Fungal organisms are a normal part of the microbiome of plants. Some of these fungal organisms produce secondary metabolites which are toxic to animals and people. Some mycotoxins are potent carcinogens . Much of the research on these and the risk to health posed by them has been done on commercial crop vegetables. There is current concern about the potentially increasing risk of aflatoxin in US maize due to climate change . Also under the microscope, so to speak, are organic crops because they are not subject to the same mycotoxin control measures as non-organic crops .
But mycotoxin contamination of herbal products also represents a possible risk for human health. One study, the aim of which was to assess the extent of this problem, analysed a variety of herbal-based supplements from Czech and US markets – including milk thistle, red clover, flax seed, soy, green barley, nettle, goji berries and yucca – for the presence of 57 mycotoxins. Mycotoxin contamination was found to be more prevalent than expected, with 96% of the 69 herbal supplement samples containing detectable mycotoxins. The highest mycotoxin concentrations were found in milk thistle-based supplements (up to 37 mg/kg). 
Caldeirao et al. (2021) carried out a study of fourteen mycotoxins in a range of herbs in the Brazilian market, as dried herbs and infusions. They found mycotoxin contamination in 42 out of 58 herb samples (72%). In herbal infusions however, the occurrence of mycotoxins was 88% lesser than those in raw products. But despite this, the authors assessed that in the case of HT-2 toxin and in the case of some herbal infusions also aflatoxins and ochratoxin A, the levels found in infusions posed potential health risks. 
Hassan et al. (2022) evaluated the prevalence of aflatoxin B1 (AFB1) and ochratoxin A (OTA) in thyme and thyme-based products, related dietary exposure, and cancer risk for regular and high consumption. From the abstract:
A total of 160 samples were collected, and 32 composite samples were analyzed. AFB1 and OTA were respectively found in 84% … and 38% … of the samples. AFB1 exceeded the limits in 41% … and 25% … of the samples according to the Lebanese and European standards, respectively. OTA was unacceptable in only 6% … and 3% … of the samples according to the Lebanese and European standards, respectively.
After estimating daily exposure levels for Lebanese consumers on the basis of a food-frequency questionnaire, the authors calculate that the AFB1 exposure from dried thyme as a herb would account for 0.41 additional cancer cases per 100,000 persons, and from thyme-based products 0.35 additional cases per 100,000. 
However, a Latvian study concluded that while 90% of dry herb samples were contaminated with 1-8 mycotoxins, and despite high levels of extraction into infusion for some of them, intake risk assessment indicated that the tested herbal infusions were safe for consumers. 
And in a study of seven mycotoxins in supplements made with fifteen different single herbs and two compound supplements for two kinds of complaints, Pallarés et al. (2022) found that, although mycotoxin contamination was quite prevalent (58.3% of organic versus 41.1% of conventional samples) estimated daily intakes for mycotoxins, calculated according to the supplement manufacturers’ recommended dosages, were in general far below Tolerable Daily Intakes (TDIs). 
Moreover, Do et al. (2015) point out that exposure and toxicities can be diversely influenced by the other constituents of medicinal plants. They comment:
Although the occurrence and exposure of mycotoxins in most medicinal herbs and spices in various countries are inevitable, many of the bioactive components in these agricultural commodities have been known to regulate the fungal growth, mycotoxin production and their toxic actions in the plant and its herbivores, including human beings and domestic animals. These endogenous components are thus crucial attenuators by reducing the inevitable exposure and toxicities when taken in together with the contaminated mycotoxins.
Nevertheless, they also warn that:
… since some natural endogenous components could enhance the toxicity of mycotoxins via metabolic activation or retarded secretion by complex formation with mycotoxins, extensive investigations into these interactions is warranted. 
Pallarés et al. (2022), despite their encoraging findings (see above), also warn that:
The rising market of herbal products in Europe and worldwide makes necessary the control of mycotoxins and other chemical contaminates in such products. Poor practices during harvesting, handling, storage, and distribution stages affect the quality and safety of medicinal herbs, so the implementation of good manu-
facturing practices is essential to reduce mycotoxins presence. [My highlighting] .
This brief post cannot pretend to be anything approaching a comprehensive review of this subject. It simply aims to sample a few recent papers in order to alert the herbal community to what may be a lesser known or under recognised issue.
