You’ve heard of ozone before, likely in regards to the ozone layer that protects us from life-enabling, yet also damaging ultraviolet radiation from the sun, and the fact that there’s a sizeable hole in the layer, allowing more harmful wavelengths to irradiate Earth. But ozone isn’t just in a depleting layer of the stratosphere. It’s all around us, just in trace concentrations. And whereas the normal, more stable oxygen we require for life has two oxygen atoms, the ozone molecule contains three oxygen atoms. That’s it.
Ozone is used industrially for several diverse applications, such as its use as a gaseous machete for cleaving carbon-carbon bonds, which has utility in the pharmaceutical industry, or in the synthesis of molecules.  More relevant to our discussion here, though, is ozone’s characteristic use for killing microorganisms like bacteria, molds, or yeast, including in food products, where its GRAS (generalized recognized as safe) status, penetrability, and degradation to non-toxic oxygen (O2) make it an attractive option for disinfection.  Even using ozonated water has provided antimicrobial properties against a variety of food-related, and cannabis-relevant bacteria, including Salmonella and Escherichia coli. 
Microorganisms can be a problem in cannabis. Recent surveys indicate that 15% of all commercially grown flower has microbial contamination. This amounted to $150 million of contaminated flower in Colorado alone, and by 2020, this is projected to be an industry-wide, $3 billion problem.
Immunocompromised people can develop aspergillosis, a serious illness that can spread throughout the lungs and bloodstream to the heart, liver, kidneys, or brain. People who are allergic to mold can have symptoms like sneezing, runny nose, skin hives or rashes, to full on asthma attacks. Patients who aren’t allergic to mold but are often inhaling mold spores can develop asthma or allergic symptoms over time.
I spoke with Jill Ellsworth, Founder of Willow Industries, an industry leader in post-harvest microbial decontamination to find out why they’ve chosen ozone as their proposed method of decontaminating cannabis flower.
“Three years ago, during our initial R&D, we tested multiple decontamination methods, but ozone proved to be the best for cannabis,” Jill explained. “It’s safe, effective, and has been used in agriculture and for water treatment since the 70s, so there was research supporting our original idea.”
“Scientifically speaking,” she elaborated, “ozone (O3) is a highly unstable molecule that is produced from oxygen. In addition to being an extremely effective antimicrobial agent, ozone rapidly decomposes into oxygen, so it cleans the flower without leaving a residue. And perhaps most importantly, our ozone system reduces microbial levels without altering the flower’s medicinal properties; in other words, we clean cannabis without oxidizing cannabinoids or terpenes.”
Jill mentioned this vital aspect of their technology since there have been skeptics of this methodology, including Jill herself, especially considering that delta-9-tetrahydrocannabinol (THC) can transform to cannabinol (CBN) through oxidation. So, how does Willow utilize ozone constructively and not destructively?
“Considering I had the same concern three years ago, I completely understand why some people would be worried about ozone oxidizing their cannabinoids or terpenes”, Jill responded. “In fact, that was our initial design challenge: we knew ozone could be used to control microbials, but we needed to find a centration of ozone that would reduce microbial counts without oxidizing the medicinal properties of the plant. It took multiple iterations of testing to find the perfect level, but as you can see by the data on our site, our system successfully remediates cannabis while protecting the most valuable chemical properties of the plant.”
Regarding the well-known transformation of THC to CBN, Jill discussed the permeability of cannabis resin: “Although there are methods for using ozone in oil remediation,” she explained, “in our current system, the ozone does not penetrate these oils so the cannabinoids and terpenes are protected.”
Some terpenes were detected in the treated flower that were not measured in the starting biomass, but these differences can stem from the variability in lab testing and the natural variation of terpenes distributed within cannabis. Trichomes (and therefore, terpenes) are not evenly distributed or localized on cannabis flowers, so, different areas on the plant will have different terpene concentrations. “Within the context of our data set,” Jill added, “this concept helps explain, for example, why a trace amount of geraniol was detected in a sample treated by our machine. Another factor to consider is that our treatment process can reduce moisture in the flower, so the relative proportion of each terpene can increase as the sample weight decreases. Therefore, we also sometimes measure slight increases in THC weight percent in treated samples.”
Microbials can produce mycotoxins, highlighting why sending infected biomass off to the extractor to save a few bucks isn’t ever truly worth it. While the extraction process can “blast off” the microorganisms, did those microorganisms produce mycotoxins that can co-extract with the targeted cannabinoids and terpenes? Is ozone also effective against mycotoxins? “At the right concentrations,” Jill explained, “ozone is capable of destroying mycotoxins, like aflatoxin, that were created by mold spores or other microbials. Willow uses a concentration that is efficient at destroying these toxins.”
The analytical data Willow provides points to their technology as cannabinoid- and terpenoid-friendly since cannabis can be effectively cleaned without deleterious effects to the native chemovar. Additionally, ozone is an organic oxidizer, so it rapidly reverts to breathable oxygen, leaving no residue.
Jill compared Willow’s ozonation treatment to using UV light. “UV light destroys mold spores that it contacts on the surface, as it can’t penetrate fully into the flower”, she explained. “Gaseous ozone, however, is capable of getting into every area of the flower, so you don’t need to break up the product to insure it is effectively being treated. Larger buds require longer run times to ensure the ozone has achieved this penetration.” Even with the added time, ensuring that a batch of flower is devoid of microbes should be well-worth the wait.
But what about seriously contaminated biomass? Can this at all be salvaged using ozonation? Is it even worth trying?
The technology is capable of effectively treating very contaminated product,” Jill advised, “but in the case of Botrytis cinerea or bacterial contamination in the plants’ water source, it’s unlikely to be able to fully decontaminate. Botrytis, commonly referred to as “bud rot,” is a systemic mold that can contaminate entire plants beyond repair if not caught early enough. This fungus is sometimes referred to as ‘gray mold’ and visibly appears as a gray fuzz inside or outside of the bud. Early signs of Botrytis can appear as bruises on the bud, which eventually collapse into rot.”
Jill also advised that bacterial or heavy metal contamination of a plant’s water source is harder to catch and will only be revealed through test results from a qualified lab. Willow can help diagnose the issue, and potentially install an ozonated water system, if necessary, to ensure clean water reaches the grow rooms.
References Brown, T. et al. In Nicole Folchetti (ed.). Chemistry: The Central Science (9th ed.)., 2003, Pearson Education. pp. 882–883.  Kim, J. et al. “Application of Ozone for Enhancing the Microbiological Safety and Quality of Foods: A Review”, Journal of Food Protection, Vol. 62, No. 9, 1999, pp. 1071–1087. [journal impact factor = 1.510; cited by 534]  Restaino, A. et al. “Efficacy of Ozonated Water against Various Food-Related Microorganisms”, Applied and Environmental Microbiology, Vol. 61, No. 9, 1995, pp. 3471–3475. [journal impact factor = 4.077; cited by 327]