Mold and microbial contamination in cannabis

TL;DR

• Water activity (aw) matters more than raw moisture content for mold growth.
• Aspergillus species pose the biggest health risk, especially for immunocompromised people.
• Total aerobic count (TAC) and yeast/mold count (TYMC) are standard screening parameters.
• Proper drying (slow, ventilated) prevents most problems preventively.

Why water activity matters more than moisture

Moisture content (%) measures the total mass of water, while water activity (aw) indicates how 'available' that water is for microbial growth. Two samples with identical moisture content can have different aw values. Molds like Aspergillus generally need aw > 0.7, which is why a 0.55–0.65 aw target range is considered safe. Hygrometer-based aw meters are therefore more informative than plain moisture scales.

How is microbial testing done?

Standard panels include total aerobic bacterial count (TAC), yeast and mould count (TYMC), targeted PCR or culture tests for Aspergillus species, plus coliforms, E. coli and Salmonella spp. as hygiene indicators. Limits vary by route of use: inhaled products (flower, concentrates) face stricter thresholds than orally ingested extracts, because the lung defends against spores less effectively than the gastrointestinal tract.

• TAC: total aerobic bacterial count
• TYMC: yeast & mold count
• Aspergillus spp. via PCR or culture
• E. coli / Salmonella as hygiene indicators

Regulatory requirements in Switzerland

In the area of microbiological safety of cannabis, Switzerland relies on strict quality standards that differ depending on the intended use. For medical cannabis, which has been allowed to be prescribed without an exceptional license from the BAG since August 2022, the strict specifications of the Pharmacopoea Europaea (Ph. Eur.) apply. Swissmedic monitors compliance with the limit values for germs and fungi in order to protect patients with weakened immune systems from health damage. In the government-approved pilot trials such as Züri Can or Weed Care in Basel, all dispensed flowers must also undergo complete laboratory testing before they go on sale.

For CBD cannabis with a total THC content of less than one percent, which is not legally subject to the Narcotics Act (BetmG), the provisions of the Food and Utility Articles Act apply. The Federal Food Safety and Veterinary Office (BLV) and the cantonal enforcement authorities are responsible for random checks. If limit values for yeasts, molds, or pathogenic bacteria are exceeded, sales bans and recalls may be imposed. The cantonal laboratories consistently implement these guidelines, which is why producers must have every batch microbiologically analyzed to ensure marketability.

• Strict limit values according to the Pharmacopoea Europaea for medical cannabis.
• Regular batch testing in the ongoing Swiss pilot trials.
• Responsibility of the BLV and cantonal pharmacists for CBD products under one percent THC.
• Risk of distribution bans if microbiological limits are exceeded.

Sources of contamination in the cultivation process

Microbiological contamination rarely occurs only during storage, but usually already during cultivation and harvesting. High humidity in the late flowering phase promotes the unnoticed growth of fungal spores inside the dense inflorescences. Dust, insufficiently filtered supply air and contaminated work clothing of the staff contribute massively to the microbial load. Organic fertilizers or standing water in the irrigation system also provide an ideal breeding ground for bacteria such as E. coli or Salmonella, which later lead to the rejection of the batch in the laboratory.

A critical moment is the harvest and the subsequent manicuring of the flowers. Tools, scissors and trimming machines must be continuously disinfected to prevent the carry-over of spores. If the plant material is stacked too densely on the drying racks, a microclimate with stagnant air is created, which offers optimal conditions for molds. Professional operations therefore rely on validated cleaning plans, controlled airflows and trained staff to keep the microbial load at a minimal level from the very beginning.

• Insufficient air circulation in the late flowering phase as the main cause of mold.
• Introduction of bacteria due to poor hygiene of staff and tools.
• Risk of microclimates if drying areas are packed too densely.
• Need for HEPA filters in supply air systems of indoor cultivations.

Mycotoxins and the invisible danger

Even if molds are killed by heat or drying, their metabolic products remain in the plant material. These so-called mycotoxins, in particular aflatoxins and ochratoxin A, are extremely heat-stable and highly toxic. When consumed by burning or vaporizing, these toxins enter the bloodstream directly through the lungs. They can damage the liver and kidneys and are considered carcinogenic in the long term. Since mycotoxins can neither be smelled nor seen, avoiding visibly moldy flowers alone is not sufficient protection against these chemical residues.

In case of suspected acute poisoning from consuming contaminated products, Tox Info Suisse offers an initial medical point of contact under the emergency number 145. Typical symptoms after inhaling contaminated flowers are respiratory complaints, coughing, fever or allergic reactions. Since the data on the long-term consequences of inhaled mycotoxins is still limited, medical professionals call for extreme caution. Only certified laboratory analyses can guarantee the absence of these dangerous toxins, which is why the black market without quality controls represents a significant health risk.

• Heat stability of mycotoxins prevents their destruction during smoking or vaporizing.
• Aflatoxins and ochratoxin A as primary, health-hazardous mycotoxins.
• Direct absorption via the alveoli bypasses the filter function of the gastrointestinal tract.
• Tox Info Suisse (emergency number 145) as the cantonal and national contact point for symptoms of poisoning.

Prevention through proper storage

After successful drying and laboratory testing, the product remains susceptible to microbial spoilage if storage conditions are unsuitable. Oxygen, light, and fluctuating temperatures promote the formation of condensation inside the packaging. This locally increases water activity back above the critical value, allowing dormant spores to germinate. The use of airtight glass containers or special composite foils protects the flowers from moisture fluctuations in the ambient air while preserving the sensitive terpenes from degradation.

A common mistake in practice is the artificial rehydration of overly dry flowers using unsuitable means such as fruit peels or damp cloths. These methods introduce uncontrolled moisture and foreign mold spores directly onto the material. Instead, only two-way humidity control packs should be used, which are salt-based and keep the relative humidity in the container constant. In addition, storage rooms should be kept cool and dark, as heat massively accelerates the growth of almost all relevant microorganisms.

• Avoidance of condensation through constant storage temperatures without fluctuations.
• Avoiding home remedies such as fruit peels for the subsequent rehydration of the flowers.
• Use of two-way humidity regulators to stabilize water activity.
• Storage in opaque, airtight containers at cool temperatures.

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