Mycotoxins are poisons that some moulds make on a crop that stays warm and damp. On cannabis flower, they form mostly during slow drying and humid storage, so the Thai rainy season is the highest-risk period. Killing the mould later does not remove the toxin, so growers, clinics and shops have to prevent it rather than try to fix it afterwards.
The moulds and what they make
Several moulds can grow on flower. Each one brings a different risk. Some make toxins, and some are dangerous mainly because people breathe in their spores (the tiny seeds moulds use to spread).
| Mould | Where it usually grows | What it makes | Main concern |
|---|---|---|---|
| Aspergillus flavus, A. parasiticus | Warm, humid drying and storage | Aflatoxins (B1, B2, G1, G2) | Aflatoxin B1 causes liver cancer when people eat it over time |
| Aspergillus fumigatus and related species | Drying rooms, storage, compost | Spores | Lung infection in people with a weak immune system |
| Aspergillus ochraceus, some Penicillium | Storage | Ochratoxin A | Damages the kidneys |
| Fusarium (also a root disease) | In the field | Deoxynivalenol (DON), zearalenone, T-2/HT-2 toxins | Mainly a field infection |
| Mucor | Very wet material | Spores; its toxins are little studied | Rare but severe infection in very ill people |
You cannot see or smell the toxin itself. Some moulds leave signs, such as green-yellow or black powder or a musty smell. Toxin can still be present after these signs are gone.
Botrytis (bud rot) is a different problem. It shows as grey fuzz and brown rot inside the bud. Remove buds with bud rot too, because the wet, damaged tissue gives storage moulds an easy place to grow.
Why the rainy season raises the risk
Mould needs free water. Water activity (aw) measures how much of the water in the flower is free for microbes to use. The scale runs from 0 (completely dry) to 1.0 (pure water).
Most fungi grow best at 0.83–0.99 aw. Drying flower below 0.70 aw stops most of them, but some Aspergillus and Penicillium moulds can survive at 0.62–0.70 aw. The standard ASTM D8197 sets 0.55–0.65 aw for dried flower. Storage at 55–62% relative humidity (RH) keeps flower in that range.
In Thailand, the rainy season breaks this range in several ways. Outdoor humidity is often far above 62% RH. Buds cut while wet from rain or dew bring water into the drying room. Dense buds dry slowly in the middle, and temperatures of 30–35 °C help mould grow there. A power cut stops the dehumidifier, and flower in unsealed containers takes up water again. For drying setups that work in these conditions, see Drying Cannabis at 85–95% Ambient Humidity.
What inhaling contaminated flower can do
There are two separate risks.
Live spores. Healthy lungs usually clear inhaled spores. People with a weak immune system can develop a serious lung infection (aspergillosis). This group includes people on chemotherapy, organ-transplant patients and people taking high-dose steroids. Published case reports have linked this infection to smoking contaminated cannabis, including a leukaemia patient in 2001 and two patients with chronic lung aspergillosis in 2011. Spores can also trigger allergy and asthma in sensitive people.
Toxins. Most evidence of harm from mycotoxins comes from eating contaminated food. A 2023 scientific review found no studies on what happens to these toxins when cannabis is burned, so no one knows how much reaches the body when flower is smoked or vaped. Medical standards still set strict limits, because patients inhale the flower and many of them are already ill.
Shops in Thailand may sell flower only with a PT 33 prescription, so every buyer is a patient. Clinics that treat patients with a weak immune system have the strongest reason to ask suppliers for test results.
Why killing the mould is not enough
Decontamination treatments such as gamma irradiation kill live mould and bacteria. They help flower pass the microbe count. Mycotoxins, however, are stable chemicals. Treatment and further drying do not reliably remove toxin that has already formed. A 2025 study still found aflatoxin in cannabis buds after gamma irradiation at the doses normally used for medical cannabis.
A batch can therefore pass the microbe test after treatment and still fail the aflatoxin test.
The same applies to extraction. Mouldy buds sent to oil or concentrate production are not made safe by extraction. Toxins carry over into the extract and can become more concentrated. In a 2021 lab study, extracts made from flower spiked to the European limits held aflatoxins and ochratoxin A at several times the levels found in the starting flower. Keep mouldy material out of extraction biomass.
How mycotoxins are tested
Two different tests answer two different questions:
- The microbiology test counts live microbes. TYMC (total yeast and mould count) is given in CFU/g, colony-forming units per gram. For inhaled flower, the Australian regulator’s limit is ≤20 CFU/g.
- The mycotoxin test measures the toxin chemicals directly. Labs usually use liquid chromatography, a method that separates and measures each compound.
You need both tests. Clean microbiology does not prove the batch is free of toxin.
For European buyers, monograph 3028 of the European Pharmacopoeia (Europe’s official book of medicine standards) tests flower for aflatoxin B1, with a limit of 2 µg/kg (micrograms per kilogram). The test method is in chapter 2.8.18. Buyers often also ask for total aflatoxins. The monograph reportedly has no explicit ochratoxin A requirement, but buyers often add one in the contract.
The results appear on the certificate of analysis (COA), the lab’s report for each batch. Make sure the lab’s ISO/IEC 17025 scope (the list of tests it is accredited for) includes aflatoxins and ochratoxin A in cannabis or herbal material. For the full list of export limits and how to choose a Thai lab, see Testing Cannabis Flower Before Export.
What to do this season
- Harvest only when the buds are dry on the surface, after rain and dew have gone. This lowers the amount of water you bring into the drying room.
- Break open the densest buds during drying. Throw away any bud that has visible mould or smells musty, and keep it out of extraction. Mould often starts inside the bud, where you cannot see it.
- Measure water activity before you seal each batch, and aim for 0.55–0.65 aw. You can use your own meter or send a sample to a lab. The reading shows whether the batch is safe to store, and you have a record if someone questions it later.
- Store flower in sealed containers. Then a power cut that stops the dehumidifier does not let humid air back into the flower. For more, see Storage: Humidity, Light, Oxygen, and Shelf Life.
- Test one rainy-season batch and one dry-season batch for TYMC, aflatoxins and ochratoxin A. Comparing the two shows whether your drying and storage still work in the worst season.
- Wear a well-fitting N95-type respirator when you handle mouldy material or clean the drying room. This protects your own lungs from spores.
- Clinics and shops: ask each supplier for a COA that shows both TYMC and aflatoxin results. Keep stock sealed and away from heat.
Spraying is not a fix for mould that forms during storage. If you use any crop-protection product during growing, use only products registered with the Department of Agriculture (กรมวิชาการเกษตร), as the label directs. See What You May Legally Spray Under Thai GACP.
If you have questions, or results from your own water-activity readings or mycotoxin tests, please post them below this topic.
References
- European Pharmacopoeia, monograph 3028 Cannabis flower (Cannabis flos); chapter 2.8.18, EDQM.
- ASTM D8197, water activity range for dried cannabis flower.
- TGA, Microbiological quality of medicinal cannabis products, Version 1.0, February 2020.
- IARC Monographs on the Identification of Carcinogenic Hazards to Humans (aflatoxins; ochratoxin A).