Factbox: How much radiation is dangerous?

A Swiss Agency for Development and Cooperation (DEZA) worker packs instruments for the measurement of nuclear radiation to send to Swiss Rescue workers in Japan, at the DEZA logistic - center in Wabern March 14, 2011.

(Reuters) - Health experts urged governments in the Asia Pacific to monitor radioactivity levels after Japan's quake-damaged nuclear power plant exploded and sent radiation into the air.

Radiation is measured using the unit sievert, which quantifies the amount of radiation absorbed by human tissues.

One sievert is 1,000 millisieverts (mSv). One millisievert is 1,000 microsieverts.

Below are some facts about the health dangers posed by higher radiation levels:

* Japan's Chief Cabinet Secretary Yukio Edano had, at one point, said radiation levels near the stricken plant on the northeast coast reached as high as 400 millisieverts (mSv) an hour. That figure would be would be 20 times the annual exposure for some nuclear-industry employees and uranium miners.

* People are exposed to natural radiation of 2-3 mSv a year.

* In a CT scan, the organ being studied typically receives a radiation dose of 15 mSv in an adult to 30 mSv in a newborn infant.

A typical chest X-ray involves exposure of about 0.02 mSv, while a dental one can be 0.01 mSv.

* Exposure to 100 mSv a year is the lowest level at which any increase in cancer risk is clearly evident. A cumulative 1,000 mSv (1 sievert) would probably cause a fatal cancer many years later in five out of every 100 persons exposed to it.

* There is documented evidence associating an accumulated dose of 90 mSv from two or three CT scans with an increased risk of cancer. The evidence is reasonably convincing for adults and very convincing for children.

* Large doses of radiation or acute radiation exposure destroys the central nervous system, red and white blood cells, which compromises the immune system, leaving the victim unable to fight off infections.

For example, a single one sievert (1,000 mSv) dose causes radiation sickness such as nausea, vomiting, hemorrhaging, but not death. A single dose of 5 sieverts would kill about half of those exposed to it within a month.

* Exposure to 350 mSv was the criterion for relocating people after the Chernobyl accident, according to the World Nuclear Association.

*"Very acute radiation, like that which happened in Chernobyl and to the Japanese workers at the nuclear power station, is unlikely for the population," said Lam Ching-wan, a chemical pathologist at the university of Hong Kong.

For the latest news on Japan's nuclear crisis, click

Sources: the New England Journal of Medicine, World Nuclear Association and Taiwan's Atomic Energy Council

(Compiled by Richard Borsuk, Kim Coghill, Tan Ee Lyn)