The Importance of Measuring Contaminants in Cannabis

Despite the resurgence in interest and use of cannabis for medical purposes, our understanding of cannabis contaminants and their effects on human health and bioavailability remains incomplete.

Cannabis Analysis

Cannabis. Image Credit: Mitch M/

The increase in global cannabis usage and the lack of quality standards

Cannabis is increasingly used for both medicinal and recreational purposes with an estimate of over 180 million users annually. Cotemporally, many countries are looking towards increasing the size and scope of their medicinal cannabis operations. Such approaches aim to produce high-quality products for clinical applications, and must therefore meet a high level of quality assessed through requisite analytical standards.

However, on a global stage, such analytical standards for quality testing of medicinal cannabis preparations are lacking, and improving them requires a more comprehensive evidence base of the contaminants of cannabis. For instance, testing the maximal limits for contaminants harmful to human health is of particular importance but has yet to be done.

The origin and effects of cannabis contamination

In a systematic review aiming to assess the extent of cannabis contaminants and their effects on human health, a study by Dryburgh et al., 2018 highlighted the most common cannabis contaminants which include microbes, heavy metals, and pesticides. Their direct human toxicity profiles are yet to be quantified but known effects include bacterial infection, carcinogenicity as well as reproductive and developmental impacts.

Firstly, microbial contamination occurs generally during the improper preparation and storage of products. If harvesting is wet, the drying and storage process under humid conditions can often lead to fungal infections including powdery mildew and botrytis as well as budworm or mite proliferation.

Infections of Aspergillus have been known to cause aflatoxins, carcinogenic mycotoxins, which have been detected in cannabis preparations and smoke. In a case-control study, the comparison of two species of Aspergillus, flavus, and parasiticus, cultured either with an American cannabis or a natural flora substrate, demonstrated that growth on the cannabis substrate produced harmful aflatoxins.

Secondly, heavy metal contamination originates from three pathways through which cannabis may be contaminated with heavy metal substances. Cannabis can bioaccumulate heavy metals from surrounding substrate soil, through cross‐contamination during processing or post‐processing when metals may be added to the preparation to increase weight and thereby increase its valuation.

These are all problematic, yet the first method through bioaccumulation is the most notable. In fact, cannabis bioaccumulates heavy metals in its tissues so effectively that hemp crops have been used for bioremediation. A study by Chinese scientists in 2021 used cannabis as agents for phytoremediation to rid areas of heavy metal contamination, demonstrating the extent to which cannabis can absorb contaminants from the soil, but also the risk of contaminating plants.

Finally, pesticides are also frequent sources of contamination, as products are used to prevent pests and pathogens from infecting plants. Human consumption of pesticides may confer substantial sequelae, including malignancy, developmental issues, reproductive, neurological, and endocrine disorders. Recent reports indicated the prevalence of pesticide contamination in Washington State, where laboratory analysis revealed that 84.6% (over 26 samples) of legalized cannabis products contained significant quantities of pesticides including insecticides, fungicides, miticides, and herbicides.

The severity of contamination effects is known to vary according to cannabis dosing formulations and administration routes. Moreover, the interaction between pharmaceutical compounds and contaminants may also pose risks to human health, yet these are not yet identified nor quantified due to the paucity in the literature describing the prevalence and human impact of cannabis contaminants.

Nevertheless, the potential human toxicity profile for contamination of cannabis encompasses acute morbidity; myocardial infarction, cerebellar infarction, infections, or psychomotor changes, as well as longer‐term morbidity; pulmonary disease, immune dysfunction, cancer, reproductive and developmental issues.

Measuring cannabis contamination

To avoid such effects, measuring methods of contaminants have improved. The qualification and quantification of contaminants now encompass a range of analytical methods discussed in a study by Craven et al. from 2019.

In this review, scientists focused on the analytical challenges and method development for the detection of pesticides and toxic elements in cannabis to meet international medical guidelines. The authors compared the costs, duration, effort required, and effectiveness of methods used for trace elemental analysis including atomic adsorption spectroscopy (AAS) and inductively coupled plasma (ICP), with atomic emission spectroscopy (AES) or MS.

