Heavy Metals in Cannabis: The Big Four
What exactly is a heavy metal? There is no precise technical definition. In the Periodic Table of Elements there is a staircase darkened line that starts to the left of boron and then moves down and to the right between aluminum and silicon, between germanium and astatine, between antimony and tellurium, and between polonium and astatine.
The elements on either side of that jagged line are called “metalloids” because they have properties that fall somewhere in between the typical properties of metals and non-metals. The elements to the left of the metalloids are considered metals and ones to the right are non-metals. The “heavy metals” are those that are dense and/or toxic.
Another definition sometimes used is “a metal with a density of around 5g/cm3 or greater.” So according to that definition, the metals on the periodic table from titanium to gallium, from zirconium to tin, and from hafnium to bismuth can be considered heavy metals (we’ll ignore the much heavier and/or rarer elements). Hey, wait a minute, what about arsenic? Right. Arsenic is technically a metalloid. It is included because it is heavy and toxic.
People have been using cannabis for a long time with no regulations or quality control measures in place to ensure product safety. That changed with the advent of legalization.
Starting in 2018 the state of California began testing inhaled and edible cannabis products for four heavy metals of primary concern: arsenic, cadmium, mercury, and lead. Since then, many states with legal cannabis markets (but not all) have implemented quality testing programs to include metals testing.
| Element | Chemical Symbol |
Atomic Number |
Density (g/cm3) |
Form in the Environment | Typical Flower Limit (mg/g) |
|---|---|---|---|---|---|
| Arsenic | As | 33 | 5.73 | Arsenic Oxides | 0.2 |
| Cadmium | Cd | 48 | 8.65 | Fertilizer Contamination, Sulfides, Carbonates, Phosphorites | 0.2 |
| Mercury | Hg | 80 | 13.54 | Chlorine and Sulfer Salts, Oxides, Methylmercury | 0.1 |
| Lead | Pb | 82 | 11.34 | Many Inorganic and Organic Forms | 0.5 |
The legal limits for edibles are generally a bit higher than those allowed for inhaled products. Distillation of cannabinoids usually reduces metal concentrations greatly, often to below their limits of detection. All the way to zero? No.
A sensitive analysis will likely find trace (or greater) amounts of many metals in a plant sample, so why the focus on just those four (arsenic, mercury, cadmium, and lead)? Let’s take a closer look at each of them, in order of atomic number (the number of protons in the nucleus).
Arsenic (As)—Arsenic is a known carcinogen affecting the skin, lungs, and bladder. It also causes heart disease and immune system disorders. Arsenic occurs naturally in the environment due to volcanic activity and weathering of rocks. It has also been released by smelting and mining processes and was widely used in pesticides for a long time. Arsenic is translocated into the plant through phosphate transport proteins in the roots.
Cadmium (Cd)—Cadmium is widespread in the environment due to mining, smelting, and other industrial activities. Short but high exposure to cadmium causes flu-like symptoms but chronic exposure increases the risks of several cancers, kidney malfunctions, and bone loss. Unlike lead and mercury, cadmium in plants can be toxic to humans at concentrations that are not visibly distressing to the plant. Cadmium is brought into roots by metal transporter and channel proteins designed to take in other nutrient metals such as iron, zinc, and manganese.
Mercury (Hg)—Hg? Hg comes from the Latin term hydrargyrum, meaning “liquid silver.” That makes sense, since mercury is the only metal that is a shiny liquid at standard temperature and pressure. Most people are exposed to mercury from methylmercury, which is formed by microbial transformation of mineral forms of mercury in the environment. Burning fossil fuels and industrial processes also release mercury. Organisms tend to bioaccumulate mercury, and it can cause a raft of problems ranging from skin rashes to neurological problems and even death. Mercury binds to sulfur compounds and proteins throughout the plant.
Lead (Pb)— The symbol Pb comes from the Latin word for lead, plumbum. Consumption of lead is especially harmful to children leading to learning difficulties, behavior problems, and damage to organs and the nervous system. In adults, exposure can cause organ damage, headaches, cramps, body aches, and general malaise. Almost everyone has a detectable concentration of lead in their blood because it is everywhere in the environment. Much environmental lead is of anthropogenic origin resulting from mining, industrial processes, and decades of using leaded gasoline and paint. Lead infiltrates plants via the extracellular space between root cells and eventually though calcium channels into the plant.
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Greenhouse-grown plants are expected to have lower metals concentrations due to some protection from wind and more control over the soil composition.
Generally, metals enter a plant in one of two ways: absorption through the roots or by atmospheric deposition.
Much of the lead and other heavy metals in soil binds to roots but are not well translocated to the stems, leaves, and flowers. In general, under normal conditions, the concentration of a heavy metal will be highest in the roots, less so in the stems and leaves, and even less in the flowers — but it won’t be zero.
Since metals are constantly present in the environment, metal-containing particles are deposited on the surface of plants by wind. Since cannabis flowers are sticky with resin, the particles tend to stick and stay put. Experiments have shown that an aggressive washing of plants in a detergent, followed by a chelating agent and by several rinses will remove metals from the flowers but also tends to reduce the cannabinoid concentration as trichomes are lost during the process.
Although there are a few true hyperaccumulators of metals in the plant kingdom, most plants can absorb an appreciable amount of metal if the concentration in the soil is high enough. Cannabis is no exception and has been used to remove metals from soil in a few trials. But it’s a stretch to consider it a true hyperaccumulator.
Consider the following: The soil lead concentration in Colorado ranges from about 10 parts per million (ppm, more properly expressed as μg/g) to more than 1,000ppm with an average of around 40ppm. Yet, outdoor-grown flowers in Colorado often test in low single digit ppm or less for lead. That’s barely accumulation let alone hyperaccumulation. Higher metal concentrations in outdoor-grown cannabis flowers due to both absorption and atmospheric deposition can be expected in plants grown in regions where the metal concentrations in the soil are high.
Greenhouse-grown plants are expected to have lower metals concentrations due to some protection from wind and more control over the soil composition.
Hydroponic plants grown indoors should have the least amount of metals since quality nutrient solutions are low in heavy metals and the plants are protected from atmospheric deposition.
It is impossible to produce cannabis products that are completely free of metals, heavy or otherwise. However, testing can go a long way toward reassuring consumers that the products they purchase and consume are as safe as possible. With properly set limits and validated testing programs, cannabis producers can monitor products and take steps to reduce contamination with not only heavy metals but microbes and pesticides as well.
