions in real time.
Native Roots has instituted a drip-watering system in all its flowering rooms, and the system can be controlled by the minute, MacDonald says. Drip watering helps ensure that the plants are not overwatered. In addition, the company makes all its fertilizers in-house in concentrated form. Commercially available fertilizers are typically 90–99% water and some salts, MacDonald says.
The substrate that cannabis is grown in also affects how much water is used. Native Roots uses fiber from the outer husk of coconut, called coir, which is popular because of its water retention, aeration, and drainage properties.
“We work pretty hard to be water conscious,” MacDonald says. “That being said, most of the water we use in this facility is used in our evaporative cooling towers, which we work very hard to keep tight. We have an engineer on staff who monitors that system,” he says.
Cannabis companies do not have to monitor their air emissions for volatile organic compounds (VOCs), such as fragrant terpenes. Many plants, including pine, lavender, and citrus, produce terpenes, and each compound has a unique smell and taste. Terpenes are not hazardous, but they can react with sunlight and nitrogen oxides in the air to form ground-level ozone, which is an air pollutant. Nitrogen oxides come primarily from combustion.
“If you drive around Colorado, especially downtown Denver in the industrial warehouse district, you will smell where the marijuana grow operations are,” says Kaitlin Urso, an environmental consultant for small businesses at the Colorado Department of Public Health and Environment.
The strong odor of cannabis led Urso, an air quality expert, to question whether cannabis production in Denver emits enough VOCs to harm the region’s air quality. In 2019, she and her colleagues quantified terpene emissions from the exhaust at three cannabis cultivation facilities. They used the resulting emission rates along with data on locations and harvest weights from cultivation facilities in the Denver area to model cannabis cultivation’s potential impact on ozone formation.
The research revealed that terpene emissions from cannabis cultivation are not a concern for ozone formation in the Denver area (J. Air Waste Manag. Assoc. 2022, DOI: 10.1080/10962247.2022.2046206). At most, cannabis cultivation would contribute to a 0.009% increase in the city’s ozone level, the study suggests. The researchers estimate that 4.5 kg of VOCs is emitted for every 0.9 metric ton of cannabis produced. A gallon (3.8 L) of paint emits about 3.6 kg of VOCs, Urso says.
These emission factors suggest that the largest facility in the Denver area “would have to grow by four times the size to meet our reporting threshold,” Urso says.
Chemical engineer and air quality expert William Vizuete, a professor of environmental sciences and engineering at the University of North Carolina at Chapel Hill, also investigated the potential air quality impacts of cannabis cultivation in Denver. That work showed that terpenes emitted from cannabis can harm air quality by a small amount, but the range of possible emissions is large (Atmos. Chem. Phys. 2019, DOI: 10.5194/acp-19-13973-2019).
Vizuete, who is also the chief scientific officer at Byers Scientific, a company that makes odor-mitigation technology, and colleagues measured the emissions from cannabis plants using enclosures that trap volatile compounds onto solid adsorbent cartridges (J. Atmos. Environ. 2019, DOI: 10.1016/j.atmosenv.2018.10.049). The compounds are desorbed and analyzed using gas chromatography (GC) with mass spectrometry and flame ionization detection. During peak growth, the researchers detected three dominant terpenes—β-myrcene, eucalyptol, and d-limonene—for all strains they tested. The composition of terpene emissions varied widely among cannabis strains, however.
In collaboration with air quality and climate-modeling expert Alex Guenther, a professor at the University of California, Irvine, the researchers characterized the aroma of the compounds using GC olfactometry. Guenther is also one of Byers’s senior scientists.
It’s a common misunderstanding that the skunk-like odor of cannabis is caused by terpenes, Vizuete says. A group of organosulfur compounds called thiols is actually responsible, he notes.
The researchers identified skunky-smelling 3-methyl-2-butene-1-thiol as one of the key thiols contributing to the bouquet of cannabis. But the odor assessment of cannabis is complex because the plant emits more than 200 compounds, Vizuete says. Byers engineered its odor mitigation system to capture all thiols, he adds.
Odor ordinances are typically implemented at the county and city levels. Local ordinances can be more stringent than state regulations, Urso says. Every county in Colorado has a strict odor ordinance, she says. Because of those rules, cannabis facilities have had to install carbon filtration systems and other odor mitigation technology, she says.
Most cannabis odor complaints are related to emissions released when the top vents on greenhouses are opened to regulate temperature and humidity, says Josh Rembusch, vice president of business development at Byers. The vents are typically closed at night, causing odorous thiols and other VOCs to build up in the greenhouses. When the vents are first opened during the day, a massive amount of these VOCs is released into the air, he says.