Overall, researchers demonstrated that the cost of AAS is low but has slow sample throughput whereas ICP-AES offers higher sensitivity and sample throughput but at a higher cost. Ultimately, ICP-MS represents the best method and is being increasingly used as it possesses the highest sensitivity and throughput. Additionally, alternative techniques from other industries using ICP-MS may be adapted to cannabis testing easily.

Concerns, issues, and challenges of cannabis contamination

Despite the increasing awareness of cannabis contamination and the efforts to eliminate potential issues through international consortiums, more challenging issues are arising. One such problem that is particularly complex is the issue of increasing drug potency, which was discussed in a study by McLaren et al. This study reviews the international evidence on patterns of cannabis potency and contamination and potential associated harms before discussing their implications for prevention and harm reduction measures.

The increased potency of cannabis has been observed in some countries for decades through anecdotal evidence, with very limited documentation both locally and globally. After reviewing available literature, the review indicates that there is enormous variation in potency between samples from the same origin than across countries, meaning that cannabis users may be exposed to greater variation in a single year than over years or decades.

However, the limited literature meant that the sources of variation could not be further explained. Nevertheless, several documents indicate that the tampering of cannabis occurs frequently and may increase the risks of contamination.

For instance, recent reports of tiny glass beads added to cannabis sales to add bulk and mimic the crystalline appearance of the resin glands, which contain large amounts of THC. This cannabis appeared across the United Kingdom and poses considerable health risks to unaware consumers, yet little is known to reduce the spread of such products.

Addressing contamination in the future

The multifunctional benefits and socioeconomic interest of cannabis are making it a plant under increasing scrutiny, from a pharmaceutical perspective but also a health hazard view.  

Recent studies found 13 of the 14 cannabis samples contained fungi, with evidence of exposure to Aspergillus fungi in many other consumers (13 of 23 individuals sampled). Another study found fungal and bacterial contamination in all 24 samples. Such problems indicate the extent of cannabis contamination, yet the limited scientific literature and inadequacy of risk control are increasing the risk of consumers being exposed to potential hazards.

Looking forward, the rise of cannabis consumption requires an associated increase in quality control and testing. Studies have suggested the elaboration of international quality standards, enforced on global and local levels to ensure the reduced risks of contamination. Further measures including control of production and processing could further limit contamination, yet the extent of cannabis contamination is yet to be fully discerned.


  • Craven, C. B., Wawryk, N., Jiang, P., Liu, Z., & Li, X. F. (2019). Pesticides and trace elements in cannabis: Analytical and environmental challenges and opportunities. Journal of Environmental Sciences, 85, 82–93. doi:10.1016/j.jes.2019.04.028
  • McLaren, J., Swift, W., Dillon, P., & Allsop, S. (2008). Cannabis potency and contamination: a review of the literature. Addiction, 103(7), 1100–1109. doi:10.1111/j.1360-0443.2008.02230.x
  • McPartland J.M., McKernan K.J. (2017) Contaminants of Concern in Cannabis: Microbes, Heavy Metals, and Pesticides. In: Chandra S., Lata H., ElSohly M. (eds) Cannabis sativa L. - Botany and Biotechnology. Springer, Cham. doi:10.1007/978-3-319-54564-6_22
  • Wu, Y., Trejo, H. X., Chen, G., & Li, S. (2021). Phytoremediation of contaminants of emerging concern from soil with industrial hemp (Cannabis sativa L.): a review. Environment, Development, and Sustainability. Published. doi:10.1007/s10668-021-01289-0

Further Reading

Last Updated: Aug 12, 2021

James Ducker

Written by

James Ducker

James completed his bachelor in Science studying Zoology at the University of Manchester, with his undergraduate work culminating in the study of the physiological impacts of ocean warming and hypoxia on catsharks. He then pursued a Masters in Research (MRes) in Marine Biology at the University of Plymouth focusing on the urbanization of coastlines and its consequences for biodiversity.  


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