Yu et al. (2021) have provided a review , the abstract of which reads as follows:
Herbal medicines have been applied in clinical treatment worldwide, whose significant curative effect attracts considerable global research attention. However, as the herbal medicine industry
develops continuously in recent years, new challenges including monitoring the quality and safety of herbal medicines appear in this industry. Numerous cases of fungal and mycotoxin contamination have been reported that affected the quality and safety of herbal medicines. The main mycotoxins found in herbal medicines include aflatoxin, ochratoxin A, and fumonisin B, which cause substantial harm to human health. They are mainly produced by species from Aspergillus, Penicillium, and Fusarium. Various reports have focused on studying the conditions for fungal growth and mycotoxin synthesis to provide references for prevention. The chemical compounds and antagonism microorganisms were also explored to inhibit fungal growth, and decrease mycotoxin accumulation. This review discusses natural occurrence of three main fungal genera (Aspergillus, Penicillium, and Fusarium) and three main mycotoxins (aflatoxin, ochratoxin A, and fumonisin B) in herbal medicines, analyzing the endogenous and exogenous factors that affect fungal growth and mycotoxin production. Moreover, the prevention methods of fungal contamination are included.
Important questions remain for the small scale herbalist wildcrafting or growing herbs to make medicines for family and friends. Laboratory testing is expensive and may not be locally available. It is clearly impossible to remove all possibility that mycotoxins will be present in their herbal preparations. So what steps can be taken to minimise the risk? At the present time I can offer only the following:
- Inspect all fresh herbs closely for any signs of mould, disease or or less than perfect quality.
- Use them fresh or process them immediately.
- If drying the herbs, make sure they are fully dried.
- Store in airtight jars in a cool, dry space out of direct sunlight.
- Inspect regularly for signs of deterioration.
- Use dried herbs within two years.
- When making ethanolic tinctures make sure the ethanol content is sufficient to prevent microbial growth.
- Make sure herb material is completely covered by the menstruum.
- Allow as little air space in the jar as possible.
- Shake well regularly.
- Strain at no more than 6 weeks.
 Wikipedia. Mycotoxin. N.d. https://en.wikipedia.org/wiki/Mycotoxin. Accessed 11th July 2022.
 Bailee Henderson. The Future of Aflatoxin in U.S. Corn. 23rd June 2020. Food safety Magazine. https://www.food-safety.com/articles/7844-the-future-of-aflatoxin-in-us-corn. Accessed 11th July 2022.
 Wageningen University and Research. EU project Safe Organic Vegetables. N.d. https://www.wur.nl/en/show/EU-project-Safe-Organic-Vegetables.htm. Accessed 11th July 2022.
 Veprikova Z, Zachariasova M, Dzuman Z et al. Mycotoxins in Plant-Based Dietary Supplements: Hidden Health Risk for Consumers. 2015. J. Agric. Food Chem. 2015; 63(29):6633–6643. Abstract: https://pubs.acs.org/doi/10.1021/acs.jafc.5b02105.
 Caldeirão L, Sousa J, Nunes LCG et al. Herbs and herbal infusions: Determination of natural contaminants (mycotoxins and trace elements) and evaluation of their exposure,
Food Research International. 2021; 144(110322). https://doi.org/10.1016/j.foodres.2021.110322
 Hassan HF, Koaik L, Khoury AE et al. Dietary Exposure and Risk Assessment of Mycotoxins in Thyme and Thyme-Based Products Marketed in Lebanon. Toxins. 2022; 14(5):331. https://doi.org/10.3390/toxins14050331
 Reinholds I, Bogdanova E, Pugajeva I, Bartkevics V. Mycotoxins in herbal teas marketed in Latvia and dietary exposure assessment. Food Additives & Contaminants: Part B. 2019; 12(3):199-208. DOI: 10.1080/19393210.2019.1597927
 Pallarés N, Berrada H, Font G, Ferrer E. Mycotoxins occurrence in medicinal herbs dietary supplements and exposure assessment. 2022. J Food Sci Technol. 2022; 59(7):2830–2841. https://doi.org/10.1007/s13197-021-05306-y
 Do KH, An TJ, Oh S-K, Moon Y. Nation-Based Occurrence and Endogenous Biological Reduction of Mycotoxins in Medicinal Herbs and Spices. Toxins. 2015; 7(10):4111-4130. https://doi.org/10.3390/toxins7104111
 Yu J, Yang M, Han J, Pang X. Fungal and mycotoxin occurrence, affecting factors, and prevention in herbal medicines: a review. Toxin Reviews. 2021. DOI: 10.1080/15569543.2021.1925696
Copyright (c) Robert Hale 2022.