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CANNABIS CULTURE – Previously stockpiled by the US military, chloropicrin was used as a chemical weapon with effects similar to tear gas.  Minimal exposure has been associated with acute eye irritation, coughing, and severe gastrointestinal effects, such as extreme nausea, colic, and diarrhea.  

Health Canada’s Pesticide Management Regulatory Agency (PMRA) has added chloropicrin to the list of pesticides permitted to be used in outdoor cannabis farms. As a soil fumigant, it is cheaper and more effective than currently approved overground pesticides, but is it also more dangerous than Health Canada is leading us to believe?

Many countries have banned chloropicrin’s use in agricultural settings due to its severe health risks, yet North America has continued to use it for soil fumigation and in stored grains to prevent infestation, despite its ability to be systemically absorbed through ingestion, inhalation, and contact with skin.  In fact, the Canadian Environmental Protection Agency recommends against exposure greater than 0.1ppm.  


Chloropicrin would be injected into the fields well before cannabis crops are planted. 

Although it would seem that this would make its use safer, the off-gassing from chemical breakdown and exposure to heat can have devastating effects.  A study published in 2009 out of California called Reviews of Environmental Contamination and Toxicology found “decomposition of chloropicrin may release the following toxic gases and vapors: phosgene, chlorine, carbon monoxide, nitrosyl chloride, and oxides of nitrogen,” all potentially very dangerous to humans, animals, outlying soil, and water sources.  

According to the CDC, off-gassing of the vapors, which are heavier than air, can lead to “hazardous concentrations that may develop quickly in enclosed, poorly-ventilated, low-lying, or confined areas (e.g., sewers, basements, and tanks).”  The danger seems to be made all the more real by reading the CDC guidelines for preparing an on-site morgue if workers applying the chemical are fatally exposed.  


Iron, zinc, and other light metals also corrode with exposure to chloropicrin, and metal contact, in general, can actually lead to flammable hydrogen gas.  Extreme heat or introduction to oxidants may also lead to explosive reactions.  Some forms of plastics, rubber, and other coatings are vulnerable to deterioration as well, which may make it challenging to chemical barriers around grow locations that would prevent seepage into the nearby property and water sources. 

At the current threshold limit value (TLV) set for chloropicrin of 0.1 ppm for an 8-hr time-weighted average exposure, California still reported over 1000 cases of people becoming ill from chloropicrin use between the years 2000 and 2015.  During human tests with an exposure slightly higher than 0.3 ppm, despite no one being able to sense the presence of the chemical, the maximum tolerance was between 3 and 30 seconds before serious symptoms began to develop.  

Health Canada maintains that chloropicrin is not a carcinogen but many scientists disagree. 


Health professionals and scientists in California banded together to urge the Department of Pesticide Regulation (DPR) to re-evaluate the evidence that chloropicrin indeed does contain cancer-causing agents.  In a collective statement, the group said, “When chloropicrin was evaluated as a Toxic Air Contaminant, the toxicologists from DPR who wrote the Human Health Risk Assessment, the scientists from the Office of Environmental Health Hazard Assessment (OEHHA), and the Scientific Review Panel (SRP) who peer-reviewed it all concluded that chloropicrin is a potent carcinogen with a low threshold for acute eye and respiratory irritation. A comprehensive chloropicrin risk assessment completed by DPR toxicologists last year reaffirmed DPR scientists’ conclusion that chloropicrin is a carcinogen and severe irritant.”

The research was done by the California scientists and professionals also found that chloropicrin exposure was only tested on young, healthy adults with no history of chronic respiratory illnesses, and having not had smoked, used recreational drugs, or had any evidence of sickness within a year of the study.  Children, the elderly, and vulnerable populations were not included in the testing.  In addition, exposure was limited to 60 minutes, which was then translated into a recommended exposure over an 8-hour period, without this period of time actually being observed.  Concentrations of soil fumigation vary over time, and at peak levels were found to be well above safe limits, which means the results of the study over 60 minutes “will not be protective of the health of even healthy individuals, and could pose very serious risks for vulnerable populations.”

The OEHHA reiterated the dangers of chloropicrin in its response to DPR’s Risk Management Directive, writing: “OEHHA respectfully disagrees with DPR’s conclusion that evidence on the carcinogenicity of chloropicrin should be viewed as equivocal.  Chloropicrin has been observed to induce gene mutations and chromosomal damage.  The DPR chloropicrin document, the OEHHA chloropicrin findings, and the (Scientific Resolution Panel) chloropicrin findings all state that chloropicrin is a genotoxic carcinogen and can be assigned a cancer potency factor of 2.2 (mg/kg-day).”  The letter continues, “The combination of the animal studies and the in vitro studies indicate that there is no question that there is evidence of carcinogenicity.”


Clearly, the fact that carcinogenicity is being denied by Health Canada, yet insisted upon by other health organizations, is a massive red flag of chloropicrin use.  Another concern is that soil fumigation renders the earth essentially dead, killing off not only the bad bugs and microbes but also the good ones.  Many farmers prefer to have healthy living soil, which leads to better and stronger crops.  The cost of fumigation, however, is significantly less than that of overspray pesticides, so some growers opt to sacrifice quality for overall profit.  

Ultimately farmers will have the choice of whether or not to use chloropicrin in their crops. 

Many will likely continue to use overspray pesticides to protect their agriculture, soil, and provide the best quality products, but undoubtedly there will be those swayed by the low-cost and effective chemical application, especially since labeling of products grown in fumigated soil is not required.  If cannabis quality decreases due to these cost-cutting measures, this could lead to an increase in black market sales as consumers continue to search for the most potent products available grown in ways that impact the environment less substantially.  

In the interim, Health Canada has said that their last review of chloropicrin was in 2018 and that they “continue to monitor for new information related to pesticides and will take appropriate action if any risks of concern to human health or the environment are identified.”  Considering how many risks have already clearly been identified, and since the main motive of fumigants over safer and more sustainable pesticides seems to be monetary in nature, it seems that only time will tell how the use of this chemical compound in our soil will truly affect both the short-term and long-term welfare of our agriculture, pesticide applicators, and everyday citizens. 

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