Heavy Metals Testing in Cannabis: Everything You Need To Know is Here

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Heavy metals testing in cannabis is essential and mandatory. When heavy metals accumulate in cannabis products they pose a serious health risk for the consumer. 

Cannabis businesses are obligated to test their products for the presence of heavy metals. If heavy is detected beyond a set threshold the cannabis is declared unsafe for human use and therefore illegal. Consequently, it is critical to ensure that cannabis does not contain heavy metals. This article explains everything you need to know about heavy metals in cannabis; what they are, how they occur, how they are tested, and how to get rid of them. In the end, you will get a few tips on when to test your cannabis plants for the presence of heavy metals. 

What are Heavy Metals in Cannabis?

Cannabis is safe for human consumption. However, when contaminated by heavy metals or other contaminants it can pose a serious risk. 

Heavy metals are natural metals that are characterized by a high atomic mass and density. There are several heavy metals that are found in cannabis, however, four are classified under the US Federal Drug Administration (FDA) under class 1 category. This means that they are harmful for human use with no known therapeutic benefits. 

These are cadmium (Cd), arsenic (As), lead (Pb) and mercury (Hg). 

These metals are very toxic to humans when inhaled or consumed because the body is not able to eliminate them. When they accumulate in the body they can be fatal. The FDA recommends that these four metals be screened in cannabis and cannabis products on a regular basis. 

The FDA recommends the following as the upper limit for heavy metals:

Heavy Metal Inhaled cannabis Ingested cannabis
Cadmium 0.2 0.5
Arsenic 0.2 1.5
Lead 0.5 0.5
Mercury 0.1 3.0


Other heavy metals that are tested include silver, zinc, copper, cobalt, antimony, and thallium.

Source of Heavy Metals in Cannabis

Heavy metals may occur naturally in cannabis soil. They can also be as a result of industrial and agricultural pollution. The use of chemical fertilizers (phosphate fertilizers) may introduce heavy metals into soil. Industrial processes such as mining, coal burning, plastics, nuclear power stations, and petroleum refining also contribute to heavy metal contamination in soil.

Heavy metals will seep into the cannabis plants from the roots, stalks, and leaves. 

Generally, cannabis flower will have the lowest amounts of heavy metals. On some occasions, heavy metals may also be found in low-quality packaging material. Fertilizers and growing media can be sources of heavy metals in soil as well. 

Lastly, elemental mercury which is found in the atmosphere can find its way to the soil and be absorbed through the plants' root system.

Even when cannabis has been grown “organically,” it can still contain heavy metals that are present in the terrain. A lot of cannabis is grown in large-scale farms that border mines and industrial sites. This may have an impact on the safety of the cannabis

How to Test for Heavy Metals in Cannabis

Heavy metals are usually screened with the acceptable limit measured in parts per million (ppm). The Bureau of Cannabis Control (BCC) oversees the testing of heavy metals in cannabis. 

The BCC mandated compulsory testing of cannabis for these four metals as of December 2018. Cultivators have to test their plants regularly to rule out the presence of these heavy metals. Farmers have to test the soil, growth medium, nutrient medium, water system, plants, and packaging materials. 

There are two common methods of measuring the amount of heavy metals in cannabis:

–         Inductively Coupled Plasma Mass Spectrometry Detector (ICP-MS) 

–         Atomic Absorption Spectrophotometers (AAS) 

Each state sets regulations on how heavy metals should be tested for.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Many states have imposed very low detection limits for heavy metals. The ICP-MS is able to detect such low limits and is, therefore, one of the most effective instruments that can be used in heavy metal testing in cannabis. Prior to testing, a representative sample has to be created and taken through a “digestion” process where the matter is broken down and heavy metals are separated for analysis. When excess levels of a given heavy metal are detected the source has to be ascertained. This could be soil, water, or packaging material. 

