On Tuesday, the Japanese government said radiation from the stricken Fukushima Daiichi nuclear plant was at levels harmful to human health. Radiation levels in parts of the plant briefly reached 400 millisieverts per hour, though they later fell. An average chest X-ray usually gives a dose of about 0.1 millisieverts of radiation.
The rise in radiation prompted the plant operator to briefly evacuate the emergency staff working on the stricken reactors. At the radiation levels recorded, 75 minutes exposure would lead to acute radiation sickness.
Several explosions have damaged the outer structures housing the nuclear reactors, and fires have broken out in the plant's fuel storage tanks. There are also fears that the containment vessels of two plant reactors have been damaged. The function of the steel vessel is to protect the nuclear reactor and fuel and prevent the spread of radioactivity into the atmosphere.
In the worst case scenario, the nuclear fuel melts inside the reactor and damage to the containment vessel allows large amounts of radiation to be released into the atmosphere through the breach.
In Tokyo, 170 miles south of the stricken plant, radiation levels on Tuesday briefly rose to 20 times normal levels, though this is not thought to pose an immediate health threat.
While the Japanese government has evacuated people living within 12 miles of the plant, the health impact of the radioactive emissions will depend on the type of radioactive isotope released, how long it remains radioactive and the degree of environmental contamination.
So far, radioactive iodine-131 and caesium-137 have been detected around the nuclear plant. Although radioactive iodine has a half-life of only eight days, it is readily taken up by the thyroid gland and can lead to cancer.
Radioactive caesium, with a half life of 30 years, can cause long term environmental contamination.
Consequences of radiation exposure
The immediate consequence of significant exposure is radiation sickness, which can include nausea, vomiting, diarrhoea and fatigue.
Later these symptoms may be followed by headaches, mouth and throat inflammation, hair loss, chest pain, skin darkening and bleeding spots under the skin, haemorrhage and anaemia.
In the case of extreme radiation exposure - suffered by those in the immediate vicinity of a radioactive release, such as plant workers - death can occur in weeks.
Immediate measures
Those residing near the plant have been told to remove and dispose of clothes and to wash with soap and water to minimise additional contamination.
The Japanese government has also distributed 230,000 doses of iodine tablets to those living near the plant. The iodine saturates the thyroid gland, preventing the gland from storing radioactive iodine, which can later develop into thyroid cancer.
A range of other treatments, from drugs that counter bone marrow damage to those minimising the impact on internal organs, could also be used.
Long term impact
The long term impact of radiation may not reveal itself until years after the accident.
In Chernobyl, those most affected were emergency workers, with 47 dying from acute radiation. About 4,000 children from the area developed thyroid cancer, and 9 died.
A 2005 consensus report of eight United Nations agencies and the governments of Belarus, Ukraine and Russia predicted that 4,000 emergency workers and residents of the most contaminated areas may die from radiation-induced cancer in the long term.
The authorities in Japan are aware of the risks and have reduced the size of the emergency team working at the damaged nuclear plant to a minimum.
Those living within 12 miles of the plant have been evacuated, while those between 12 and 18 miles from the plant were told to stay inside their homes, with windows closed and ventilators off. ( yahoo.net )
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