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Why does cannabis smell like skunk? This Iowa State professor has answers.

Iowa State professor Jacek Koziel's exposure to research on the smell of marijuana happened while assisting a former Ph.D. student with her mapping of the substance's compounds.

Koziel remembers thinking "Oh yea, this is intense," as he saw three large canvas bags of police-confiscated marijuana, donated to their research at the State Forensics Laboratory in Ankeny. The student, Somchai Rice, now an Iowa State researcher, delved into similar projects with heroin and cocaine, creating an extensive library of compounds in the substances.

The paper published in 2015 that came from this project received 20,000 hits, and nearly a decade after that project began, Koziel is back digging into marijuana compounds. 

The recent research project, led by Byers Scientific, singled out what compound causes marijuana's strong, skunky smell, a compound Koziel recognized from his research with Rice. 

Marijuana plant in hands.


"Now with the project with Byers it's like, 'Oh my gosh, we saw this compound. It's right there," Koziel said. "Now, we have a confirmation of exactly what this compound is."

Rice did not participate in the Byers research, Koziel said, but is using the same methodology she used with drugs to research the smell of wine. 

As marijuana is legalized in states across the country, this research is meant to help with odor mitigation efforts where marijuana is grown and also measure if odor complaints are genuine or motivated by bias against weed. 

"The sort of legal climate between 2016 and 2020 in many states is dramatically different," Koziel said. "With the expansion and legalization of the cultivation of hemp, the interest in solving or minimizing that impact of objectionable skunky odors has taken a different importance." 

Those states that legalized it have faced some growing pains when it comes to odor. Colorado, the first state to legalize recreational marijuana, was unprepared for the strong stench of agricultural production, Byers Senior Project Manager Emily Long said.

"I think Colorado didn't anticipate the odor issues prior to legalizing,"  Long said.

Byers Vice President of Business Development Jim Rembusch said experts see large-scale cannabis producers attempt to use carbon systems to mitigate odor which would really only work on small-scale production. 

"We see time and time again where municipalities get stuck in a bit of a box where they legalized, they want to move forward and they think they have good plans in place," Rembusch said. "But they don't look at geographical areas ... There's this perception that if you just put some quote-unquote carbon scrubbers and that odor will go away."

The recent discovery is the first step to solving the odor issue plaguing legalized states, Long said. 

So, what exactly causes marijuana's 'skuny' smell?

The compound researchers positively identified, 3-methyl-2-butene-1-thiol, or 321MBT, is believed to cause the skunky smell associated with marijuana. A thiol contains sulfur, which gives it its unique smell. The identified compound is one of roughly 400 found in marijuana, over 60 of which are cannabinoids and cause drug-like effects when consumed. 

The thiol component is also used to give natural gas its detectable "rotten egg" smell and is what causes a bottle of beer left in the sun to have a skunky smell, Koziel said.

Beer's "skunky" odor comes from hops that are in the same plant family as cannabis and hemp, according to a Byers news release. MBT is also found in actual skunk oil taken from the anal glands of skunks.

Positively identifying this compound opens up many possibilities for mitigation, Koziel said, including using genetics to breed cannabis with less MBT or finding a chemical, biological or physical way to remove the compound. 

For example, Koziel said his team has used ultraviolet light and biochar to mitigate the manure smell from animal agriculture.

Aside from mitigation, Long and Rembusch said identification of this compound makes it possible to measure the effectiveness of odor mitigation. If a parent is concerned a marijuana operation opened up down the road, Long said they may be complaining before there even is a smell.

"Because it's cannabis. There's a stigma around the plant," Long said. "It's federally illegal still, and people are not behind it and don't support it."

As smell is heavily based on perception, this makes it difficult to tell when a complaint is genuine. 

Since researchers can detect the quantity and quality of molecular compounds, they will be able to measure how many of those compounds a filtration device picks up, Rembusch said. This allows them to mathematically prove the system is working and a complaint is disingenuous. 

While marijuana became legal just over the Mississippi in Illinois last year, Iowa Gov. Kim Reynolds said in 2019 she will not be the governor to legalize recreational marijuana in Iowa but signed a bill to expand the medical use of marijuana last year.

Rembusch said he thinks legalization across the country is inevitable. 

"Maybe this is an opportunity for (Iowa) to be prepared because it's going to come to Iowa," Rembusch said. "It may be the last state, but it's gonna happen."

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