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Monday, 23rd May 2011

A scientific perspective of the radiological effects of the Fukushima nuclear accident to the Hong Kong environment

The unfortunate accident at the Fukushima Daiichi nuclear power plant triggered by the devastating earthquake and tsunami in Japan on 11 March 2011 hit the headlines for several weeks in a row. While we mourn the tragic loss of lives in Japan, some of us also worry about the possible effects that the transport of radioactive materials through the atmosphere can bring to Hong Kong. Now, with many weeks' worth of data on environmental radioactivity collected by the Hong Kong Observatory, we are in a better position to illustrate the effects of the accident.


Keeping a safe distance

In the Fukushima accident, the geographical distance of more than 3,000 km between the site and Hong Kong offers the best protection. Theories of atmospheric dispersion tell us that the dilution process over such an enormous distance will be so effective that even if the winds are blowing in our direction, the concentration of artificial radioactive substances reaching Hong Kong will be extremely insignificant. Moreover, with Japan situated at the higher latitudes under a belt of westerly winds aloft, it is just as likely that the radioactive substances released to the atmosphere will be blown eastwards across the Pacific in the first instance, in which case radioactive substances travelling through such a round-the-globe route before reaching Hong Kong will become dispersed and diluted further along the way.

Similarly, the vast expanse of ocean between Japan and Hong Kong provides a very effective buffer that helps to disperse and dilute any artificial radioactive substances transported by sea. It is also noted that the prevailing sea current near Fukushima is predominantly drifting towards the northeast. As such, the chance of sea waters in Hong Kong having a significant concentration of radioactive substance originating from Japan is extremely remote.


The Observatory as the sentinel in radiation monitoring in Hong Kong

Subsequent to the Fukushima accident, the Observatory soon stepped up its radiation monitoring efforts in Hong Kong. Air samples were collected at King's Park and Yuen Ng Fan every day, and analysed for their radioactivity content. Within two to four weeks after the first major release of radioactivity at Fukushima, radioactive iodine and caesium normally associated with nuclear power generation were successively detected in the air in Hong Kong. However, up to now since 14 April, artificial radionuclides were no longer found in the air samples in Hong Kong.

Figure 1     Activity of iodine-131 detected in air samples in King's Park from late March to late April 2011.
Figure 1      Activity of iodine-131 detected in air samples in King's Park from late March to late April 2011.
(The concentration level required for taking protective actions is called "Derived Intervention
Level" (DIL) [1]. The DIL for inhalation of iodine-131 from the air is 661.38 Bq/m3.)


The Observatory's radiation monitoring not only tells us whether artificial radionuclides are present, but it also provides scientific data for ascertaining their likely effects upon Hong Kong. Here are the facts from the scientific data gathered:
  • At the height of the event locally, 828 μBq/m3 of iodine-131 was detected in the air samples in Hong Kong. (1 μBq = 1 microbecquerel = 1 millionth of a becquerel. Becquerel is the SI unit of radioactivity. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second.)


  • Keep on inhaling the air with 828 μBq/m3 of iodine-131 for one year gives a radiation dose of about 0.05 μSv to a human being. [1 μSv = 1 microsievert = 1 thousandth of a millisievert (mSv) = 1 millionth of a sievert. Sievert is the SI unit of dose (the committed effective dose in scientific terms), which is a measure of the biological effect to a person, taking into account factors such as the type and energy of radiation, the tissue or organ affected, the retention of the radioactive substance in the human body after intake, etc.]


  • Despite the presence of a tiny amount of radioactive iodine and, to an even smaller extent, radioactive caesium for a short period of time, the environmental gamma radiation levels as measured at the ten monitoring stations in various parts of Hong Kong have remained normal since the beginning of the accident.

For the purpose of putting such facts and figures in the proper context, here are some interesting comparisons:
  • One will need to breathe in air with such iodine-131 concentration level for hundreds of years to get a radiation dose similar to that of a chest X-ray (around 10 - 50 μSv).


  • On average, a person in Hong Kong receives a radiation dose of about 7 μSv/day from all natural sources of radiation (cosmic ray, gamma ray from building materials and soil/rock, food and drink, as well as inhaling the naturally occurring radioactive gas radon).


  • Based on the Derived Intervention Levels provided by the Hong Kong Radiological Protection Advisory Group[1], the peak concentration of iodine-131 as measured in Hong Kong will need to increase by 800,000 folds for the health impact to become an issue and protective actions need to be taken.

The relative insignificance of the Fukushima-related radioactive dosage to Hong Kong can also be gauged from the schematic diagram below.

Figure 2     Schematic scale illustrating the radiation dose from different origins and their potential health effects[2].
Figure 2      Schematic scale illustrating the radiation dose from different origins and their potential health effects[2].


Conclusion

A full assessment of the impact of the Fukushima accident, like the one in Chernobyl, will require the hard work of scientists in the years to come. But for now, the scientific data clearly indicate that, despite the presence of artificial radionuclides in the air in Hong Kong for a short period of time, the amounts are so minuscule that there is practically no health impact to speak of. Nevertheless, the Fukushima accident, tragic as it is, does provide an opportunity for us to learn more about radiation and to review our state of preparedness. Ultimately, it is only through sound scientific knowledge and rigorous radiation monitoring that we can respond sensibly to any nuclear emergency.



W.M. Ma and W.H. Leung


References:

1. Report No. 1 of the Hong Kong Radiological Protection Advisory Group: Derived Intervention Levels

2. Parts of the information are derived from the website of the Radiation Health Unit, Department of Health:
http://www.info.gov.hk/dh-rhu/english/html/understand_rad_english.htm



Last revision date: <17 Jan 2013>