Introduction to Marijuana

Marijuana is an annual, dioecious (having separate male and female plants), flowering herb in the Cannabaceae family. There are three species of Cannabis: sativa, indica, and ruderalis. Hemp is a variety of Cannabis sativa developed to produce fiber and contains very low amounts of the bioactive THC (tetrahydrocannabinol); varieties of all three species that produce higher amounts of THC are available for cultivation for medicinal or recreational use. THC is produced by trichomes on flowers borne on the female plants; as such, the flower buds are harvested and processed for use. For information on marijuana products, chemical constituents, health effects, history, and more, see our article, Marijuana.

Legal Status of Marijuana in the U.S.

In the United States, marijuana is classified as a Schedule 1 controlled substance under the US Controlled Substances Act, which is the highest classification under the Act. As such the use, possession, sale, cultivation, and transportation of marijuana is illegal under federal law, but various states have laws permitting the possession (in small quantities), sale, transport, and/or cultivation of marijuana. Medicinal and recreational marijuana use are fully legalized in the states of Alaska, Colorado, Oregon, and Washington; use of medicinal marijuana is legalized in 25 states and the District of Columbia. Most states that permit the sale of medicinal marijuana also allow for its cultivation.

Map of the US Showing US Cannabis Laws

Environmental Health Impacts of Marijuana Cultivation

Energy Use

Marijuana can be grown indoors, outdoors, or in greenhouses. Indoor and greenhouse marijuana can be grown in soil, growing mixes (various organic and mineral ingredients substituting for soil), or hydroponically (in water enriched with nutrients). As a whole, the most significant environmental impact of marijuana cultivation is the energy intensity of indoor cultivation. The majority of the energy is used for lighting and ventilation (to remove heat and humidity); marijuana requires about 2000KWh of energy per pound of product (BOTEC). This level of energy intensity is very high compared with other horticultural products, and energy can account for over one-third of total production costs. One study from 2012 estimates that indoor marijuana cultivation accounts for approximately 1% of the total electricity used in the United States (Mills).

Lighting is usually provided by a mix of metal halide (MH) and high-pressure sodium (HPS) lamps, which are replaced every three to four growing cycles. Carbon dioxide generators fueled by natural gas or propane are often used in indoor grow houses to boost plant growth. Illegal operations that run on off-grid power may have altered energy use profiles due to the use of diesel or gasoline generators. Indoor growing, despite the cost of energy use, has generally been the preferred cultivation method due to the ability to minimize and hide the cultivation area (in illegal operations) and to control environmental conditions to produce larger amounts (multiple harvests per year) of high-grade, profitable products. An additional concern about lighting is that both metal halide and high-pressure sodium lamps contain mercury; it is important for these lamps to be properly recycled after use.

Greenhouse cultivation requires significantly less energy than indoor cultivation; the primary use of energy is for heat, though not all greenhouse designs require supplemental heat. The operation of greenhouses to grow marijuana is similar to that of certain flowers and out-of-season vegetables.

Water Use

The water required to grow marijuana is also significant, especially in hydroponic production. One study estimates that one cultivation room of 22 square meters (about 240 square feet) requires 151 liters (40 gallons) of water a day (Mills). Another estimate is that individual marijuana plants can use up to 6 gallons of water per day, depending on the season (Moxley).

Colorado State University Extension states that for outdoor cultivation, marijuana requires 25-35 inches per year (as opposed to 12-15 inches for hemp). This is in a similar range as common agricultural crops such as corn, alfalfa, and peaches.

Fertilizer Use

Colorado State University Extension states that nutrient requirements of marijuana are variable during different growth stages and that reliable information is not available. However, hemp requires nitrogen-rich soils and in general the nutrient requirements of Cannabis are similar to that of corn (BOTEC). Hydroponically grown marijuana requires higher levels of nutrients than soil-grown marijuana (BOTEC). 

Pesticide Use

Regulatory Framework

Due to its federal status as illegal, the Environmental Protection Agency provides no guidance on the use of pesticides for marijuana cultivation, and no pesticides are registered (permitted) federally for use on marijuana. States that permit marijuana cultivation have taken various approaches to regulating pesticide use: 17 states and the District of Columbia have adopted policies regulating pesticide use. Five of these states (Delaware, Massachusetts, New Hampshire, New Jersey, and Vermont) outlaw the use of any federally registered pesticide, while three states (Delaware, New Jersey, Vermont) have policies requiring marijuana to be grown without any pesticides. Four states (Illinois, Nevada, New Hampshire, Washington) have created or plan to create (in the case of Nevada) lists of pesticides approved for use. State regulations on testing of marijuana products for pesticide residues and on labeling of pesticide contaminants on marijuana products vary. A summary table of pesticide laws in states with legalized marijuana cultivation is available at the end of the report Pesticide Use in Marijuana Production: Safety Issues and Sustainable Options by Beyond Pesticides.

