A recent study looking at plastic levels in five different species of seafood consumed by humans detected traces in every sample tested.

Researchers bought oysters, shrimp, squid, crab and sardines from a market in Australia and analyzed them using a newly developed method that identifies and measures five different plastic types simultaneously.

The study – by the University of Exeter and the University of Queensland – found plastic levels of 0.04 milligrams per gram of tissue in squid, 0.07 milligrams in shrimp, 0.1 milligram in oysters, 0.3 milligrams in crabs and 2.9 milligrams in sardines.

"Considering an average serving, a seafood eater could be exposed to approximately 0.7 milligrams of plastic when ingesting an average serving of oysters or squid, and up to 30 milligrams of plastic when eating sardines, respectively," said lead author Francisca Ribeiro, a QUEX Institute PhD student.

For comparison, 30 milligrams is the average weight of a grain of rice.

"Our findings show that the amount of plastics present varies greatly among species and differs between individuals of the same species.

"From the seafood species tested, sardines had the highest plastic content, which was a surprising result."

The researchers bought raw seafood including five wild blue crabs, ten oysters, ten farmed tiger prawns, 10 wild squid and 10 wild sardines.

They then analysed them for the five different kinds of plastics that can be identified by the new method.

All of the plastics are commonly used in plastic packaging and synthetic textiles and are frequently found in marine litter: polystyrene, polyethylene, polyvinyl chloride, polypropylene and poly (methyl methacrylate).

Polyvinyl chloride was found in all samples, while the plastic found in highest concentrations was polyethylene.

Microplastics are very small pieces of plastic that pollute much of the planet, including the sea where they are eaten by marine creatures of all types, from small larvae and planktonic organisms to large mammals.

The new testing method is a step towards defining what microplastic levels can be considered harmful and assessing the possible risks of ingesting microplastics in food.

The paper, published in the journal Environmental Science & Technology, is entitled: "Quantitative Analysis of Selected Plastics in High-Commercial-Value Australian Seafood by Pyrolysis Gas Chromatography Mass Spectrometry."