About one in 20 secondary school students could be at risk of mercury poisoning from eating large amounts of tuna or swordfish, a government study has found. Their diets gave a mercury exposure of 6.41 micrograms per kilogram of body weight per week - about 28 per cent more than the five-microgram safety limit set by the World Health Organisation. Dr Philip Ho Yuk-yin, consultant in community medicine at the Food and Environmental Hygiene Department, said the main source of mercury was predatory fish such as tuna, shark and swordfish. He said excessive mercury could affect the nervous system. Dr Ho unveiled findings of the first study on students' dietary exposure to dioxins, mercury, arsenic and cadmium at a joint meeting of the Legislative Council's health services and food and environmental safety panels yesterday. He said the dietary patterns of 903 secondary school students were analysed from a food-consumption survey conducted by the department last year. Dr Ho advised students to eat less food containing mercury. The limit for vulnerable groups, such as children and pregnant women, is 60 grams of tuna a week, or four to six pieces of sushi. As long as students do not exceed the limit over a long period, 'temporary excursions' above the limit will not have adverse health effects. The same study found that the average student's exposure to dioxins and the three heavy metals was well below the safety limits. Among high consumers - students whose exposure to the contaminants are in the top five per cent - only mercury exceeded the limit. This means students generally have a low risk of suffering toxic effects from the food they eat. Chronic exposure to dioxins and arsenic can cause cancer, while excessive cadmium affects kidney function. Legislator Dr Lo Wing-lok said that, while the study was generally reassuring, the government should investigate mercury content in food. Apart from predatory fish, the major sources of the contaminants are meat, milk, dairy products and shellfish.