Gamma irradiation, an industry-standard sterilization method for medicinal and recreational cannabis, does not fully eliminate toxic fungi or their chemical residues, a McGill University study has found. Current testing practices may also miss contamination, raising concerns about health risks for vulnerable users, particularly those with weakened immune systems.
The researchers said this is especially significant given that 70 per cent of cannabis is consumed by smoking or vaping, which may deliver toxins directly into the lungs and exacerbate smoking-related injury to lung tissues.
These findings point to a need for stricter testing and better safeguards, they said.
Major health hazards
Gamma irradiation works by damaging the DNA and RNA of microbes and degrading mycotoxins - harmful compounds produced by certain fungi. While this process significantly reduces microbial loads, researchers found viable spores of mycotoxigenic fungi, DNA fragments and traces of toxins remained after irradiation.
These remnants pose major health hazards, with heightened risk for such populations as cancer patients, transplant recipients and individuals with HIV/AIDS. However, the study also cites multiple case reports of fungal lung infections and other opportunistic infections in healthy people exposed to contaminated cannabis products.
Combined methods can improve testing
Researchers analyzed dried cannabis buds from a licensed producer and ready-to-use products from a licensed producer that were legally on the market. They used three complementary approaches:
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Culture-based methods to grow and identify living fungi and bacteria.
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Molecular tests (PCR and qPCR) to detect fungal DNA and toxin-producing genes.
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Antibody-based ELISA assays to measure mycotoxins such as aflatoxins and ochratoxins.
The team found that ELISA, the current industry standard, is not sensitive enough on its own to detect trace amounts of mycotoxins left after irradiation, nor is mass spectrometry when used in isolation. Adding molecular and culture-based methods allows testers to track smaller fragments that ELISA or mass spectrometry alone might miss.
"A single spore can cause disease, so we had to go beyond the ELISA limit to see. To the general population, this may not have much significance, but immunocompromised people will be at much higher risk," said Saji George, study co-author and Professor at the Department of Food Science and Agricultural Chemistry.
Industry collaboration and next steps
The researchers stressed that once contamination occurs, it is extremely difficult to remove, so prevention is key. They are working with industry partners on such solutions as beneficial bacteria that prevent harmful fungi from establishing on the crop.
"Cannabis buds have sticky resins, so they are really susceptible to contamination. These fungi are everywhere, so we need to be more careful at every stage, from growing and harvesting to processing and storage," said Mamta Rani, study co-author and Research Associate at the Laboratory for Sustainable Agricultural Food and Environmental Applications of Nanotechnology (SAFE-Nano lab) at McGill.
"It is possible to produce clean cannabis. Some companies we work with have achieved this through strict hygiene practices and controlled environments," she said.
Added George: "We are not trying to tarnish the industry, but to help make it more sustainable and provide guidelines for safer products. We need stricter safety standards, especially for medicinal cannabis."
About this study
"Detection of Mycotoxigenic Fungi and Residual Mycotoxins in Cannabis Buds Following Gamma Irradiation," by Mamta Rani, Mohammad Jamil Kaddoura, Jamil Samsatly, Guy Chamberland, Suha Jabaj and Saji George, was published in Toxins.
The research was funded by Tetra Biopharma Inc. (G253375).
