On a daily basis, the average person around the world is subjected to doses of heavy metals and other contaminants from a variety of sources. One of the most insidious sources of routes of exposure is through food, beverages, and other sources of oral consumption (that is, nutraceuticals, pharmaceuticals, and so on). The World Health Organization contends that food may be the source of the largest contribution to the intake of contaminants. Many agricultural products may naturally contain heavy metal and other contaminant compounds, from either natural biochemical processes or from bioaccumulation from the environment. Other products become contaminated by natural, agricultural, or industrial sources or poor hygiene methods of production and storage. Finally, there are food and beverage products that are intentionally adulterated or counterfeited with materials containing contamination. In this column, we look at different sources of potential contamination exposure that may be of concern to both the cannabis industry and the cannabis consumer from the perspective of an analytical industry professional with decades of experience in metals analysis. The guest author of this installment, Robert Thomas, has worked as an analytical chemist in the field of trace element analysis for more than 45 years, including 24 years for an inductively coupled plasma-mass spectrometry (ICP-MS) manufacturer and 19 years as principal of his own consulting company.
An added complication is that the cannabis and hemp plant can not only absorb heavy metals from the soil, but also from contaminants in fertilizers, nutrients, pesticides, and the growing medium as well as from other environmental pathways (11). Additionally, the process of cutting, grinding, and preparing the cannabis and hemp flowers for extraction can often pick up elemental contaminants from the manufacturing equipment. Finally, the cannabinoid extraction process will extract different amounts of heavy metals, depending on the solvent or the super- and sub-critical extraction process used and could possibly end up in the finished products (12). It’s also worth pointing out that the equipment used to deliver these products to consumers such as inhalers, vaporizers, and transdermal patches can mean the user is exposed to additional sources of elemental contaminants, apart from what’s in the cannabinoid compound itself.
Phytoremediation Properties of Cannabis and Hemp
Cannabis and hemp are known to be hyper-accumulators of contaminants in the soil (13). That is why they have been used to clean up toxic waste sites where other kinds of remediation attempts have failed. In the aftermath of the Chernobyl nuclear melt down in the Ukraine in 1986, industrial hemp was planted to clean up the radioactive isotopes that had leaked into the soil and ground waters (14). Of course Chernobyl is an extreme example of heavy metal and radionuclide contamination, but as a result of normal human (anthropogenic) activities over the past few decades including mining, smelting, wood processing and treatment, electroplating, gasoline exhaust, energy production, use of fertilizers, pesticides, waste treatment plants, lead-based paint and plumbing materials, and so on, heavy metal pollution has become one of the most serious environmental problems today.
Phytoremediation using certain plants is emerging as a cost-effective technology to concentrate and remove elements, compounds, and pollutants from the environment. Within this field of phytoremediation, the use of cannabis and hemp plants to concentrate metals from the soil into the harvestable parts of roots and above-ground shoots (phytoextraction) has great potential as a viable alternative to traditional contaminated land remediation methods (15). However, the natural inclination of these plants to absorb heavy metals from the soil and growing environment could potentially limit its commercial use for the production of medicinal cannabinoid-based compounds. A number of studies have now been carried out on cannabis and hemp that provides convincing evidence that they are active accumulators of heavy metals such as lead, cadmium, arsenic, mercury, magnesium, copper, chromium, nickel, manganese, and cobalt, as the result of human activities (16).
Other Factors for Metal Uptake
The high concentrations of heavy metals accumulation achieved in cannabis cannot be explained exclusively by passive ion uptake. The hyper-accumulating properties of cannabis are dependent upon several factors including soil pH, availability of metal ions in solution, the nitrogen, potassium, and phosphorus nutrient content, and the ability of natural or added chelating compounds, such as humic acid and biochar, to bind with the heavy metals to stop them being taken up by the plant. These are some of the most important factors for the scavenging of heavy metals from the growing environment. It should also be noted that the plant’s natural polyamine compounds (amino acid functional groups) will strengthen the defense response of plants and impact their activity against diverse environmental stressors including metal toxicity and oxidative stress.
Based on evidence in the public domain, there are about 15 heavy metals found in natural ecosystems (soil, water, air) that could be potential sources of contaminants accumulated by the plant including Pb, As, Hg, Cd, Ni, V, Co, Cu, Se, Ba, Ag, Sb, Cr, Mo, Mn, Zn, and Fe. Their levels of toxicity would need to be investigated further, but there is a case to be made that the majority of them could be the future basis of a federally-regulated panel of elemental contaminants in cannabis and hemp.
Potential of “Real-World” Sources of Metal Pollutants
With all the diverse and varied conditions used for growing cannabis, it is very difficult to eliminate all the potential sources of elemental contaminants to reduce their impact on the plant. However, it is well recognized that heavy metal pollution has been problematic over the past few decades, which if not minimized, can result in enhanced levels in cannabis and hemp. They might not all have a negative impact on the health of the plant during cultivation, but the chances that they will end up in the flowers and the final manufactured products are very high. It’s therefore worth listing some of the many potential “real-world” sources of elemental contamination, both from a cannabis and hemp plant cultivation perspective as well as the cannabinoid manufacturing process.
