Factbox: How much radiation is dangerous?
(Reuters) - Radiation levels in Japan remained a huge worry on Wednesday following explosions and fires at the Fukushima power plant. But there has been no indication that people away from the plant's immediate vicinity have been exposed to harmful amounts.
In Tokyo, only very insignificant amounts -- one that pose no danger -- have been detected.
Here are some facts about the health dangers posed by radiation:
* On Tuesday evening, radiation levels around Tokyo were less than 1 microsievert. While that is nearly 10 times normal readings, experts say that amount of radiation is very minimal and even smaller than a dental x-ray, which is about 10 microsieverts.
* Even if a person was exposed to that level of radiation in Tokyo all year round, that amounts to about one-third of the radiation from a single organ CT scan.
* People are constantly exposed to some level of natural radiation. They get exposed to tiny amounts through sitting in airplanes, routine chest or dental X-rays, and larger amounts through medical tests such as CT-scans and MRIs.
Depending on the flight route, flying at a height of 40,000 feet exposes the passenger to radiation of between 3 and 9 microsieverts per hour -- far more than the levels seen in Tokyo so far.
* Generally, people are exposed to about 1 to 10 millisieverts of radiation a year from natural background radiation, caused by radioactive substances in the air and soil. One thousand microsieverts make 1 millisievert.
* A whole body CT scan, for example, gives a radiation dose of 20 to 30 millisieverts, while a single organ CT involves a dose of less than 10 millisieverts.
* Radiation is measured using the unit sievert, which quantifies the amount absorbed by human tissues. One sievert is 1,000 millisieverts.
* On Wednesday morning, levels at the Fukushima plant reached 10 millisieverts an hour before falling to around 3 millisieverts, Kyodo News Service quoted Japan's nuclear safety agency as saying. Early on Tuesday, the level peaked at 400 millisieverts an hour -- 20 times the annual exposure for some nuclear-industry employees and uranium miners.
Below are different levels of radiation exposure -- all measured in millisieverts -- and their likely effects on humans, as published by the U.S. Environmental Protection Agency:
* Exposure to 50-100 millisieverts: changes in blood chemistry.
* 500: nausea, within hours.
* 700: vomiting
* 750: hair loss, within 2-3 weeks
* 900: diarrhoea
* 1,000: haemorrhage
* 4,000: possible death within 2 months, if no treatment
* 10,000: destruction of intestinal lining, internal bleeding and death within 1-2 weeks *20,000: damage to the central nervous system and loss of consciousness within minutes, and death within hours or days.
Sources: Taiwan Atomic Energy Council, World Nuclear Association, US Department of Transportation, US Environmental Protection Agency
(Reporting by Richard Borsuk and Tan Ee Lyn in Singapore, editing by Jonathan Thatcher)
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