Metals in Cannabis and Related Substances—Regulations and Analytical Methodologies

March 1, 2018
Abstract / Synopsis: 

Evolution in the legislation regarding the production and consumption of cannabis and related products for medicinal and recreational uses has led to emerging regulations regarding the potency, terpene profiles, and impurity content of such products. Among the impurities, or contaminants, that require testing are metals. Because of their toxicity and carcinogenicity, most jurisdictions are, at a minimum, requiring testing for arsenic, lead, cadmium, and mercury. Metals accumulate in plant material through normal metabolic processes. Furthermore, some plants are capable of hyperaccumulating metals to concentrations well above those in the soils and waters that nourish them. Some such metals are beneficial or nutritional in nature, whereas others show varying levels of toxicity. As such, testing for metals, particularly ones that are toxic, are important for products destined for human consumption. Quantitation of metals within cannabis materials can be accomplished through a variety of analytical techniques. Such amenable technologies include atomic absorption (AA) spectroscopy, inductively coupled plasma-optical emission spectroscopy (ICP-OES), and inductively coupled plasma-mass spectrometry (ICP-MS). As with all aspects of analytical instrumentation, each method has its advantages and disadvantages. Here, we present an overview of analytical methodologies, challenges facing the analyst, and notes on regulatory stipulations.

Recent shifts in legislation efforts in the United States and other countries have led to the use of cannabis and related products (for example, cannabis-based extracts, tinctures, and “edibles”) for medicinal or recreational purposes. In the United States, legalization efforts have been concentrated at the state level, with nine states and the District of Columbia allowing the full use of cannabis for medicinal and recreational purposes, whereas 29 states as well as Guam and Puerto Rico allow various levels of medicinal use of cannabis. In addition, 13 other states have decriminalized cannabis, which means criminal penalties from simple possession are removed. However, there are still three states that completely prohibit any possession or use of cannabis. As of the start of 2018 with legalization taking effect in California followed shortly thereafter by Vermont, approximately 1 in 5 United States residents (~21%) live in states with completely legalized cannabis. Abroad, several countries offer various levels of legality, including full legality (such as in Uruguay), partial legality (in the Netherlands), or have decriminalization statutes that remove criminal penalties for cannabis possession and use.

Because the final products are destined for human consumption through inhalation, ingestion, or topical application, jurisdictions in which cannabis is legal have recognized the need for control of various chemical aspects of the plant and its derivative products. Such regulations focus on potency, pesticide contamination, residual solvent content, moisture content, and metal content, and they necessitate the use of a variety of analytical techniques including high performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), gas chromatography (GC), GC-MS, and elemental spectroscopy methods (1). These regulations have been recognized as particularly important for medicinal cannabis because consumers may have weakened metabolic systems or compromised immune responses, limiting the effectiveness of their bodies to tolerate or process extraneous chemicals, toxins, metals, or biological material (2). For the purpose of this article, we explore the nature of, regulations on, and analytical methods for determining the metal content of cannabis and associated materials.

Health Effects of Metals

Metallic elements are known for their complex biochemistries and variety of both beneficial and deleterious health effects within the human body. Although many metals serve nutritional and metabolic purposes within the human body, others can act as toxins or carcinogens (3). For example, iron consumed in food is used by the body to transport oxygen within blood and is also used in cellular respiration, whereas consumption of mercury or lead can cause damage to the nervous system (4). Because of these effects, both positive and negative, the content of metals in products destined for human consumption (for example, food and pharmaceutical products) is often regulated by a local or federal oversight agency (such as the US Food and Drug Administration [FDA] or the US Environmental Protection Agency [EPA]).

Metals in Cannabis

Like all plants, cannabis uptakes metals from its environment as a result of normal plant metabolic functions. Some of these metals are naturally occurring, and leach into groundwater that the plant uptakes from the soils and minerals in which it grows. Other metals precipitate in rainwater as metal-laden atmospheric aerosols that are dissolved into precipitation. Most of these atmospheric metals arise from anthropogenic sources, such as smelting, refining, and other industrial processes. Finally, metals may be introduced into the plant’s environment as constituents of fertilizers, pesticides, herbicides, and fungicides used to increase crop yield. Regardless of their provenance, when metabolized, metals are absorbed and transported through the plant roots and into plant tissue, where they often reside as metal complexes within proteins and extracellular fluids (5).

Cannabis is so effective at uptaking metals from its environment that hemp (a nonpsychoactive variety of cannabis) has been used as a tool in the bioremediation of metal-contaminated sites. For example, hemp was used to leach radioactive strontium and cesium from the soils surrounding Chernobyl (6). In fact, cannabis has been called a hyperaccumulator of various trace metals, including lead, cadmium, magnesium, copper, chromium, and cobalt (7), which leads to concern that these elements may occur in high concentrations in cannabis plants cultivated for human consumption.

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  2. A. Hazekamp, Cannabinoids 1(1), 1-9 (1996).
  3. C. Reilly, The Nutritional Trace Metals (Wiley-Blackwell, Oxford, UK, 2004).
  4. L. Järup, Br. Med. Bull. 68(1), 167-182 (2003).
  5. I. Raskin, R.D. Smith, and D.E. Salt, Curr. Opin. Biotechnol. 8, 221-226 (1997).
  6. E. Charkowski, “Hemp ‘Eats’ Chernobyl Waste, Offers Hope for Hanford,” in Central Oregon Green Pages (1998). Republished:
  7. M. Girdhar, N.R. Sharma, H. Rehman, A. Kumar, and A. Mohan, Biotech. 4(6), 579-589 (2014).

How to Cite This Article

A.P. Fornadel, D.L. Davis, R.H. Clifford, and S.A. Kuzdzal, Cannabis Science and Technology 1(1), 36-41 (2018).