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.
Ma Wai-man, acting assistant director of the observatory, said the dust had travelled a long way around the world via the westerly wind belt and its radioactivity had been greatly diluted to almost negligible levels.
'It would take 800 to 2,500 years of continued exposure to the detected levels in order to receive the radiation dosage equal to one X-ray,' he said.
At the observatory's King's Park radiation laboratory, air samples were being taken for 22 hours a day, instead of every three hours before the crisis, to identify the sources of radiation.
Chan King-ming, an environmental science professor from Chinese University said it would normally only take one working day to finish testing and he hoped the observatory could release the results faster.
But Ma said the test results had to be verified manually by staff and the process could not be replaced by a computerised or automated system. The observatory would announce the results of the tests on samples taken on Monday and Tuesday today.
Ma said the ongoing radiation data released hourly on the observatory website only showed the intensity of radiation levels in the air at 10 monitoring sites in Hong Kong, without pinpointing the composition and sources of the radioactive substances.