Contaminants of Concern in Cannabis : Microbes, Heavy Metals and Pesticides John M. McPartland and Kevin J. McKernan, 2017

Contaminants of Concern in Cannabis : Microbes, Heavy Metals and Pesticides

John M. McPartland and Kevin J. McKernan

in S. Chandra et al. (eds.), Cannabis sativa L. – Botany and Biotechnology, Chapter 22, 2017, 457-474.

Doi : 10.1007/978-3-319-54564-6_22


Microbiological contaminants pose a potential threat to cannabis consumers. Bacteria and fungi may cause opportunistic infections in immuno-compromized individuals. Even dead organisms may trigger allergies and asthma. Toxins from microbial overloads, such as Shigla toxin and aflatoxins, may pose a problem—unlikely, but possible. The Cannabis plant hosts a robust microbiome; the identification of these organisms is underway. Cannabis bioaccumulates heavy metals in its tissues, so avidly that hemp crops have been used for bioremediation. Heavy metals cause myriad human diseases, so their presence in crops destined for human consumption must be minimized. Pesticide residues in cannabis pose a unique situation among crop plants—the Environmental Protection Agency (EPA) will not propose pesticides guidelines, because Cannabis is illegal on the federal level. The use of illegal pesticides is a rising crisis, and a breakdown in ethics. Testing for pesticide residues and maximal limits are proposed.


22.1 Introduction

Cannabis (the plant) and cannabis (the plant product) may be contaminated by microbes, heavy metals, or pesticide residues. The first two contaminants, microbes and heavy metals, present a Janus-face or “flip-side of the coin” in relation to Cannabis. Some bacteria and fungi are part of the plant’s microbiome. They provide benefits to Cannabis. See the book chapter by Parijat Kusari and Oliver Kayser for more about the Cannabis microbiome. On the “flip-side of the coin,” other bacteria and fungi cause disease, and these must be controlled.

Heavy metals are harmful to humans, and these contaminants must be minimalized in cannabis destined for human consumption. Cannabis pulls heavy metals from soil with great efficiency. Therein lies a second Janus face: the plant has great potential as a tool for bioremediation. Bioremedial plants extract pollutants from soil and accumulate the pollutants in their tissues, for harvesting and removal. Pesticide residues have no “flip-side of the coin,” they are just bad. Growth in the cannabis industry, from outdoor hippie gardens to indoor commercial warehouses, has multiplied pesticide usage. Pesticide regulation in the USA is primarily a responsibility of the Environmental Protection Agency (EPA). The EPA will not register pesticides for use on Cannabis or set tolerance levels because the crop is illegal on the federal level (Stone 2014). For that same reason, no cannabis can be labeled as “Organic” by the USA Department of Agriculture.

This chapter focuses on microbes, heavy metals, and pesticide residues in cannabis inflorescences and seed oil. Other contaminants exist, such as butane residues in cannabis extracts. For these, the reader is directed elsewhere (Upton et al. 2013; Farrer 2015). Adulterants—deliberately added contaminants—are a separate issue, particularly hashish diluents and psychoactive adulterants (Bell 1857; Dragendorff and Marquis 1878; Indian Hemp Drugs Commission 1894; Perry 1977; Wilson et al. 1989; McPartland and Pruitt 1997; McPartland 2002; Caligiani et al. 2006; McPartland et al. 2008; Busse et al. 2008; Venhuis and de Kaste 2008; Scheel et al. 2012).

22.2 Microbial Contaminants

Cannabis is often characterized as a “disease-free” crop. In fact, a plethora of plant pathogens attack the plant. At least 88 fungal species cause diseases in Cannabis (McPartland 1992), as do eight patho-varieties of plant pathogenic bacteria (McPartland et al. 2000). Some phytopathogens are unique to Cannabis (McPartland 1984), and some organisms are ubiquitous. The most threatening diseases of flowering tops are caused by three ubiquitous fungi—Botrytis cinerea (the cause of gray mold), Trichothecium roseum (white mildew or pink rot), and Alternaria alternata (brown blight). Phyto-pathogens cannot infect humans, except perhaps immuno-compromized individuals. Opportunistic infections by A. alternata have been reported in patients receiving chemotherapy, recent organ transplant patients, and people with AIDS. Airborne conidia (spores) of B. cinerea and A. alternata cause mold allergies and asthma, particularly in greenhouse workers (Jurgensen and Madsen 2009). From a consumer perspective, a separate population of bacteria and fungi is of greater concern than phyto-pathogens: post-harvest storage microbes (McPartland 1994a). Storage organisms are saprophytes, rather than pathogens. They can only invade dead plants after harvest. Fungi are the primary cause of storage contamination. They thrive under low oxygen levels, limited moisture, and intense competition for substrate.