A "very minute amount" of radiaoactive Caesium-137 was detected in air samples taken from the city on Saturday, together with Iodine-131 from air samples, said a statement from the Hong Kong Observatory.
"Since the amount of radioactive Iodine-131 and Caesium-137 detected are both very small, there will not be any impact on public health and no precautionary measures are required," the statement added.
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The samples were taken over a 24-hour period over 8 to 9 April, it said.
"The environmental gamma radiation level in Hong Kong remains normal," the statement said, adding that monitoring of the radioactivity levels will continue.
Countries across Asia have stepped up precautionary checks on radiation since the March 11 9.0-magnitude quake that triggered a massive tsunami which killed thousands and destroyed swathes of Japan's northeast coast.
Emergency crews are battling to contain a nuclear disaster at atomic reactors in the battered region.
