Skunky VOC Emissions are Stinking up the Green-Cannabis Proposition
It’s fashionable to describe the odor of uncombusted cannabis as “skunky”. While beauty is in the nose of the beholder, exhaust can be a nuisance, and indeed causes twice as many complaints in Vancouver as waste-treatment plants. Not surprisingly, industrial odors particularly impact lower-income populations.
At any rate, underlying the bouquet of a nice dank-smelling bud are volatile organic compounds, known as VOCs, and other chemicals, such as 3-methyl-2-butene-1-thiol.
The VOCs from plants are typically referred to as “biogenic”, and take the form of terpenes, often called “terps”. The lovely smells in a pine forest (pinene), lemons (lemonine), or lavender (linalool) are carried by VOCs, although cannabis has among the very highest per-pound emissions rates among 24 crops examined. More than 150 types of VOCs are associated with cannabis plants. These “biogenic” VOC releases vary significantly by strain of plant, cultivation practices, temperature, and other factors.
Some cannabis-related VOCs/terpenes are said to have medicinal value, but VOCs are more commonly thought of as potential health hazards (which is why “green building” standards and ordinances discourage VOC-emitting carpets, paints, etc.). I haven’t located any studies looking at health impacts of cannabis-related biogenic terpenes on humans, but a study from two decades ago found adverse impacts of limonene and pinene in upper airways and lungs at total-VOC levels of routinely breathed by cannabis workers.
VOC emissions are also important to track because some “species” are chemical precursors to other air pollutants such as formaldehyde, ground-based ozone, and particulates (PM 2.5) which are components of what is commonly called “smog”. Since the ozone formation process involves the reaction of VOCs with nitrogen emissions from sources such as vehicles, the tendency to locate cannabis factory farms along highways is particularly problematic. However, studies to date have identified very little of the key ozone-generating terpene (isoprene) in the air where cannabis is grown. More needs to be know about the link to ozone formation.
A parallel source of cannabis-related VOCs are the non-biogenic emissions from cannabis extraction facilities. Petroleum products such as propane and butane are often used in the extraction, resulting in high rates of VOC emissions, their build-up inside, and their eventual exhaust to the outdoors. Extraction operations are sometimes co-located with cultivation sites, which means that emissions are a blend of biogenic and non-biogenic VOCs
While filtration systems at the point of exhaust can remove some VOCs from exhaust streams, they are not a panacea. Firstly, they are of no help to workers “upstream” from the filters. Moreover, the resistance they post to exhaust air-flows require larger fans and greater energy use and cost to achieve the same flow rates. Filtration is generally not required by local ordinances, and is quite costly–both in terms of materials and maintenance–to the cultivators. Meanwhile, outdoor farms cannot control emissions, but they can have a more diffuse spatial distribution in areas with smaller populations.
So, VOCs are of concern for outdoor air quality as well as indoor environmental conditions for workers, but to what extent do cannabis farms and the extraction process contribute to VOCs that people breathe?
We are fortunate to have researchers examining this question. They have had to overcome a lot of inertia in doing so, since government agencies like the EPA or OSHA shun the research, given the legal status of cannabis at the federal level. Several studies have been done within the 16,000-square-mile Denver Metro/Northern Front Range Ozone Non-attainment Area (shown here), where air pollution levels are currently characterized as “severe”.
In 2019, Samburova et al., provided some of the first published measurements inside grow rooms, and associated worker exposures. The chart below provides a sense of how much higher those indoor concentrations are than the “natural” background levels outside, and also how the types of VOCs differ. In this particular case, extraction was also performed at the site (indicated by the purple bars).
This chart shows the types and quantities of VOCs from cannabis cultivation and processing compared to background levels in Denver. This is for one particular facility out of four assessed by Samburova et al. in 2019.
VOC production and emissions have been observed to vary widely depending on which stage of cultivation and processing the measurements are taken. A 2023 paper by Urso et al found that those during peak periods of trimming and drying were far higher than indoor averages. Measurements found levels on the order of ten-times higher than those during cultivation, with the peak concentrations in the rooms (40,000 to 70,000 parts per billion) were roughly 8 to 13-times higher, respectively, than the exhaust from cultivation rooms (before filtration).
Levels of VOCs peak at nearly 60,000 ppb in one trimming room measured by Urso et al, which is about twenty-times the average levels found by Samburova (previous chart)
An earlier paper by Urso et al sought to estimate the role of cannabis-caused VOCs in the formation of Ozone and PM 2.5. Their method was to measure emissions from three facilities and scale up those numbers in proportion to the total production of all 250 cannabis-cultivation locations in the city. Resulting concentrations of Ozone were assumed to be “well-mixed” and thus averaged over “4km” areas (i.e. each six square miles), in Denver. The three facilities contained a wide range of strains at various plant-development stages. In one of the facilities, measurements were taken from the air exiting curing rooms (which appear to be the highest). Two of the sites were measured prior to carbon filtration in the exhaust stream. The researchers estimated a very negligible increase in ozone formation, attributed to the lack of isoprene in the strains cultivated in the facilities they monitored. Two prior studies (here and here) had, however, found isoprene in the VOCs emitted by cannabis plants, albeit in small amounts.
Importantly, disproportionately non-white workers in these facilities receive low wages while breathing risky volatile organic compounds (VOCs) 500-times higher than outdoor levels and 100-times worse than in homes. In a later study, Urso et al noted that emissions have been seen to be highest while workers are present (likely due to disturbances of the plants).
In contrast to ozone, Wang et al’s work in 2020 sought to understand exposures to VOCs themselves, and took a more spatially granular approach using field measurements of emissions at 34 locations around Denver, ranging from immediately outside cultivation sites to progressively greater distances into the surrounding neighborhoods (up to a bit over a mile away). The levels of VOCs at the point of exhaust generally agreed with those found by Urso et al. They found VOC concentrations in outdoor air near the facilities (before mixing into larger air bodies) to be 4–8 times higher than “background” levels in a city park. Again, even that park is located in the city, and will thus contain non-biogenic VOCs from nearby traffic and industrial processes. Key among their findings is that the assumption of the pollutants being “well-mixed” over even small areas is not a realistic representation of what is occurring in practice near the facilities (see figure).
This chart, per Wang et al (2020), shows how the concentration of VOCs (“monoterpenes”) declines rapidly with downwind distance from the cultivation facility. Note that the greatest distance shown in the chart (2000 meters) represents one fourth the area (and a far smaller proportion of the air volume) of the 4-km grid assumed in the aforementioned simulation-based study by Urso et al, particularly given that these measurements are along the plume created by wind directionality.
These results are important for many reasons. First is general public health and nuisance odor. When cannabis is grown in outdoor farms, the dilution potential is large and the surrounding populations are typically small. In cities where large indoor cannabis farms (and processing centers) are licensed in hundreds of locations, relatively large amounts of VOCs are emitted over small areas where many people live. More poignantly, the neighborhoods surrounding cannabis cultivation sites tend to be inhabited by low-income and non-white populations in far higher proportions to those in the cities more generally. This is certainly true for the prominent cannabis-cultivation cities such as Denver and Oakland, California. Denver’s 249 licensed indoor cannabis grows are located primarily in non-white and low-income neighborhoods, while Oakland’s ironically named “Green Zone,” hosts about 200 cultivators who operate in the city’s poorest neighborhoods. Thus, the issue is not only one of environmental quality broadly, but also one of environmental justice.