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Pyrethrins: A challenging cannabis contaminant

by Mike May
Published: Sep 28, 2017   
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Cannabis growers often use the insecticide pyrethrins to control pest species and protect their crops. These come from a natural source—flowers of the chrysanthemum plant. Various forms of insecticidal pyrethrins can be extracted from the blooms of these plants. Being a natural product, some people think that pyrethrins must therefore be safe. Natural or not, everything is composed of chemicals. Pyrethrins should not be found on edibles or smokables at any significant concentration. But, when these chemicals are used on cannabis, they can be incredibly difficult to quantify.

Pyrethrins kill insects on contact. But these chemicals hurt more than insects. Pyrethrins can also be harmful to humans, especially in cannabis products designed for smoking, because these chemicals can be easily absorbed through the lungs. Compared to many pesticides, pyrethrins are not very toxic to humans, and the most common problem is an allergic reaction, but it can last for days. The reaction gets worse with an increase in the dose.

The trouble in detecting pyrethrins in cannabis, says Adrian Devitt-Lee, Senior Research Associate at CannaCraft, “stems from the chemical similarities between cannabinoids and pyrethrins.” To try to get a better reading, chemists often dilute samples to reduce so-called matrix effects—basically things that get in the way of what a scientist is trying to measure. “Matrix effects are the various interactions between the solution, which can be 20% THC [tetrahydrocannabinol] in cannabis flower, and the analyte—pyrethrins, in this case,” Devitt-Lee explains. “But because pyrethrins are only at concentrations around 1 part per million, we can’t dilute the solution very much.” He adds, “Low concentrations don’t prevent chemicals from being detected, but they reduce a chemist’s flexibility.”

Beyond the low concentrations, the chemical similarity between cannabinoids and pyrethrins amplifies the challenge. As Devitt-Lee says, “Because pyrethrins are very similar to cannabinoids, we are looking for a signal 500,000 times smaller than the background in this 20% THC example.”

Despite those challenging numbers, Devitt-Lee is optimistic. “Eventually a lab will validate a method for quantifying low levels of pyrethrins in cannabis, but I don’t know of any in California that have,” he says.

There needs to be a good method, though, because some states regulate the use of pyrethrins on cannabis. For example, a wide range of forms of pyrethrins are listed on Oregon’s “Guidelist for Pesticides and Cannabis,” along with an alert that states: “Recent laboratory results show high levels of the active ingredients pyrethrins and/or piperonyl butoxide in some cannabis samples.” It adds that the state is “investigating why some samples indicate levels of one or both of these pesticides, which far exceed the Oregon Health Authority (OHA) Action Levels, and yet others do not.”

The answer might be this: With pyrethrins being so difficult to detect in cannabis samples, it’s easy for a laboratory to produce an inaccurate result. Moreover, the lack of a validated method for analyzing these insecticides leaves laboratory scientists to decide on their own how to run the analysis. At the very least, that contributes to irreproducibility between labs. It’s possible, as well, that labs try different methods with different samples or products, which could further complicate any comparison of pyrethrin levels from one product or raw material to the next.


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