Import control measures by HKSAR Government in the event that Japan begins to discharge nuclear-contaminated water (Q1.1-1.12)Radiation and Health (Q2.1-2.5) Radiation and Food Safety (Q3.1-3.11) Import Surveillance and Control (Q4.1-4.7)Iodine Prophylaxis and Health Concern (Q5.1-5.10) Radiological Standard for Bottled/Packaged Waters under Routine Condition (Q6.1-6.5)

Radiation and Health

2.1 What is the background radiation dose?

Humans are constantly exposed to different kinds of radiations, especially natural radiation. The annual dose received by the public in Hong Kong from natural background radiation is about 2 mSv. In general, the annual dose received by the public in the world ranges from 1 mSv to 10 mSv. However, different areas in the world may have different background radiation levels. For example, the background radiation in some regions in Iran and India may reach 200 mSv per year. According to the report of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in 2000, the worldwide average annual effective dose through ingestion (including foods and drinking water) is 0.3 mSv. Moreover, people are also exposed to radiation in other ways, for example, 0.4 mSv per year from cosmic rays, 0.5 mSv per year from terrestrial gamma rays and 1.2 mSv per year from inhalation (mainly radon).

2.2 What is "half-life" for radionuclide?

Half-life is the time required for a radioactive substance to lose 50% of its activity by decay. Each radionuclide has a unique half-life.

2.3 What are the possible radionuclides being released in a nuclear emergency? How long these radionuclides will remain in the environment?

If a nuclear power plant does not function properly, radioactivity may be released into the surrounding area by a mixture of products generated inside the reactor ("nuclear fission products"). These include radioactive iodine-131 (I-131), caesium-134 (Cs-134), caesium-137 (Cs-137), strontium-90 (Sr-90), ruthenium-103 (Ru-103), ruthenium-106 (Ru-106) and cerium-144 (Ce-144). The main radionuclides representing health risk are radioactive caesium and radioactive iodine which have been covered in CFS's surveillance programme for radioactivity in food.

Since the duration of radionuclides remains in the environment may be affected by factors like weather conditions, the type and concentration of radionuclide involved, it is difficult to estimate accurately the persistence of these radioactive materials. Radionuclides with longer half-life such as Cs-137 may remain in the environment for years.

Radionuclide Half-life
I-131 8.04 days
Cs-134 2.06 years
Cs-137 30 years
Sr-90 29.1 years
Ru-103 39.3 days
Ru-106 1.01 years
Ce-144 284 days

2.4 What are the possible health effects of the exposure to radioactive caesium?

Radioactive caesium, caesium-134 (Cs-134) and caesium-137 (Cs-137), can be released during nuclear emergencies. External exposure to large amounts of radioactive caesium can cause burns, acute radiation sickness, and even death. Exposure to radioactive caesium can increase the risk for cancer. Internal exposure to radioactive caesium, through ingestion or inhalation, allows the radioactive material to be distributed in the soft tissues, especially muscle tissue, increasing cancer risk.

2.5 What is the measurement for human exposure to radioactive substances?

The exposure is expressed as effective dose which represents the total detriment to an individual resulting from an exposure to ionising radiation. The effective dose is equal to the energy deposited by ionising radiation per unit mass of irradiated material (in grays) multiplied by the radiation weighting factor which accounts for the harmfulness of different types of radiation (alpha, beta or gamma) and multiplied by the tissue weighting factors which account for the different sensitivities to radiation induced biological effects of different human organs or tissues. Its unit is Sievert (Sv), or the more commonly used millisievert (mSv). The annual dose received by the public in Hong Kong from natural background radiation is about 2 mSv.