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The Observatory Detected Very Minute Amount of Radioactive Iodine from Air Sample Collected at King's Park
29 March 2011)


The Observatory detected very minute amount of artificial radionuclide Iodine-131 from air samples collected at King's Park from 26 March to 27 March 2011 (noon to noon) and from 27 March to 28 March 2011 (noon to noon).  The concentrations of Iodine-131 measured from these two samples were 0.0000625 Bq/m3 and 0.0001869 Bq/m3 respectively, which were respectively 1 / 10,000,000 and 1 / 3,500,000 the concentration level required for taking protective actions* (Figure 1).


Since the earthquake and tsunami event in Japan on 11 March, the Observatory has strengthened the monitoring of concentration of radioactive substances in the air. The frequency of sample collection from the air sampler at King's Park has increased from once weekly to once daily so as to operate round-the-clock for radioactivity measurement. Owing to the very low radiation dose, the Observatory needs to extend the measurement time from normally a few hours to 22 hours to meet the reliability requirement.  Even with this measuring method, the amount of radioactive iodine detected is still very minute that will not affect public health and no precautionary measures are required.  Radiation level in Hong Kong remains normal and the public needs not worry.  The Observatory will continue to monitor closely and routinely announce the measurement results.


The procedures for measuring Iodine-131 is illustrated below:


The procedures for measuring Iodine-131


Figure1 The activity of Iodine-131 in air samples measured at King's Park. Please note that the date in the figure is the starting date for air sampling. For example, 26 March corresponds to the air sample taken between noon, 26 March and noon, 27 March.

* The concentration level required for taking protective actions is called "Derived intervention level" (DIL).  The DIL for inhalation of Iodine-131 from the air is 661.38 Bq/m3. (Reference: Report No. 1 of the Radiological Protection Advisory Group, HKSARG: Derived Intervention Levels)

Last revision date: <21 Dec 2012>