Indoor and Outdoor Growing Sources
- Heavy metals in soil and growing media, particularly for plants that are cultivated outdoors
- Areas surrounding active or abandoned mines, particularly gold and silver mines that used mercury for extraction
- Poor water quality used in hydroponics for the growing of cannabis plants indoors (think of Pb-contaminated water in Flint, Michigan)
- Chromium, arsenic, and copper compounds used to treat and preserve wood
- Use of low-grade fertilizers or nutrients made from phosphate rocks, which contain significant amounts of heavy metals
- Inorganic pesticides that contain, arsenic, copper, lead, and mercury
- Emission of elemental mercury into the atmosphere from industries such as coal-fired power plants, metal refineries, petrochemical plants, and cement works (approximately 100 tons of Hg are emitted by U.S. industries annually)
- Decades of adding tetra ethyl lead to gasoline as an engine anti-knocking agent, which has ended up in the soil, particularly around major highways
- Decades of using lead, cadmium, and arsenic based pigments in paint
Manufacturing and Production Sources
- Manufacturing equipment used to produce the myriad of cannabis-based products such as stainless steel, plastic, polymers, and glassware
- Solvents and chemicals used to extract, infuse, and concentrate cannabinoids from the plants (super- or sub-critical extraction temperature and pressure will have an impact how much heavy metal is extracted into the final product)
- Leaching of heavy metals from delivery devices such as vaping sticks, inhalation devices, and infused transdermal patches
- Fillers and mineral-based raw materials added to tablet formulations
- Materials used in gel cap formulations
- Topical and transdermal cream formulations
- Recipe ingredients for edibles such as cookies and brownies
- Any products or ingredients sourced in Asia can potentially be a source of contamination (think melamine in infant formula and pet food, lead paint on toys, pewter in fake silver jewelry, and metallic components including Pb-based solder used in vaping devices to deliver products via inhalation)
- The Global State of the Hemp Industry, Hemp Business Journal, a division of New Frontier Data Analytics (2019).
- Cannabis Consumer Report, New Frontier Data Analytics (2019).
- “Preventing Lead Poisoning in Young Children,” U.S. Department of Health and Human Services, Public Health Services, Centers for Disease Control (1991).
- R. Thomas, Spectroscopy 34(2), 22–32 (2019).
- R.J. Thomas, Measuring Heavy Metals in Cannabis and Hemp: A Practical Guide (CRC Press, Boca Raton, Florida, to be published in summer 2020).
- Marijuana Policy by State: https://www.mpp.org/states/.
- M.A. Khan, et.al., J. Chem. Soc. Pak. 30(6), 805–809 (2008).
- General Chapter <232> “Elemental Impurities in Pharmaceutical Materials– Limits,” 2nd supplement to United States Pharmacopeia 37–National Formulary 32 (USP37–NF32) (United States Pharmacopeial Convention, Rockville, Maryland, 2014).
- General Chapter <2232> “Elemental Contamination in Dietary Supplements,” 2nd supplement to United States Pharmacopeia 37–National Formulary 32 (USP37–NF32) (United States Pharmacopeial Convention, Rockville, Maryland, 2014).
- General Chapter <233> “Elemental Impurities in Pharmaceutical Materials – Procedures,” 2nd supplement to United States Pharmacopeia 37–National Formulary 32 (USP37–NF32) (United States Pharmacopeial Convention, Rockville, Maryland, 2014).
- W. Chen, et. al., J. Environ. Qual. 37(2), 689–95 (2008).
- B. Whittle, C.A. Hill, I.R. Flockhart, D.V. Downs, P. Gibson, and G.W. Wheatley, US Patent Number, US7344736B2, “Extraction of pharmaceutically active components from plant materials,” GW Pharmaceuticals.
- D.V. Gauvin, et.al., Pharmaceutical Reg. Affairs 7(1), 202. doi: 10.4172/2167-7689.1000202 (2018).
- P. Soudek et. al., in Advanced Science and Technology for Biological Decontamination of Sites Affected by Chemical and Radiological Nuclear Agents, N. Marmiroli, B. Samotokin, and M. Marmiroli, Eds. (IOS Press, Amsterdam, and Springer in conjunction with the NATO Public Diplomacy Division, 2007) pp. 139–158.
- R. Ahmad et. al., Clean Soil Air Water 44(2), 195–201 (2016), https://doi.org/10.1002/clen.201500117.
- D.V. Gauvin, et.al., Pharmaceut Reg. Affairs 7(1),1–99 (2018).
- R. Pappas, et al., J. Anal. Toxicol. 38, 204–211 (2014.
- P. Ziarati, Z. Mousavi, and S. Pashapour, J. Med. Discovery 2(1), jmd16006; doi:10.24262/jmd.2.1.16006 (2017).
- P. Olmedo, et. al., Environ. Health Perspect. 26(2), https://doi.org/10.1289/EHP2175 (2018).
- M. Halstead, et.al., J. Anal. Toxicol. in press (2019).
- P. Atkins, Cannabis Science and Technology 1(4), 40–49 (2018).
- R.J. Thomas, Measuring Elemental Impurities in Pharmaceuticals: A Practical Guide (CRC Press, Boca Raton, Florida, 2018).
About the Guest Author
Robert Thomas is the principal of Scientific Solutions, a consulting company that serves the training, application, marketing, and writing needs of the trace element user community. He has worked in the field of atomic and mass spectroscopy for more than 45 years, including 24 years for a manufacturer of atomic spectroscopic instrumentation. He has served on the American Chemical Society (ACS) Committee on Analytical Reagents (CAR) for the past 19 years as leader of the plasma spectrochemistry, heavy metals task force, where he has worked very closely with the United States Pharmacopeia (USP) to align ACS heavy metal testing procedures with pharmaceutical guidelines. Rob has written almost 100 technical publications.
About the Columnist
Patricia Atkins is a Senior Applications Scientist with SPEX CertiPrep and a member of both the AOAC and ASTM committees for cannabis.
How to Cite This Article
R. Thomas, Cannabis Science and Technology 2(5), 22-30 (2019).