Contaminants In Cannabis Concentrates: A Cause For Concern

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Over 80 percent of the 57 concentrate samples tested in a recent study contained some amount of pesticides and or residual solvents

According to a recent study in the Journal of Toxicological Sciences, cannabis concentrates may contain pesticides and toxic solvents which remain as residue from the manufacturing process.
In order to address the scarcity of research on cannabis concentrates, The Werc Shop, a cannabis testing lab, collaborated with the University of South California to analyse cannabis concentrates for their cannabinoid, pesticide, and solvent content. They utilized 57 samples of cannabis concentrates from California medicinal cannabis users. Forty-eight of these being solvent-based concentrates while the remainder were dry or water-based hash samples.

The THC content of the 9 hash samples ranged from 50-65%, while the concentrates varied more widely. Most of the concentrates had THC content between 65-75%, while seven of them had less than 40% THC, including one sample with less than 5%. Five samples in the concentrate group had high CBD content, but otherwise the median was 1% CBD for both hash and concentrates.While the hash samples were “exceptionally clean” and did not contain any residual solvents or pesticides, the same could not be said for the concentrates. Over 80% of the concentrates were found to contain residual solvents, including isopentane, butane, heptane, propane, and other solvents. Additionally, nearly 40% of the concentrate samples contained pesticides. The most common was paclobutrazol, a plant growth regulator, and bifenthrin and myclobutanil were also detected.


A second experiment was conducted to evaluate how efficiently THC is transferred from a cannabis concentrate to the vapor stream inhaled by the user during dabbing, in which cannabis concentrates are applied to a hot platform and the resulting vapor inhaled through a bong or water pipe. Using a mechanical lung system, the researchers determined that approximately 40% of the available THC content was captured, though there are differences in transfer efficiency depending on the kind of concentrate that is used. Most importantly, transfer efficiency is extremely dependent on individual user variables such as lung surface area and how long the breath is held. Nonetheless, this result should be a useful benchmark for doctors and medical cannabis patients looking to determine how to dose when working with dabbing as a method of ingestion.

The high proportion of samples containing residual solvents, many of which are toxic, demonstrates that producers are still preferring solvent-based methods over carbon dioxide extractions, which produce a cleaner product. Rather than regulate the use of solvents to make concentrates, the authors suggest that regulations should explicitly permit the use of CO2 in order to reduce the use of solvent-based processes in cannabis concentrate production and thus reduce the prevalence of residual solvents in the final product.

The presence of pesticides in nearly 40% of the concentrates tested is also concerning. Special attention must be paid to the use of pesticides in growing cannabis, especially since medical patients may be more susceptible to their toxic effects. In addition, pesticides in smoked material are more toxic to begin with because they bypass metabolism in the digestive tract.

Cannabis concentrates have undergone a surge in popularity in the medical and recreational cannabis communities in recent years. This has been aided by the development of better extraction methods as well as the rise in popularity of dabbing. Some medicinal users find the concentrated dose provided by dabbing to be good for immediate and effective relief of their symptoms, although a majority of medicinal users prefer vaporizing flowers.
Dab users have been reported to prefer dabbing over smoking flowers for its stronger and longer effects and because it requires less inhalations to achieve the desired effect. However, reports from dab users citing higher rates of tolerance and withdrawal have suggested that dabbing may come with a greater risk for dependence.

Cannabis concentrates can be made in a variety of ways, with dry methods being the oldest. Rather than using solvents, trichomes are separated from the rest of the plant matter in a number of ways including using dry ice or a fine mesh screen. The resulting product is often called “kief” or “finger hash”. Water-based methods involve shaking ice cold water containing cannabis plant matter. The low temperature and agitation cause trichomes to become brittle and separate from the plant matter. The trichomes are then sifted and dried, making what is often called “hash”, “bubble”, or “ice-wax”.

Solvent-based methods, such as Butane Hash Oil (BHO) and Rick Simpson Oil (RSO), involve soaking cannabis in a solvent for a period of time and then boiling off the solvent to yield a sticky cannabis resin. Research on these concentrates has shown much of the plant’s terpene content is lost in the heating process, and the solvents, which are often toxic, tend to remain as unwanted residue in the final product.

Liquid gas-based extraction is another solvent-based method; it involves using butane, propane, or other solvents which are gases at room temperature but can be made liquids through cooling or pressurization. For the extraction, the liquid gas is then passed through a hose or tube containing cannabis. This has often been done “in residential areas by non-skilled operators” and has resulted in “catastrophic fires and explosions” due to the use of flammable gases under pressure. Industrial manufacturers used closed systems when working with liquid gas, thus mitigating these dangers. Some producers have turned to making concentrates using compressed carbon dioxide, a method which avoids the use of flammable and toxic solvents altogether.

Concentrates produced with liquid gas-based methods are referred to by various names depending on texture and colour, including “wax”, “crumble”, “honeycomb”, “budder”, “sap”, “shatter”, and others.

In conclusion, the study indicates that liquid gas extraction methods are safer than solvent based extraction methods and do not leave contaminants in their products. This information is invaluable to discerning doctors and their patients, whose health may depend on the cleanliness of their medicine.


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