Another radioactive substance has been detected in the air, but the Hong Kong Observatory sees the amount as too small to pose a health hazard.

It is the first time the highly toxic radioisotope cesium-137 has been found since the earthquake and tsunami in Japan on March 11 crippled the Fukushima Dai-ichi nuclear complex.

The observatory said yesterday that a "very minute amount" of cesium-137 was detected at King's Park during a 24-hour period ending Saturday.

The concentration was 0.000067 becquerels per cubic meter, only 1/37-millionth of the level that requires protective action. The danger level is 2,480 becquerels per cubic meter, according to the observatory.

Tiny traces of radioactive iodine- 131 were also found - the 10th time in two weeks - with the level at 0.000187 becquerels per cubic meter.

Senior scientific officer Ma Wai- man said: "The amounts were very small and will have no effect on health. The public need not take any precautionary measures. In the coming days, even if the level rises or changes, I expect the amounts [of radiation] will remain small."

Michael Lam Hon-wah, associate professor of City University's department of biology and chemistry, agreed.

"The amounts detected were so small they will not pose any health hazard," he said. "There is no need to worry. We don't have to stay home in fear of radioactive dust."

The half-life of cesium-137 is 30 years. Lam said: "Even if the radionuclide gets into the soil and into the food chain, the amounts will be too small to harm human bodies."

Lam said there is no drug available to protect against cesium. He said cesium-137 may concentrate in muscle, blood cells or other parts of the body. Long-term excessive inhalation may cause diseases such as leukemia.

Meanwhile, the Water Supplies Department said tap water remains normal and safe. And the Centre for Food Safety tested 77 food samples imported from Japan in a 24-hour period to noon yesterday. All were satisfactory.