Pests and Pesticide Use

While hemp is considered to be a relatively pest-resistant crop, pesticide use is widespread in both indoor and outdoor cultivation. However, it is difficult to find accurate data on pest problems and management. Colorado State University Extension Service states in a 2015 fact sheet: “The specific list of insects and pathogens associated with cultiva­tion of Cannabis at any given site is not well known, due in part to the clandestine nature of production in the past. ... Much of the current pest biology and management information available from written and on-line sources was written by individuals with no training in entomology or plant pathology. This has led to a Cannabis pest management information base that is incomplete, unreliable, and/or simply incorrect." One scientific survey conducted in Mississippi found that most of the insect pests were sap feeders, while others were leaf chewers, pollen utilizers, and one possibly a root feeder. Many potential pests are host specific, specializing in Cannabis and related plants. Two potential insect pests in Colorado are the hops aphid and the hemp russet mite. The hops aphid is typically found in outdoor sites, and the hemp russet mite in indoor sites. The hemp borer is found in the eastern US. Generalist insects that also feed on marijuana include lygus bugs, stink bugs, grasshoppers, thrips, and the two-spotted spider mite. Outdoor marijuana plants are attractive to beneficial insects and insect predators such as lady bugs, minute pirate bugs, predatory thrips, and damsel bugs.

According to a dated (1996) article from the Journal of the International Hemp Growers Association, the majority of hemp pests are insects. Other pests include slugs and snails, which may damage seedlings in damp outdoor settings, birds, which eat seeds, and mammals. An additional article from the Association on Cannabis diseases states: "Serious diseases include gray mold, hemp canker, damping off, assorted leaf spots, blights, stem cankers, root rots, nematode diseases, broomrape, macro- and micronutrient deficiencies, and genetic diseases. Environmentally stressed plants become predisposed to diseases. Stress includes drought, insufficient light, untoward temperatures, or growing plants in monoculture."

In addition to financial incentives to reduce pest damage, the high-density growing conditions with irrigation and increased fertilizer use can cause an increase in pest problems, making the use of pesticides desirable to growers. Due to the lack of federal guidelines on the use of pesticides in both marijuana and hemp production, the regulatory framework in many states permitting marijuana cultivation being in various stages of development and enforcement, and the financial incentives to boost yields and product quality, it is likely that many growers are using a range of pesticides without adequate information on appropriate methods that increase pesticide effectiveness while reducing hazards to workers and the environment. The section below on pesticide residues provides information on specific pesticides detected in marijuana products and exposure pathways to consumers.

"Clean" Marijuana

Some growers emphasize the cultivation of marijuana without pesticides, or with pesticide products that are certified for organic production or are exempt from federal EPA registration (due to lower toxicity). Clean Green is a third-party certification program that certifies marijuana farms, processors, products, and retailers that meet a sustainable agriculture standard including criteria for organic and biodynamic farming practices. The Organic Cannabis Growers Society provides education, resources, and certification. However, as "certified organic" is a federal designation and marijuana is not a federally recognized agricultural or horticultural crop, marijuana grown following organic practices cannot legally be called organic.

Concerns with Outdoor Production

Studies and media reports of illegal outdoor growing operations in California show significant environmental damage: deforestation and hilltop leveling to clear land (increasing the threat of landslides and erosion), damming and diversion of streams to provide water, runoff of polluted water, spills of diesel fuel, littering, and poisoning of wildlife by pesticides (typically rodenticides or rat poison).

Pesticide Residues in Marijuana Products

Like other crops, pesticides used during production or to treat soil prior to production may remain on or in marijuana plants, including the flowers. Pesticides detected in marijuana products grown in Washington are listed below; the majority of the pesticides are not permitted for use on marijuana by the Washington State Department of Agriculture. Testing laboratories in California have reported that marijuana samples are often contaminated with pesticide residues, and in 2009 the Los Angeles City Attorney’s office found that two of three samples of marijuana available at dispensaries contained high amounts of the insecticide bifenthrin (Sullivan). In 2015 the Cannabis Safety Institute tested samples of 389 flowers and 154 concentrates from Oregon growers for 65 pesticides and found residues of 24 pesticides;14% of the flowers and 46% of the concentrates contained residues of at least one pesticide at a level above 100 parts ber billion (or 0.1 parts per million), and nearly 5% of the concentrates contained pesticides at levels above 50,000 ppb (or 50 ppm). The most commonly found pesticides in this study were bifenthrin and piperonyl butoxide (technically an insecticide synergist). A 2015 study from the Journal of Forensic Science tested 130 marijuana samples for the herbicides paraquat, glyphosate, and AMPA (aminomethylphosphonic acid) and found paraquat in 12 samples, glyphosate in three samples, and AMPA in one sample (Lanaro et al).

