Cadmium, Lead and the Missing Step: Why a Heavy Metals Panel on Hemp Is Not Evidence About Your Oil

Hemp has an awkward reputation it never asked for. For three decades it has been studied as a phytoremediator: a plant willing to draw cadmium, lead and nickel out of contaminated ground and hold them in its stems, leaves and flowers. That talent is valuable when the task is cleaning a brownfield site. It is considerably less valuable when the same plant is being pressed into a bottle of CBD oil that someone in Leeds takes before bed.

This is the soil-to-oil problem, and it is the part of a heavy metals certificate that UK buyers most often fail to examine. A panel showing cadmium at 0.02 mg/kg is reassuring, provided you know the sample was the finished oil in the retail container, tested recently, by a laboratory accredited for that method. If the sample was hemp flower, or crude extract, or material from two seasons ago, the reassurance is thinner than it first appears.

Why hemp accumulates metals at all

Cannabis sativa takes up cadmium through the same transport proteins it uses for zinc and iron, which is why accumulation occurs even on soils that would pass an ordinary agricultural screen. Lead behaves differently: it is largely immobile in soil, so the more common route into the plant is atmospheric deposition on leaves and flowers, alongside contamination from machinery, dust and handling at harvest. Nickel, chromium, copper and zinc appear for related reasons.

The practical consequence is that metal content is a property of a particular field in a particular year, not of the species. Two brands selling oil from the same cultivar can hold entirely different cadmium profiles because one sourced biomass grown on former industrial land abroad and the other contracted a farm in Lincolnshire with a documented soil history.

Which metals belong on the panel

A credible panel covers lead, cadmium, arsenic and mercury at minimum, and ideally reports inorganic arsenic separately from total arsenic, since the inorganic fraction carries most of the toxicological concern. Some laboratories add nickel, chromium and aluminium. The length of the list matters less than whether each result is quantified against a stated limit of quantification, and whether the matrix is the finished product.

What UK limits actually cover

Great Britain retained the maximum levels set out in Regulation 1881/2006 as amended. For food supplements, those levels stand at 3.0 mg/kg for lead, 1.0 mg/kg for cadmium and 0.10 mg/kg for mercury. There is no harmonised GB maximum for arsenic in supplements, which is why some brands test against an internal specification of 0.1 mg/kg while others say nothing whatever.

Two features of those figures deserve a pause. First, they apply to the product as sold and as consumed, not to the biomass or the intermediate extract. A certificate for hemp flower is a certificate about an ingredient, and an ingredient is not the item on your shelf. Second, compliance is expressed per kilogram of product. An oil diluted in a generous volume of carrier can pass a limit that a concentrated paste would fail. That is compliance, but it is not purification, and it should not be marketed as though it were.

Five details that decide whether a certificate means anything

Units. Laboratories report in mg/kg or micrograms per kilogram, and a factor of a thousand sits between them. A mercury result of 0.05 mg/kg is 50 micrograms per kilogram. Confusion at this level is common on marketing pages.

Limit of quantification. Not detected is a statement about the method, not about the sample. If the LOQ is 0.5 mg/kg, the laboratory could not have seen mercury at the 0.10 mg/kg limit even had it been present. Ask for the LOQ and compare it directly with the maximum level.

Basis. Results may be given on a dry-weight or an as-sold basis. For an oil, as-sold is what you need. For biomass, ask what moisture correction was applied.

Matrix. The certificate should name what was tested: hemp biomass, crude extract, winterised distillate, or finished oil in the retail container. Anything other than the last of these sits one step removed from your bottle.

Accreditation. A UKAS-accredited laboratory working to ISO/IEC 17025, with the specific test within its scope, is the baseline. Check the schedule of accreditation rather than the logo printed on the PDF.

Where the metals go during extraction

Supercritical CO2 is a selective solvent and carries comparatively little metal, though plant fines and waxes can drag some through. Ethanol is less discriminating and pulls more across, although winterisation, filtration and distillation subsequently remove a good deal. The honest position is that nobody can predict the finished figure from the biomass figure, which is precisely why the finished product needs its own test. A brand that tests only the plant is telling you about its supplier, not its oil.

A related tell is language. Heavy metals tested describes an activity. Cadmium below 0.02 mg/kg in batch 2026-0417, finished oil, tested on 14 September 2026 by an accredited laboratory describes a product.

Six questions worth asking before you buy

Is the heavy metals panel for the finished retail oil, and does the batch reference match the bottle in your hand? What were the LOQs for each metal, and were they below the retained GB maximum levels? Was the laboratory UKAS-accredited for that specific method? When was the sample drawn, and from which point in the batch? How does the result compare with the brand's published specification, if it has one? And if a metal exceeded the limit in an earlier batch, what happened to that batch?

The handshake, taken literally

The image of a handshake is useful because a certificate should join three things into a single chain: the field the plant grew in, the extract that went into the bottle, and the bottle itself. Where that chain breaks, where the only document is a soil or biomass report, or a report from a different harvest, or a page of green ticks with no numbers attached, the buyer is being asked to trust a reputation rather than a measurement. For a plant with hemp's appetite for metals, that is a substantial ask.