Byers offers a solution that combines molecular filtration, also known as carbon scrubbers, with dry vapor-phase technology to mitigate these enormous VOC releases from greenhouses. The carbon scrubbers capture odorous compounds by adsorption. Smelly substances that slip through the scrubbers next pass through a virtual curtain of waterless vapor emitted along the base of each top vent. The vapor neutralizes the odors before they are released into the environment, the company says.
Waste, both plant material and consumer packaging, is another growing environmental challenge facing the cannabis industry. Some states are loosening their rules to allow companies to recycle packaging and compost cannabis plant debris to address this growing waste stream.
Colorado made it easier for cannabis growers to compost waste plant material in 2021. Before that, the state required all cannabis plant waste to be mixed 50:50 with other waste, rendering the cannabis debris unusable and unrecognizable, Urso says. The mixed waste was mainly landfilled because businesses had trouble finding the amount of compostable waste to match their plant waste stream, she says.
The updated regulations in Colorado exempt cannabis waste that is low in THC content, including stems and root balls, from the 50:50 requirement for composting. Many other states have modeled their regulations for cannabis waste disposal after Colorado’s. Most still require mixing cannabis waste with 50% noncannabis waste.
For instance, Michigan has the 50:50 mixing rule for cannabis waste, says Kaitlyn Leffert, an environmental quality analyst for the Michigan Department of Environment, Great Lakes, and Energy. But the state is considering ways to make it easier to compost the plant material, she says.
“Facilities aren’t throwing out anything that would have economic value to them anyways,” particularly plant material that contains THC, she says.
Federal legalization of cannabis would give growers more disposal options, Leffert says. In some instances, states don’t have a big enough composting facility near a cannabis facility to handle large loads of plant material, she says. But it’s federally illegal to transport cannabis waste across state borders, even for composting. So this debris often ends up in landfills, she says.
Smokey’s in Colorado composts its plant waste to help regenerate the soil it uses to grow cannabis. Unlike facilities that use substrates like rockwool or coir to grow cannabis, Smokey’s does not dispose of its soil after each use. Instead, it continuously regenerates soil fertility and improves the soil’s health with cover crops.
Plant debris isn’t the only waste problem the cannabis industry faces. Most states have strict requirements on cannabis products that often result in a large amount of extra cardboard and plastic packaging. One goal of the requirements is to prevent cannabis from getting into the hands of children.
“Policy makers may not have fully considered the environmental implications of the waste that we’re going to be generating,” Kagia of New Frontier Data says.
Colorado has updated its rules to allow cannabis dispensaries to collect packaging waste for recycling and reuse, Urso says. “We can allow bulk recycling through these collection programs and have businesses incentivize it with customer loyalty programs,” she says.
Several years ago, Smokey’s dispensaries began offering loyalty reward points to customers who bring back clean packaging, such as the small plastic vials that cannabis is sold in, for recycling, Kadinger says. Today, the company has more of a closed-loop system and works with a company called Green for Green, she says. If the waste products can be sanitized, Green for Green sanitizes them and sells them back to Smokey’s. If they can’t be sanitized, Green for Green puts the products into a recycling program, she says.
Some states, including Michigan, do not allow the recycling of cannabis packaging waste. But Michigan is considering whether to allow dispensaries to collect such waste, and “hopefully we can move in that direction,” Leffert says. Otherwise the little plastic vials, tubes, and bags that cannabis is sold in have no way of being recycled, she says.
The cannabis industry also does not have a mechanism to recycle vape cartridges and batteries.
Vape cartridges “account for roughly a quarter of the sales of products in the legal cannabis market,” Kagia says. “I think there’s a very real risk for this to turn into the industry’s K-Cup problem,” he says, referring to the waste associated with disposable pods used in single-serve coffee makers. “They offer a great deal of convenience for the consumer, but at the rate that the vape sector is growing, a decade from now could end up being a very significant contributor to the total amount of waste that the cannabis industry is generating,” he says.
The legal cannabis market is almost exclusively in the US and Canada right now, Kagia says. Practices being used there, particularly the US, will be replicated around the world, he predicts.
“The urgency is not just about the environmental impacts for the US,” he says. If the US doesn’t begin to invest aggressively to build resource-efficient cannabis cultivation facilities and mitigate the environmental impact of cannabis-related waste, these unsustainable practices will be adopted in Europe, Latin America, Asia, and Africa, he says. “And we will have missed this window to build a portfolio of best practices that allow for an efficient, resource-minded, and minimally wasteful industry.”