Four states (Arizona, Colorado, Nevada, and Washington–for recreational use) and the District of Columbia require marijuana products to be labeled with the names of all pesticides used in production, but products are not necessarily in compliance.

 

Pesticides Detected in Samples in Washington*Type of PesticideApproved for Use on Marijuana by WSDA?

Boscalid

FungicideN
CarbarylInsecticideN

Carbendazim

FungicideN
DinotefuranInsecticideN
DiuronHerbicideN
ImidaclopridInsecticideN

Myclobutanil

FungicideN
Piperonyl butoxideInsecticide synergist (enhances effectiveness)Y
PermethrinInsecticideN (but pyrethrin, a similar insecticide, is approved)
PropinconazoleFungicideN
SpiromesifinMiticideN
WSDA's list of approved pesticides is available here.

 

Twelve states require that agencies test random batches of marijuana products for pesticide residues. Taking Washington as an example, in September 2016 the Washington State Department of Health filed an emergency rule requiring the testing of medicinal marijuana for the unapproved pesticides below. Concentrates and extracts must also be tested for pyrethrins and piperonyl butoxide (insecticide synergist), which are permitted.

  • Abamectin (insecticide, acaricide)
  • Bifenthrin (insecticide)
  • Chlormequat chloride (plant growth regulator)
  • Daminozide (plant growth regulator)
  • DDVP (Dichlorvos) (insecticide, organophosphate)
  • Imidacloprid (insecticide)
  • Myclobutanil (fungicide)
  • Paclobutrazol (plant growth regulator)
  • Permethrin (insecticide)
  • Propiconazole (fungicide)
  • Spinosad (insecticide)
  • Spiromesifen (miticide)
  • Uniconazole (fungicide)

 

There is little peer-reviewed research on pesticide residues in marijuana, and possible health risks to the consumer via inhalation are difficult to evaluate because toxicological data on pesticides typically do not examine exposure via combustion. However, inhaled chemicals have direct access to the bloodstream (with absorption rates varying by chemical), and a study published in 2013 in the Journal of Toxicology found that up to 69.5% of pesticide residues remain in marijuana smoke (Sullivan et al). Three smoking devices were used: a hand-held glass pipe, an unfiltered water pipe, and a filtered water pipe. The recovered amounts were highest with the hand-held glass pipe (60.3% to 69.5%), followed by the unfiltered water pipe (42.2% to 59.9%) then the filtered water pipe (0.08% to 10.9%). Since marijuana, unlike tobacco, is typically smoked without filters, this shows that exposure levels may be very high. Also, complex chemical reactions occur during combustion and users are exposed to many chemicals, some of which may not be known: "Additionally, during heating pyrolysis (breakdown) products from the plant material form a highly complex mixture of products, many of which may interact with the pesticides or pyrolysis products of the pesticides forming more toxic materials, or highly toxic pyrolysis products may form from the pesticide residues alone." Individual exposure levels also vary: "Different user behaviors including depth of breath, length of inhalation hold time, and choice of heating method may also impact overall individual exposure amounts." The authors conclude: "High pesticide exposure through cannabis smoking is a significant possibility, which may lead to further health complications in cannabis users.” The pesticides added to the marijuana (dried and processed flowers) then tested for in smoke were bifenthrin, diazinon, and permethrin, along with the plant growth regulator paclobutrazol.

One 2002 study on pesticide residues in tobacco found that 1.51% to 15.5% of pyrethroid insecticides were transferred to smoke. Data on pesticide residues on tobacco is limited: EPA regulates pesticides used in tobacco production but monitors residues only to screen for unapproved pesticides. As stated by a report by the Government Accountability Office, "EPA has concluded that the low levels of residues in tobacco smoke do not pose short-term health concerns requiring mitigation. EPA does not assess intermediate or long-term risks to smokers because of the severity of the health effects linked to use of tobacco products themselves."

An issue requiring further investigation is whether pesticide residues tend to be higher in certain marijuana products. Marijuana concentrates or dabs are increasing in popularity; a 2015 study from the Journal of Toxicological Sciences found that 80% of samples of marijuana concentrates tested were contaminated with residual solvents (used in the extraction process) or pesticides (Raber et al). Testing in 2015 by the Cannabis Safety Institute found that mean levels of pesticide residues were roughly ten times higher in concentrates than in flowers.

Ingestion is another route of exposure to pesticide residues in marijuana. Further research is required to compare the residue levels of pesticides permitted for use on food crops with residue tolerances set by EPA; as demonstrated by testing done by the Cannabis Safety Institute (mentioned above) and others, marijuana products can contain residues far above food tolerances. However, ingestion of pesticide residues within the EPA tolerances does not mean that there is no risk, but rather "a reasonable certainty that no harm will result from aggregate exposure." A few states (Connecticut, Illinois, Nevada) require that products found to contain pesticides permitted for use on food crops at levels above residue tolerances be disposed of. In Oregon, the maximum permitted amount of any pesticide on medicinal marijuana is 0.1 parts per million.

