Cannabis Odor and the State-of-the-Science
Cannabis plants transpire (or sweat) as humans do. There are over 200 compounds found in the emissions coming off the plants consisting of monoterpenes, sesquiterpenes, thiols, and more.  All plants, not just cannabis, emit these compounds (or “off-gas”) at a measurable rate known as the emission rate. As we know, terpenes are responsible for the taste and aroma profile of the cannabis plants; however, science now knows the main culprit for the skunk-like odor is 3-methyl-2-butene-1-thiol or 321 MBT. In March 2021, a research team comprised of Byers Scientific, Iowa State University, and Texas-based odor experts reported the isolation and identification of the volatile chemical that appears to be primarily responsible for the downwind skunky environmental odor complaints which have been commonly reported for commercial cannabis and industrial hemp growing operations.
The human olfactory system has evolved over time to detect substances, such as sulfur-containing thiols, at exceptionally low levels. This level of recognition is called the odor detection threshold (ODT). Our nose can sometimes detect compounds at levels that are extremely difficult or nearly impossible to detect with ambient air samples using highly sensitive analytical equipment. As cannabis emissions escape the facility and enter the airstream traveling downwind, there are physical and chemical reactions that take place. As these molecules travel, many of them disperse into the atmosphere and/or fall out of the airstream at various rates. The molecule 321 MBT seems to travel the furthest and even at minimal concentrations (i.e., parts per trillion) can be detected by the human nose.
Odor, Perceptions, and the Risk to Operations That is Here to Stay
Left unaddressed, cannabis odor is arguably the single biggest risk to commercial cannabis operations due to the potential for litigation and regulatory enforcement action. As facilities scale, so does the issue of the real and perceived smell. The number one issue is that human olfactory perceptions are subjective, meaning that our individual responses to the same odor-causing emissions can vary significantly. However, due to the rise in complaints from neighboring communities across the country, local regulators are forced to take action that can threaten commercial cannabis operations.
To compound the problem, many local regulators are unsure how to appropriately address issues related to cannabis odor as odor investigations and assessments rely on highly subjective procedures. By the time the odor complaint is processed, and an investigator is dispatched, the smell may no longer be present. Therefore, enforcing odor-related mandates and evaluating odor events objectively is undoubtedly problematic.
The Need for Future Research
Although the primary skunky odor-causing culprit has now been identified, this is only the first critical step in fully understanding this issue. As an example, leaf enclosure studies reveal other thiols and compounds present in plant emissions , and their atmospheric reactions may significantly affect the perception and measurement of 321 MBT.
Scientifically Qualifying and Quantifying Odor-Causing Profiles
A team of scientists led by Will Vizuete, Ph.D., and Alex Guenther, Ph.D., have developed a one-of-a-kind scientific and mathematical approach for quantifying odor coming from cannabis plant emissions at a given facility. The leaf enclosure method has been adapted for the cannabis industry by Dr. Guenther such that it measures the exact types and quantities of compounds emitted off the cannabis plant. Once the leaf enclosure samples are analyzed using gas chromatography-mass spectrometry (GC-MS), emissions factors are determined for each site-specific molecule allowing for the calculation of a facility’s emissions footprint.
Site-Specific, Data-Derived and By-Design Technology – Cannabis Emissions Profiler (CEP)
Due to the considerable variability in the rate and composition of emissions across different cannabis plants and among the same plants, it is critical to quantify a canopy’s unique gas-phase emission rate based on key factors such as the number of varieties, plant count, life stage, and plant weight. These volumetric measurements can then be entered into our Cannabis Emissions Profiler™ (CEP) to identify the cubic feet per minute (cfm) required for the optimal number of air exchanges and properly size equipment to sequester odor molecules. This process helps to eliminate a perpetual reliance on HVAC (heating, ventilation, and air conditioning) systems in hybrid-greenhouses, warehouses, and indoor cultivation spaces that are not engineered or optimized for cannabis odor control. Cannabis odor control at this scale, and at these concentrations of molecules, is a task for proven odor mitigation experts backed by science.
