Cadmium in scallops was found to be 28.6 parts per million, 15 times higher than Hong Kong's legal limit of two ppm, according to associate professor Dr Richard Cheung Yun-hing, who led the study. Babylon shells, commonly served to accompany alcohol, contained 21.3 ppm of cadmium, while sun and moon scallops contained 13.9 ppm.

Under the Food Adulteration (Metallic Contamination) Regulations, vendors are forbidden to sell shellfish that contains more than 2 ppm of cadmium.

Consuming too much cadmium will weaken the body's immune system and its ability to absorb calcium. There is also a risk of developing kidney stones.

World Health Organisation guidelines say a person should not consume more than 7mg of the heavy metal per week for every 1kg of their body weight. That means an adult weighing 70kg should eat no more than 10 pieces of shellfish a week, Cheung says.

'It is OK to have shellfish occasionally. It would only pose a problem when you eat a lot of them over the long term,' Cheung said.

For children, pregnant women and the elderly, the maximum portion should be halved, he added. The heavy metals can also pass through the placenta of pregnant women and lead to miscarriages or abnormal development of the foetus.

The professor said the shellfish probably originated from areas of the mainland where industrial activities generated cadmium waste. 'When there is rain, the industrial waste will be washed into the river. Since shellfish live near the shore, they are likely to absorb the toxins,' he said.

Frog shells and blood cockles were found to have cadmium content of 3 to 4 ppm, which is higher than the legal limit. But Cheung said such levels would not threaten health unless an 'unrealistic amount' of the shellfish was consumed.

Consumers can protect themselves by avoiding eating the organs of shellfish, and washing and cooking shellfish thoroughly before eating.