Opportunities to Reduce Environmental Health Impacts

Energy and Water Use

  • Develop and promote LED or other low-energy-use light bulbs and encourage recycling of mercury-containing lamps
  • Encourage outdoor growing in states with legalized cultivation, with guidelines for minimizing environmental impact

Pesticides

  • Ban or discourage the use of pesticides beyond the vegetative phase (such as in Illinois), which would reduce the levels of residues on the harvested flowers
  • Encourage the use of organic or low-pesticide cultivation methods
  • Enforce regulations in states that require organic or pesticide-free cultivation, routinely testing products for residues to determine compliance
  • In states permitting pesticide use, require labeling of any pesticides used for production and routinely test products to determine compliance
  • Develop information resources on pest management and on organic or low-pesticide cultivation methods
  • Investigate and encourage the use of filters to reduce users' exposure to contaminants
  • Encourage or require medical dispensaries to sell products grown without pesticides

References and Resources

Beyond Pesticides. Pesticide Use in Marijuana Production: Safety Issues and Sustainable Options. Pesticides and You. Vol. 34, No. 4 Winter 2014-2015.

Borel, Brooke. The Wild West of Marijuana Pesticides. The Atlantic. August 31, 2015 (accessed September 29, 2016)

BOTEC Analysis Corporation. Environmental Risks and Opportunities in Cannabis Cultivation (white paper). June 28, 2013.

Cai J, Liu B, Zhu X, Su Q.  Determination of pyrethroid residues in tobacco and cigarette smoke by capillary gas chromatography. J Chromatogr A. 2002 Jul 26;964(1-2):205-11.

Cannabis Safety Institute. Pesticide Use on Cannabis (report). June 2015.

Colorado State University Extension Service. Current Impacts of Outdoor Growth of Cannabis in Colorado (fact sheet). July 2015.

Coughlin-Bogue, Tobias. Washington State Fines Two Marijuana Growers for Using Prohibited Pesticides. The Stranger. February 11, 2016.

EPA. About Pesticide Tolerances. (accessed October 3, 2016)

Gross, Rachel E. Why Is It So Hard to Get Clean Weed? Slate. April 2016.

Lanaro R, Costa JL, Cazenave SO, Zanolli-Filfo LA, Tavares MF, Chasin AA. Determination of Herbicides Paraquat, Glyphosate, and Aminomethylphosphonic Acid in Marijuana Samples by Capillary Electrophoresis. Journal of Forensic Science. 2015 Jan;60 Suppl 1:S241-7.

Melamede, Robert. Cannabis and tobacco smoke are not equally carcinogenic. Harm Reduct J. 2005; 2:21

McPartland, J. M., 1996. A review of Cannabis diseases . Journal of the International Hemp Association 3(1): 19-23.

McPartland, J.M. 1996. Cannabis pests. Journal of the International Hemp Association 3(2): 49, 52-55.

Mills, Evan. The carbon footprint of indoor Cannabis production. Energy Policy 46 (2012): 58-67.

Moxley, Mitch. Green But Not Green: How Pot Farms Trash the Environment. Slate. April 2014. (accessed September 29, 2016)

Okanogan Cannabis Association. "The OCA supports the economic revitalization of Okanogan County by supporting small scale sun grown cannabis producers and the safe, secure production of the state’s legal cannabis crop.”

Raber JC, Elzinga S, Kaplan C. Understanding Dabs: Contamination Concern of Cannabis Concentrates and Cannabinoid Transfer During the Act of Dabbing. Journal of Toxicological Sciences. 2015 Dec;40(6):797-803.

Sullivan S, Elzinga S, Raber, JC. Determination of Pesticide Residues in Cannabis Smoke. Journal of Toxicology. Volume 2013, Article ID 378168.

Toomey, Diane. The High Environmental Cost of Illicit Marijuana Cultivation. Yale Environment 360. July 16, 2015.

US Government Accountability Office (GAO) Report: Pesticides on Tobacco: Federal Activities to Assess Risks and Monitor Residues. March 26, 2003.

Warren, Gina S. Regulating Pot to Save the Polar Bear: Energy and Climate Impacts of the Marijuana Industry. 40 Colum. J. Envt'l Law. 385 (2015)

Washing State Department of Agriculture. Pesticide and Fertilizer Use on Marijuana in Washington. (accessed Septmber 29, 2016)

Washington State Department of Health. Medical Marijuana Product Compliance. (accessed September 30, 2016)

 

(Image credits: Young Cannabis Plant in the Vegetative Stage (Wikimedia), Indoor Grow House (WSDA))

 

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