Heavy metals can pose a serious health risk when ingested in cannabis products. The Federal Drug Administration (FDA) recommends regular testing of ingredients and products to rule out the presence of heavy metals. Every state sets its own rules regulating the testing of heavy metals and other contaminants that could be present in cannabis and its downstream products.
What are Heavy Metals and Why are They Present in Cannabis?
Heavy metals occur naturally in the environment and are characterized by their high density and atomic mass.  The human body cannot eliminate heavy metals. And when they accumulate, they become toxic.
The FDA considers certain heavy metals as being highly toxic when consumed. Cadmium (Cd), arsenic (As), lead (Pb), and mercury (Hg) fall in this category and it’s these metals that are quantified and provided on certificates of analysis.
Heavy metals are commonly found in the environment especially in areas that are near mines or industrial sites. These metals can find their way to soil and water from where they may accumulate in cannabis. The roots and leaves usually have a higher concentration of heavy metals compared to the flowers.
Chemical fertilizers that are high in phosphates can be a source of heavy metals in soil and water. Packaging materials can also contribute to the presence of heavy metals in cannabis.
Heavy metals are usually immobile in the environment; they need to be ionic to be readily available for uptake by cannabis. pH also affects the accumulation of heavy metals in cannabis. Most metals are accumulated when the soil pH is low. [2,3]
The Bureau of Cannabis Control (BCC) in California imposes the following limits for heavy metals in cannabis:
|Heavy Metal||Maximum limit allowed
(inhaled, edible, and topical/ transdermal medicinal cannabis)
|Cadmium||0.2. 0.5, 5.0|
|Arsenic||0.2, 1.5, 3.0|
|Mercury||0.1, 3.0, 1.0|
How Labs Test for Heavy Metals
Heavy metals can be tested using inductively coupled plasma mass spectrometry (ICP-MS). This instrument detects trace amounts of heavy metals. Other techniques include atomic absorption spectroscopy (AAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Heavy metals are not spread evenly through a sample. A random sample must be collected through a robust sampling process to ensure that it’s representative of the batch.
Risks Associated with Consumption of Heavy Metals
- Lead poisoning: This may cause joint and muscle pain, dizziness, and gastrointestinal problems.
- 2. Cadmium poisoning: Cadmium is carcinogenic.
- Mercury poisoning: This may cause neurological damage.
- Arsenic poisoning: Arsenic is highly toxic and carcinogenic.
Use of Bioremediation and Chelant Extraction to Remove Heavy Metals from Soil
A marine microorganism known as Desulfuromonas palmitatis can be used to extract arsenic while ethylenediaminetetraacetic acid (EDTA) may be used to extract metal chelates such as zinc.  This is a combined chemical and biological method of extracting heavy metals.
EDTA-enhanced phytoremediation is also an effective and affordable method of extracting metals from contaminated soil and water since it increases their bioavailability and uptake by plants. 
Image Source: https://commons.wikimedia.org/wiki/File:Heavy_Metals_vs_REE_vs_Plant_Molecular_Biology.png
- Koeler M., et al. “Introducing Heavy Metals.” Intechopen Books, 2018, doi: 10.5772/intechopen.74783. [cited by 5]
- Yu, Haixin, et al. “Meta-Analysis of Soil Mercury Accumulation by Vegetables.” Scientific Reports, vol.8, no.1, 2018, p.1261, doi:10.1038/s41598-018-19519-3. [journal impact factor = 4.122; cited by 7]
- Zhang, Yanhao, et al. “PH Effect on Heavy Metal Release from a Polluted Sediment.” Journal of Chemistry, edited by George Z Kyzas, vol. 2018, no.7597640, 2018, doi:10.1155/2018/7597640. [journal impact factor = 1.727; cited by 8]
- Katerina V., et al. “Removal of Heavy Metals and Arsenic from Contaminated Soils Using Bioremediation and Chelant Extraction Techniques.” Chemosphere, vol.70, no.8, 2008, pp.1329-1337. [journal impact factor = 5.108; cited by 70]
- Shahid, M. et al. “EDTA-Enhanced Phytoremediation of Heavy Metals: A Review.” Soil and Sediment Contamination Journal, vol.23, no.4, 2014, pp.389-416. [journal impact factor = 0.65; cited by 90]