By Jeremy Hsu
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Love it or loathe it, marijuana’s scent is certainly distinct. Just ask the professional tennis players at the 2023 US Open who voiced discomfort about the pungent pot miasma drifting into outdoor courts from a nearby park in Queens, New York. Alexander Sverev described one court to the Associated Press as smelling “like Snoop Dogg’s living room”.
The whiff of weed has become increasingly familiar across much of the US following a wave of state-level legalisation. In Santa Barbara County – home to …
more licensed cannabis growers than any other county in California – residents and business owners have intensified complaints about the associated stench in recent years. So what is it that actually gives marijuana its smell?
Scientists had long assumed that weed’s dominant odours mainly involve naturally occurring chemical compounds called terpenes that also determine the psychoactive and medicinal properties of marijuana. But these form the underlying canvas for a cannabis strain’s overall scent rather than providing the most unique and standout smells.
Two independent research groups recently discovered a different type of chemical playing the starring role in marijuana’s distinctly skunky scent – volatile sulphur compounds known as thiols that contain a molecule called 3-methylbut-2-en-1-yl (321MBT).
“Growers might have this issue of stinking up the town, or if you have some cannabis and you’re in an apartment next to your neighbour they might be able to smell it,” says Iain Oswald at Abstrax Tech, a cannabis research and development company in California. “It is these thiols for sure that are off gassing from the flower itself.”
The same 321MBT molecule is responsible for the skunky smell that occurs in beer packaged in clear bottles after it has been exposed to sunlight, says Don Wright at Don Wright and Associates, an odour monitoring and control consultancy in Texas. He also cited the molecule’s contribution to the characteristic smell of musky spray from mammalian skunks, which they produce when threatened.
Wright and his colleagues discovered 321MBT’s role by focusing on the smallest possible group of chemicals that matched the odour characteristic of downwind smells for cannabis while still being detectable to human nose at greater distances. He explained how the molecule was previously overlooked because it exists at very low concentrations compared to other chemicals such as terpenes.
“Historically, odour problem investigations have looked at the biggest players – the highest concentration compounds,” says Wright. “But it’s the rule rather than the exception that the problems are carried in the very trace-level compounds buried in this overwhelming forest.” Meanwhile, Oswald and his colleagues performed a more data-intensive analysis of the many chemical compounds that contribute to different scent profiles in cannabis. This approach similarly identified 321MBT as the main culprit behind the “straight skunky smell” of popular cannabis strains such as OG Kush – and the researchers used an indoor greenhouse experiment to show that the concentrations of volatile sulphur compounds containing 321MBT increase significantly during the last flowering stage for cannabis plants.
But other chemicals present in similarly low concentrations can create different aroma arrangements in cannabis. Oswald’s group identified “tropical” volatile sulphur compounds that combine with the skunky scent to provide a sweeter scent for cannabis strains such as Tangie Mimosa and Papaya Peach. The researchers also highlighted the chemical compound skatole – most infamously found in human faeces – as helping to create a “savoury garlic note” for some cannabis strains when mixed with the skunky odour.
“Many people would probably say they don’t like it,” says Oswald. “But people who consume cannabis are […] looking for the most potent of whatever aroma it is.”
Oswald and Abstrax Tech hope to help the cannabis industry use the latest science of smell to selectively breed cannabis strains for specific odour profiles. Growers could also potentially breed out the skunkiest odours that are the source of some communities’ complaints about cannabis, says Wright.
At Byers Scientific, an air quality equipment manufacturer and research firm in Indiana, William Vizuete has already helped his company to engineer filtration equipment that captures skunky scents before they leave the site of cannabis farms. But as one of Wright’s research colleagues, he sees much more work to be done regarding the origins of various cannabis scents.
“I would not be surprised, given the hundreds of strains that are out there, that there would be more molecules out there being emitted that we have not yet identified,” says Vizuete.