Potency testing gets the attention. Contaminant testing is the part that determines whether a product is safe to consume, and it is where most batch failures happen.
Cannabis contaminant testing covers four broad categories — biological, chemical, elemental and physical — each with a different origin, a different analytical approach and a different fix. Understanding which is which turns a failure from a setback into a diagnosis.
Biological Contamination
Microbial screening
Microbial screening looks for bacteria, yeast and mould. Panels vary: some report total aerobic count and total yeast and mould count, others target specific organisms such as E. coli, Salmonella, and Aspergillus species. Most regulated markets require both.
Aspergillus gets particular attention because several species can cause invasive pulmonary infection in immunocompromised people — a group heavily represented among medical cannabis patients.
Methods are either culture-based, where organisms are grown on media and counted, or quantitative PCR, which detects genetic material. Culture takes days because growth takes days. qPCR is faster but detects DNA from organisms that may no longer be viable, which is a different question and occasionally a source of disagreement.
Where it starts: drying and curing conditions, facility hygiene, storage humidity. Rarely cultivation itself. Cross-reference against water activity on the same report — the two move together.
Mycotoxin screening
Mycotoxins are toxic secondary metabolites produced by certain moulds — principally aflatoxins B1, B2, G1 and G2, plus ochratoxin A. Mycotoxin screening is a separate analysis because these compounds are chemically stable and persist after the mould that produced them is dead.
A batch can therefore pass a viable microbial count and still carry mycotoxins from an earlier contamination event that was subsequently treated or simply died back. Aflatoxins are established human carcinogens, which is why the limits are low.
Where it starts: a mould problem at some earlier point — during growing, drying, or storage — even if the mould is no longer present.
Chemical Contamination
Pesticide analysis
Pesticide analysis screens for residues across a defined list of active ingredients, each with its own action limit. Lists differ substantially between jurisdictions — a product passing in one state can fail in another on identical data.
The analysis uses liquid or gas chromatography coupled to tandem mass spectrometry, because the compounds are chemically diverse and the detection limits are low.
Combustion is what makes pesticides a bigger issue in cannabis than in food. Some residues survive burning, and several degrade into more harmful compounds. Myclobutanil converts to hydrogen cyanide when heated, which is why it is prohibited on cannabis across regulated markets despite being permitted on other crops.
Where it starts: deliberate application, but also drift from neighbouring operations, clones brought in already carrying residue, reused equipment, and contaminated growing media. When a result is positive, work backwards through every input rather than assuming misapplication.
Residual solvent testing
Residual solvent testing applies to solvent-extracted concentrates. Butane, propane, ethanol, hexane and others are measured by headspace gas chromatography, which samples the vapour above the material rather than the material itself.
Where it starts: inadequate purging, usually a function of time, temperature and vacuum. Mechanically separated products — rosin, dry sift, ice water hash — do not require it, though documenting a clean result is a reasonable commercial choice.
Elemental Contamination
Heavy metal testing
Heavy metal testing screens for lead, arsenic, cadmium and mercury, measured by ICP-MS at parts-per-billion sensitivity. Some jurisdictions add chromium, nickel or copper.
Cannabis is an efficient bioaccumulator — it draws elements from soil and water and concentrates them in tissue. The property is effective enough that hemp is deliberately planted for soil remediation on contaminated ground, which is precisely why this analysis is not optional for consumable crops.
Where it starts: growing medium, water source, fertilisers and amendments — particularly cheaper phosphate-based products, which can carry cadmium. For vape products, cartridge hardware is a documented source of metal contamination independent of the cannabis inside, so testing the filled product rather than the oil alone matters.
Physical Contamination
Filth and foreign material
Filth and foreign material inspection is a visual and microscopic examination for insect fragments, hair, visible mould, and anything else that should not be present.
It is the least technical analysis on the panel and one of the most informative about handling, because it almost never reflects cultivation. It reflects what happened after harvest.
Where it starts: trim rooms, drying areas, storage containers, transport.
Water Activity, and Why It Belongs Here
Water activity is not a contaminant measurement, but it is the best predictor of whether a batch will become one.
Water activity measures how much water in the material is chemically available to support microbial growth, as distinct from total moisture content. Below roughly 0.65 aw, most moulds cannot grow. Above it, they can — which means a batch that passes microbial screening today with high water activity is a batch that may fail later in storage.
Treating water activity as an early warning rather than a box to tick prevents more microbial failures than any other single practice. See how to store cannabis properly for the storage side.
Reading a Failure
Different failures point in different directions.
- Microbial → drying, curing, storage humidity, facility hygiene. Check water activity on the same report first.
- Mycotoxin → an earlier mould event, even if nothing is visible now. Review the whole post-harvest history.
- Pesticide → inputs and environment. Audit everything applied, everything brought in, and what is happening on adjacent land.
- Heavy metal → soil, water, nutrients. Test the inputs directly rather than guessing.
- Residual solvent → purge parameters. Time, temperature, vacuum.
- Foreign material → post-harvest handling and facility cleanliness.
A laboratory that will work through the data with you is worth considerably more than one that emails a result and stops. That is one of the criteria in how to choose a cannabis testing laboratory.
Prevention Beats Retesting
By the time a batch fails, the cost is already incurred. The practices that reduce failures are unglamorous and consistent: verified inputs from documented sources, water tested at the source, controlled drying with monitored humidity, thorough cleaning between harvests, sealed storage with stable conditions, and records complete enough to trace a problem when one appears.
Contaminant testing tells you whether all of that worked. It cannot substitute for any of it.