OSAKA — As the world struggles to understand the risks of a worst-case scenario at the Fukushima No. 1 nuclear plant, Tuesday’s government order for residents living within a 20-km radius of the plant to finish evacuating and those between 20 and 30 km to stay indoors heightened tensions in the northeast and Tokyo regions.

Radiation levels of 400 millisieverts — levels significantly dangerous to humans — were detected Tuesday morning near the Fukushima No. 1 nuclear power plant’s No. 3 reactor. At the plant’s No. 4 reactor, the level registered 100 millisieverts, while the area between the No. 2 and No. 3 reactors registered 30 millisieverts.

Abnormal levels of radiation believed to be from the failing reactors were even detected as far away as Tokyo, Chiba, Tochigi and Kanagawa Prefectures, although the government and experts said the levels’ effects on humans was almost negligible for now.

The most pressing question — what radiation levels pose a danger to human health — is one that invites a variety of responses. Those can depend on whether the ones doing the answering are fundamentally pro- or antinuclear power, are nuclear scientists or medical doctors, or are policymakers from different countries.

On average, a person is exposed to approximately 3.0 millisieverts of radiation each year, of which 2.4 millisieverts is from naturally occurring sources, according to the World Health Organization. Only about 0.01 millisieverts is due to man-made radiation, specifically nuclear power.

The International Atomic Energy Agency notes one chest X-ray will give a 0.1 millisieverts dose. Flying in an airplane adds about 0.005 millisieverts per hour to the body, but a whole body CAT scan exposes a person to about 10 millisieverts.

In terms of the Fukushima plant, the WHO advised Monday that radiation-related health consequences will depend on exposure. Exposure, in turn, is dependent on the amount of radiation released, weather conditions, the distance someone is from the plant, and the amount of time someone spends in irradiated areas. Prevailing meteorological conditions such as wind direction and rain also affect millisievert levels.

The U.S. Department of Health and Human Services says there have been cases of colon cancer caused by doses of about 1,000 millisieverts, and that exposure to less than 200 millisieverts can cause leukemia, thyroid, breast and lung cancer.

Liver cancer, the department notes, can be caused by exposure to less than 100 millisieverts of radiation. Some levels may also carry the risk of cancer outbreaks more than a decade after initial exposure.

According to the Japan Atomic Energy website, short-term exposure to more than 200 millisieverts leads to the possibility of cancer.

But the greatest long-term danger may be from agricultural products in areas where radioactivity falls to the ground and is absorbed by crops and livestock.

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