Mercury in fish oil: what do laboratory tests show?

A mercury test provides information about the sample examined. To interpret the result, you need to know which mercury determination was performed, on what material and with which reporting threshold. An entry such as “<0.010 mg/kg” is not an exact value of zero.

The words “fish oil” or “purified” alone do not establish a measured mercury concentration. That requires an applicable report with an identifiable link to the product and batch.

Which form of mercury was examined?

Mercury occurs in different chemical forms. Methylmercury is one of them. The European Commission also distinguishes these forms in its information about mercury in food.^[1]

Reports can list different determinations:

A methylmercury result is not automatically a total-mercury result. A total-mercury result does not by itself establish how much of each form is present.

If the report says only “Hg”, the chemical symbol for mercury, check the description and method. Do not assume which determination it means.

Does the report apply to your supplement?

Check the product description, batch code and tested material. A report can concern ingredient oil, oil extracted from capsules or a finished supplement.

Those materials cannot be substituted for one another without explanation. A result per kilogram of ingredient oil is not a direct measurement of the complete finished product.

Also check who selected and supplied the sample. The test primarily concerns that sample; sampling helps determine which further statements can be made.^[3]

A brand name without a product or batch connection is insufficient to assign a result to your pack.

How do you read a less-than result?

The LOQ is the low level from which the method can determine the amount with performance suitable for the application.^[3]

Here, the result is reported as less than that threshold. It does not mean the laboratory measured exactly 0.010 mg/kg. Nor does it establish that absolutely no mercury is present.

Check the unit too. In this example:

<0.010 mg/kg is equivalent to <10 µg/kg.

That converts the same entry into another unit; it is not a second measurement. The less-than sign stays.

Does “not detected” mean “mercury-free”?

A limit of detection (LOD) and a limit of quantification (LOQ) are different concepts. They describe the method’s capabilities, not permitted maximum levels.^[4]

Read the legend for “not detected”, “ND” or a less-than sign. Does the entry refer to a detection limit, the LOQ or another reporting threshold? “<LOQ” alone does not establish whether a signal below that threshold was observed.

Two reports stating, for example, “<0.005 mg/kg” and “<0.010 mg/kg” do not automatically establish a measured difference between two products. One report has a lower reporting threshold; the exact concentrations remain unknown.

Do not enter those results as zero or turn them into a general claim that the product is mercury-free.

Which limit applies to food supplements?

For the food supplements category, Regulation (EU) 2023/915, Annex I, point 3.3.2, specifies a mercury maximum level of 0.10 mg/kg.^[2]

The product category matters. A limit for fish muscle meat cannot simply be used as the limit for a food supplement. Also check that the reported material and calculation basis match the comparison.

This figure is a legal maximum for that category, not a laboratory detection limit. A brand’s or customer’s specification is another reference again. Check which requirement the report actually uses.

What does “within the limit” mean on a report?

Comparing a result with a maximum requires more than two numbers. The substance, unit and tested material must match the requirement being applied.

Official controls follow reporting and interpretation rules, including measurement uncertainty and, where applicable, recovery under Regulation (EC) No 333/2007.^[4] Other conformity statements must likewise identify the requirement and decision rule used.^[5]

The fictional extract above is therefore not automatically treated as a complete product assessment. It shows one reported mercury determination for one sample. A statement of conformity needs its own supporting basis.

For the general reading order, see How to read a laboratory report for omega-3.

Which details should accompany a useful result?

Check these points before comparing a value:

• Determination: total mercury, methylmercury or another clearly identified determination.
• Product and batch: an identifiable connection between report and pack.
• Material: ingredient, oil extracted from capsules or finished supplement.
• Method and unit: what was determined, and on what basis is the result expressed?
• Reporting threshold: what do less-than signs and abbreviations mean?
• Dates: distinguish sampling, analysis and report issue dates.
• Comparison: which specification or legal requirement and decision rule are used?

If information is missing, record exactly what remains unknown. A missing date is different from a measured result above a limit.

What can’t one mercury test establish?

A mercury result does not provide results for lead, cadmium, dioxins or PCBs. Those substances require their own determinations.

Nor does the result establish an oxidation value or DHA+EPA content. A sample from one batch is not a permanent measurement of every batch of the product either.

Price and oil source cannot fill those measurement gaps. Another source, such as algae oil, does not remove the need for applicable test information.

What if no public mercury result can be found?

The measured value then remains unknown to you. That does not establish that no testing took place or that a limit was exceeded.

You can ask the brand for the report for the relevant product variant and batch. Ask for the mercury determination, tested material, result with unit and reporting threshold, and analysis and report dates.

A response containing only a product specification is not yet a measurement result. Keep the document and explanation on which any stated value is based.

Reading mercury information on SUPPFAX

SUPPFAX brings together label information and available Netherlands shop prices. Read published test information with its source, date and scope.

A mercury determination, other measurements and price per gram of DHA+EPA answer different questions. SUPPFAX does not perform laboratory tests itself or use one result to give a complete verdict on a product or brand.

Sources

Consulted on 31 October 2026.

1. European Commission — Mercury in food. For chemical mercury forms and references to food legislation. ↩
2. Regulation (EU) 2023/915, consolidated version of 19 August 2026, Annex I, point 3.3.2. For the maximum level of 0.10 mg/kg for the food supplements category. ↩
3. Eurachem — The Fitness for Purpose of Analytical Methods, third edition, 2025 (PDF). For method context, sampling, LOD and LOQ. ↩ ↩
4. Regulation (EC) No 333/2007, consolidated version of 30 April 2024, Annex, Parts C and D. For LOD/LOQ and reporting and interpretation in official controls. ↩ ↩
5. ILAC G8:09/2019 — Decision Rules and Statements of Conformity (PDF). For statements of conformity and the associated decision rule. ↩