The Hong Kong Observatory detected radioactive dust from Fukushima in Hong Kong, but levels had been extremely low, it said yesterday.
The observatory rejected suggestions it had delayed releasing the results, saying it needed time to verify the samples collected four days ago before making the data public.
The radiation level of 0.0001869 becquerels per cubic metre detected in Hong Kong on Sunday is less than 1/300,000th of the level of 661.38 becquerels that the Hong Kong Radiological Protection Advisory Group believes warrants health concerns.
It was the first time Hong Kong had detected radioactive dust since the devastating earthquake in Japan on March 11 which led to leaks from a nuclear power plant in Fukushima.
Coastal provinces on the mainland also reported finding traces of iodine-131 on Monday, after Heilongjiang province reported the first such find on Saturday.
The observatory said minute amounts of radioactive iodine-131 - a man-made radioactive substance believed to be originating from the Fukushima nuclear plant - were detected in air samples taken on Saturday and Sunday. No other radioactive materials, such as the more toxic plutonium, were detected.