Pesticides in Cannabis – an Explainer

By Jamie Toth and Dr. Jeff Rawson

Defining Pesticides

Let’s start by defining what a pesticide is, so we are all on the same page. A pesticide is a treatment for crops that prevents disease and protects against competition. These include fungicides, herbicides, plant growth regulators, and insecticides. Some pesticides may be sprayed onto plants and do their work on the surface (“contact”), and others can go into the plant and do their work from the inside (“Systematic”).

Types of Pesticides

Fungicides – prevent the growth of fungi and their spores, and they can be biological or chemical.

Examples in Cannabis: Chemical: Potassium bicarbonate, Biological: Bacillus Amyloliquefaciens, (harmless bacteria).

Herbicides – prevents or controls unwanted vegetation / plants (weeds).

Examples in Cannabis: Acetic Acid, Capric Acid.

Plant Growth Regulators (PGR) – Plant Growth Regulators are chemicals that modify the growth of crop plants and limit the growth of intrusive plants. For more information about PGR, check out this awesome recent article from Sensi Seeds:

Examples in cannabis: Harpin Protein, Gibberellic Acid.

Insecticides – kill or deter insects. Miticides, which kill arachnids such as mites, are often lumped in with insecticides.

Examples in cannabis: Isaria Fumosorose, Azadirachtin.

How Pesticides are Regulated in the USA

Pesticides are regulated at the federal level by the Environmental Protection Agency (EPA) via the Federal Food, Drug and Cosmetic Act (FFDCA) and the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). These laws put EPA in charge of the sale and use of pesticides, and of setting safe limits for their residues on food or animal feed. The limits are enforced by various state departments of agriculture and the federal Food and Drug Administration (FDA).

The key challenge for regulating the use of pesticides in cannabis is the way FIFRA was designed and written. Every pesticide must be approved individually for each specific crop. Since it has been illegal ever since FIFRA passed (1941), no pesticides were permitted for use on cannabis for decades.

This makes it hard for states to set reasonable limits. Any detection of any pesticide is a violation of a law that states are supposed to enforce. Testing labs are usually required to report positive results for any pesticide on cannabis to state authorities.

Setting Limits

Restrictions for the use of pesticides are not based upon assumptions of how they might harm humans, but rather upon evidence of their damage to organisms and the environment. The EPA states that it makes its determinations by considering ‘the toxicity of the pesticide and its break-down products, the aggregate exposure to the pesticide in foods and from other sources of exposure, and any special risks posed to infants and children.’ This scientific evidence is challenging to accumulate.

The only agricultural product for smoking that has been regulated by the US federal government is tobacco. Tobacco leaves are cured and smoked, so the science of pesticides in tobacco can instruct us about how we should regulate them in cannabis.

A particularly clear account of the steps which must be considered is “The Nature and Significance of Pesticide Residues on Tobacco and in Tobacco Smoke (1968)” by F.E. Guthrie, which outlines five questions that scientists must answer to put a number on how much pesticide is safe to use, using the example of tobacco cigarettes with TDE (Dichloro-2,2-bis[p-chlorophenyl]ethane).

1) How much pesticide stays on/in the plant when it’s applied?

This was determined by measuring the weight of TDE residue on a certain amount of tobacco leaves after the plants were sprayed with a controlled amount of TDE. When there were really small percentages, they were expressed as “parts per million” (ppm), which means “millionths of a gram pesticide per gram leaves”.

2) How does curing affect the amount of pesticide or byproducts?

This was calculated by measuring how different curing processes changed the amount of pesticide on the leaves. Depending on how it would be used, some tobacco is cured at room temperature and some at high temperature, so both had to be checked.

3) How much of the pesticide residue gets into the smoke when it’s burned?

This is measured by determining what percentage of pesticide ends up in the smoke when a cigarette burns. It turns out that the contents of mainstream smoke, which is pulled through the cigarette and inhaled by the smoker, are different from the contents of sidestream smoke, which is emitted to the surroundings from the cigarette. In general, a “steam distillation” process carries whole molecules of pesticide through the cigarette to the smoker, while breakdown products of the pesticide end up mostly in the sidestream.

4) How much pesticide stays inside a person when they inhale the smoke?

This measured how much pesticide stays inside people when they inhale it in smoke. Guthrie’s summary includes the story of an experiment with human smokers inhaling radioactively-tagged TDE, then having their bodies measured to quantify how much TDE stayed inside them.

5) How much pesticide does a person have to take in for it to affect their health (a “Point of Departure”, POD)?

This measured how much pesticide it takes to impact the health of a person. It’s never been considered ethical to deliberately poison humans for science, so this information almost always comes from studies of mice and rats.

When we consider Guthrie’s summary, it becomes clear how hard it will be to set scientific limits on how pesticides are used in cannabis cultivation. You need to figure out how close to flowering (or close to harvest) it’s safe to spray a given pesticide, based upon each one’s unique persistence. You need to consider how curing or flash freezing affect the residue on the flower, and how it transfers from the flower during extraction. You need to consider all the ways cannabis might be smoked. What happens with pesticides might be different in a bowl than a joint than a vaporizer, and some might be trapped in a bubbler. You need a bunch of information about the consequences of human exposure, and nobody will let you do any of those experiments on people.

You would need all of this information, for every product that will be made from cannabis and consumed by a person. Then you could finally set a scientific standard and label a pesticide for use on cannabis. Since all of that ranges from hard to impossible, it’s easier to prohibit all pesticides, at least while plants are in flower. Because some pesticides are “systematic” and brought into the plant’s tissues, even avoiding sprays during flowering may not be enough to protect consumers.

How to Consider the Risks of Pesticides on Cannabis

It’s important to put all of this within context. Hazard assessments of tobacco pesticides often found POD, the lower limit for health impact, at ppm, millionths of grams of pesticide per gram of tobacco. State testing labs usually detect pesticides on cannabis at the levels of parts per billion (ppb), one thousand times lower in concentration. Efforts to estimate the hazards of pesticides on cannabis that used the available data have suggested that the risks of noticeable impacts to health are low for regular smokers.

Regardless, I wouldn’t want this on my weed!

We don’t have enough data to really know. It’s hard to measure the impact of chronic exposure, for one thing.

More significantly, workers in cannabis handle thousands of times the weed that any user does. Their bodies bear the burden of every hazard that cannabis contains, and we are likely to see health outcomes for this workforce emerge in the years to come.

All of these challenges with permitting pesticides for use on cannabis have made cultivators secretive about them. It’s hard to get good information about what pesticides are really used on cannabis, and how, when growers are afraid to admit to their unpermitted use.

Now that you know a bit more about pesticides, read about some of the pesticides that intersect with cannabis!

Pesticides Found in Cannabis / Marijuana During Recent Shelf Tests (under development)

Pesticides Found in Cannabis / Marijuana In Literature (under development)

Pesticides Identified by States for Testing in Cannabis / Marijuana (under development)

Pesticides Found in Cannabis / Marijuana In Recalls (under development)