Research on the presence of carcinogenic compounds in Nigerian smoked fish

by Aaradhana Natarajan ’20

Caption: Dried Ethmalosa fimbriata or “bongo” fish was found to contain toxic amounts of polycyclic aromatic hydrocarbons.

Smoked fish makes up nearly 61% of the dry fish produced in Nigeria. While smoking is a common technique for meat preservation, wood smoke typically produces Polycyclic aromatic compounds (PAHs), which then saturate the fish meat. Professor Isioma Tongo and researchers at the University of Benin in Nigeria studied the concentration of PAHs in four local species of smoked fish and their risk to human health.

Randomized samples of four fish species were obtained from three of the major markets; Oreogbeni, New Benin, and Santana. After soaking 10 grams of each dehydrated fish sample in extraction solvent, the residual extracts were obtained using column chromatography. Each extract was tested for known carcinogenic PAHs.

The health risk for humans was calculated by relating the PAH concentration in each sample to the fish ingestion rate of the average Nigerian adult. The PAH4 index equation was then used to make the results compatible with the European Union Commission (EU) standards and hypothesize cancer risk. Results indicated that the carcinogenic PAH concentrations in mg/kg were 0.446, 0.222, 0.467 and 0.201, for Clarias gariepinus, Tilapia zilli, Ethmalosa fimbriata and Scomber scombrus , respectively. The differences could have resulted from interspecies variation in fat, moisture and skin nature. These values were significantly higher than those reported from Ghana, India and Kuwait, consistent with the higher concentration of PAHs found in the samples.

All these values greatly exceed the limit of 0.03 set by the EU. The researchers thus concluded that continued consumption of these smoked fish could pose a serious health risk to humans. This issue is not limited to Nigeria, and other governments should be encouraged to conduct similar studies on their own aquatic food sources in order to ensure the health of their general populations.



  1. I. Tongo, Human health risk assessment of polycyclic aromatic hydrocarbons (PAHs) in smoked fish species from markets in southern Nigeria. Toxicology Reports 4, 55-61 (2017). doi: 10.1016/j.toxrep.2016.12.006
  2. Image retrieved from:

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