Control Technologies and Recommendations by Grow Style
Where appropriate, outdoor cultivation facilities may benefit from deployment of Byers Scientific’s patented vapor-phase perimeter system which uses no water to control fugitive odors, making it less resource intensive and more environmentally sustainable. This system manifests the power of adsorption in the airstream to eliminate odors on contact using a proprietary odor neutralizer made up of organic plant-based oils that is formulated specifically for cannabis malodor compounds.
Byers Scientific recommends a comprehensive approach for greenhouse operations using its perimeter vapor-phase system in conjunction with its interior MT-6™ Molecular Filtration units. Recent advancements in understanding the proximity of high-odor emissions concentrations for greenhouse operations have led to the latest vapor system deployment strategies, including ridge-vent application, to help maximize the efficiency and efficacy of odor control strategies. For example, as a greenhouse is in black-out (light deprivation) and ridge vents are closed, the operator can utilize MT-6™ units to scrub the air inside of the greenhouse, which significantly reduces the emission load, so that when vents are opened, the ridge-vent application of odor neutralizing vapor tackles any left-over escaping odor molecules.
The MT-6™ Molecular Filtration unit is designed with three phases for optimal emissions control. Phase 1 is a Camfil proprietary pleated panel filter that works to remove airborne contaminants, improving indoor air quality while maintaining the integrity of the activated carbon media before scrubbing odor in Phase 3.
Phase 2 is an optional photocatalytic oxidation phase (PCO), which removes airborne contaminants and kills microbials and other microorganisms including a broad range of pathogens, resulting in improved indoor air quality as well as plant health.
Phase 3, the final step, consists of Forty-Eight 24” LGX-048 Camfil coconut shell activated carbon canisters. This carbon media was selected due to its superior surface area for adsorbing the optimal number of odor molecules. To illustrate, a tablespoon of this carbon media has the equivalent surface area of an entire football field. Additionally, Byers Scientific can run Butane Life Tests, in accordance with ASTM D5742-95 , on this carbon media to identify the optimal lifespan of the carbon before replacement is needed to preserve resources, reduce waste, and prevent odor break-through based on the specific application.
Byers Scientific offers two effective solutions for indoor grows – its MT-6™ units and MicroCool IBEX Technology (atomization) systems. While the MT-6™ works to remove the majority of odor causing compounds on the inside of the facility, the IBEX atomization unit is engineered for an exterior application on exhaust fans or HVAC exhaust points. The unit deploys ultra-fine (<10 microns) fog droplets of specially formulated odor neutralizer that remain airborne and move with the airstream adsorbing malodor compounds and mitigating odor. The sheer volume of ultra-fine droplets creates significant surface area, increasing the IBEX system’s ability to effectively adsorb odor-causing cannabis emissions. Similar to the vapor/scrubbing approach for greenhouses, Byers Scientific takes a comprehensive approach when partnering with its clients to mitigate risk through the mitigation of odor.
Cloud-based Technology Monitoring Odor Events in Real-Time with Indisputable Data
All Byers Scientific equipment is automated through its proprietary cloud-based SCADA system for remote control, management, and monitoring. Real-time data allows for scheduling maintenance and service when necessary, and the instantaneous availability of critical data reports equips operators with the information necessary to prove compliance to regulators and licensing agencies 24/7.
The risk and potential liability associated with both real and perceived cannabis odors is significantly reduced by the power of Byers Scientific’s industry-leading data-derived odor mitigation and monitoring solutions.
References Rice S, Koziel JA. Characterizing the smell of marijuana by odor impact of volatile compounds: An application of simultaneous chemical and sensory analysis. PLoS One. 2015;10(12):e0144160. [journal impact factor = 2.740; times cited = 36]
 Vizuete W. Estimated emissions, concentrations, and deposition of monoterpenes from an outdoor cannabis farm. Byers Scientific.
 ASTM D5742-95(2005), Standard Test Method for Determination of Butane Activity of Activated Carbon, ASTM International, West Conshohocken, PA, 2005.