How to get a Representative Sample

Different tests have shown that heavy metals are rarely spread evenly through a batch. Consequently, it is important to carry out a sampling process to find a representative sample that can be tested. The quality of the test is determined by the sample size and how well it represents the entire batch. 

A random sample is obtained by collecting several small samples from different areas and combining them. From this 1 gram of the representative sample is then analyzed for the presence and levels of heavy metals.

What Are The Risks Associated with Heavy Metals in Cannabis?

Cannabis is a super bioremediator; this means that it absorbs many compounds from the environment, toxic compounds included. Listed below are some of the risks associated with the ingestion of heavy metals:

  1. Lead: Lead is a common contaminant in soil and water. Lead poisoning can be fatal. It presents with symptoms such as joint and muscle pain, headaches, dizziness, and gastrointestinal problems.
  2. Cadmium: This is a byproduct of zinc, so where there is lead there is also a high likelihood of cadmium as well. Cadmium is carcinogenic, it has also been linked to neurological, reproductive, renal, and respiratory challenges. 
  3. Mercury: Mercury can be found in contaminated soil or water. It is easily absorbed through lung tissues as the lungs are great absorbers. Mercury poisoning may cause neurological damage which may present as tremors, seizures, memory loss, impaired motor control, and confusion. 
  4. Arsenic: Arsenic has very high toxicity levels. It is commonly found in contaminated water. It is linked to the development of skin cancer and in severe cases, it may cause death. 

Heavy metals are present in higher amounts in cannabis concentrates such as wax and shatter as compared to the cannabis dried flower.

 Soil pH affects heavy metals in cannabis

The presence of heavy metals in cannabis does not guarantee absorption or accumulation in cannabis. The soil might have heavy metals but they will not be taken up into the plant unless the pH is ideal.

Soil pH affects how heavy metals are taken up and concentrated.  Most heavy metals are absorbed when the pH is low. However, arsenic and mercury are more available when the pH is increased. 

Other than the pH, the form of the metal will also determine how well it is absorbed into the cannabis plant. Most heavy metals are immobile in soil and need to be converted to their anion/ cation form to be absorbed into cannabis. 

Lastly, the concentration of the heavy metals in soil or water will also affect how fast they are absorbed into cannabis. The higher the concentration of heavy metals the faster the absorption. 

What is Phytoremediation?

This is a natural way to eliminate heavy metals from polluted soil using plants. The plants absorb toxins from the soil such as heavy metals, pesticides, and other contaminants. Cannabis which has high abruption capabilities can be used for phytoremediation. However, when this cannabis is consumed it will pose a serious health risk to the consumers. 

Certificate of Analysis for Heavy Metals

All cannabis sellers must subject their products to independent labs for third-party testing. This test screens for potency and the presence of contaminants such as heavy metals. These results must be made available to consumers upon request or can be posted on the website.

Unfortunately, there is no standardization for COAs across testing labs. Many testing labs are not regulated and consequently they do not adhere to set standards for testing procedures and equipment. This makes it very hard to trust third party results that are displayed by most cannabis sellers. But as the industry becomes mainstream there is likely to be better regulation and standardization which will result in enhanced patient safety. 

 Tips on Reducing Heavy Metals in Cannabis

States are becoming more vigilant in testing for heavy metals in both medicinal and recreational cannabis. You can check with your state’s Bureau of Cannabis Control for more information on this. Meanwhile, here are a few tips to help limit the levels of heavy metals in cannabis.

  1. Regular testing throughout the growing cycle
  2. Eliminate the use of chemical fertilizers
  3. Use RO water
  4. Use high-quality ingredients 
  5. Avoid using equipment that may introduce heavy metals 

How often should you test for heavy metals?

You should test for heavy metals every time there is a major change such as a new growing medium, water source, fertilizer, or new strain. When testing, ensure that you get a random sample that is representative of the batch. You should also comply with the set standards for testing that have been stipulated in your jurisdiction. 

Heavy metal testing in cannabis is involving and costly, but it is the best way to ensure that the cannabis in the market is safe for human consumption, 


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