Vegetables imported from the mainland are safe to eat - but fewer than 20 percent of the stalls claiming to sell organic vegetables have the certificates to back up their claims.
The Hong Kong Organic Resource Centre said the metal content in all 117 samples collected from various locations in December and January were within Hong Kong limits, though four exceeded the more stringent limits set by the European Union, Australia and the Codex Alimentarius Commission.
The center said two of the four were Shanghai cabbages - one choi sum and the other broccoli.
The tests carried out were for the eight heavy metals arsenic, chromium, cadmium, copper, lead, zinc and antimony. The choi sum and one of the Shanghai cabbages contained 0.8 milligrams of cadmium per kilogram while the other Shanghai cabbage contained 0.6mg of cadmium and 17.8mg of copper.
The broccoli sample contained 0.36mg of cadmium and 156mg of copper per kilogram.
Center director Jonathan Wong Woon-chung said the normal consumption of vegetables from the local markets poses no immediate harm to health.
But he said if a child consumed more than 48 grams of the most copper-tainted vegetable sample, he or she would take in a dosage higher than the daily intake levels set by World Health Organization.
The vegetables collected are commonly consumed locally and included pak choi, carrots, cucumbers, broccoli, choi sum and Shanghai cabbage.
Wong said parents should encourage their children to eat a variety of vegetables instead of just one or two types. The survey also found that fewer than 20 percent of stalls claiming to sell organic vegetables actually sold certified organic vegetables.
"When you buy organic vegetables, ask if they have the certificates. If they don't, ask from which farm they came," Wong said.
The center also found that only two of the 20 stalls claiming to sell organic fish had the certificates to prove their claims.