Food safety

Formalin in fish: why it is used, why it is harmful, and how to screen for it

VidyuthLabs · 14 October 2026

Formalin is sometimes added to fish to make it look fresh for longer. It is a serious food-safety problem in India and across South and Southeast Asia, and it cannot reliably be detected by looking at or smelling the fish.

What formalin is

Formalin is a solution of formaldehyde in water, typically 37 to 40 percent formaldehyde. It is used to preserve biological specimens and in industry. Formaldehyde is classified by the International Agency for Research on Cancer as carcinogenic to humans (Group 1). It is not a permitted preservative for fish.

Why it ends up in fish

Fresh fish spoils quickly without an unbroken cold chain. Where ice and refrigerated transport are short, some traders dip or spray fish with formalin to slow spoilage and keep it looking firm and fresh during long journeys to market. The buyer has no easy way to tell.

The health risk

Eating formaldehyde-treated food can irritate the mouth, throat and stomach, and repeated exposure is a long-term health concern given its classification as a carcinogen. Children and people who eat fish daily are the most exposed.

A complication: natural formaldehyde

Some fish contain formaldehyde naturally. In certain marine species, an enzyme breaks down a compound called trimethylamine oxide into formaldehyde after the fish dies, especially during frozen storage. Bombay duck and some members of the cod family are known examples. This means a test cannot simply ask "is any formaldehyde present?". It has to tell low natural levels apart from the much higher levels typical of deliberate treatment.

How fish is screened today

• Colour-change kits. A swab or paper strip reacts with formaldehyde and changes colour. ICAR's Central Institute of Fisheries Technology has developed a rapid kit of this kind. These kits are cheap and quick, but reading colour by eye is subjective and only roughly quantitative.
• Laboratory methods. Spectrophotometric and chromatographic methods give accurate numbers, but need a lab and time.

The case for electrochemical screening

An electrochemical sensor turns the presence of formaldehyde into an electrical signal measured by a potentiostat. Compared with colour reading, that offers a number instead of a shade, results stored with time and place, and the same handheld instrument used for other tests. That makes it suited to frequent checks at markets, landing centres, cold-chain checkpoints and export processing units.

How VidyuthLabs approaches it

VidyuthLabs is developing a dedicated formalin-detection sensor in the NanoX family for screening fish and seafood with the AnalyteX portable potentiostat. As with any screening test, positive results that lead to enforcement or rejection should be confirmed by an accredited laboratory.