By Mayumi Negishi and Tan Ee Lyn
TOKYO (Reuters) - Radioactive substances have been leaking from Japan's tsunami-damaged Fukushima Daiichi nuclear plant for nearly two months, and the plant is likely to continue emitting radiation into the atmosphere, and possibly into the ocean and ground water, for many months to come.
People living within a 20 km (12 mile) radius of the plant have been evacuated, while those in five towns downwind from the plants have also been told to prepare to leave their homes.
Following are some questions and answers about the health risks from continued radiation exposure from the plant, 240 km (150 miles) from Tokyo, as operator Tokyo Electric Power Co struggles to bring it under control:
HOW MUCH RADIATION HAS BEEN MEASURED OUTSIDE THE EVACUATION
The highest radiation reading outside the evacuation zone on Thursday measured 0.046 millisieverts an hour in Namie, Fukushima, which is one of the towns whose residents are preparing for evacuation.
Most areas outside the evacuation zone logged readings around 0.001 millisieverts an hour or less.
Japan's National Institute of Radiological Sciences calculated that in the month from March 14 to April 11, following the earthquake and tsunami, a person in Tokyo who spent eight hours a day outdoors and drank tap water would have been exposed to about 0.12 millisieverts.
In absolute terms, these quantities are extremely small. People in Japan are on average exposed to 1.5 millisieverts of natural background radiation a year.
HOW MUCH EXPOSURE IS DANGEROUS?
Experts' opinions vary. One benchmark from the Symposium on the International System of Radiological Protection recommends maximum exposure levels of 1 millisievert per year for the general public.
Children, whose cellular activity is more active than in adults, are especially vulnerable, and public health officials say they should be shielded as much as possible from unnecessary exposure.
CAN LONG-TERM EXPOSURE TO LOW LEVELS OF RADIATION CAUSE
Yes. Radiation is cumulative, and in theory, every radioactive particle that makes its way into the body increases the risk of cancer.
But there is no conclusive study that links a rise in cancer to cumulative radiation doses of less than 100 millisieverts. Experts are divided on the policy implications for these lower dosages.
Researchers are stumped in part because it can take decades and even generations before cancer emerges, and because so many other lifestyle choices can increase cancer risk. Researchers also had very poor quality data on dosage measurements after Chernobyl.
Japan's National Institute of Radiological Sciences says that cumulative exposure to 100 millisieverts of radiation raises the risk of death from cancer by 0.5 percent.
Every day, people are exposed to a vast array of other cancer-causing substances through cigarette smoke, eating and drinking habits, chemicals, viruses and bacteria.
IS SOME RADIATION MORE HARMFUL THAN OTHERS?
Yes. Radioactive particles are most harmful when inhaled or ingested, and particles that are easily absorbed and which have longer half-lives can cause more damage to cells and genetic material inside.
When examining radiation from the Fukushima plant, health officials focus especially on Iodine-131. Inside the body of an adult, it has a half-life of 7 days, but it accumulates quickly in the thyroid gland. Children are especially at risk because their thyroids are still developing.
Another by-product, Cesium, spreads throughout the body, concentrating in muscle tissue. Cesium-137 has a half-life of 30 years, but inside an adult, the amount will be halved in 90 days.
Strontium and plutonium are rarer by-products of nuclear fission. But if they are ingested, they tend to collect in bones, where they are likely to stay put. Strontium can cause bone cancer. Plutonium is more dangerous when inhaled, increasing the risk of lung cancer.
HOW CAN RISK BE MINIMIZED?
Everything depends on how good the Japanese government is in monitoring radiation levels and how thoroughly it samples food products.
"The Japanese are very sensitive when it comes to food safety, so I am confident that children will not be exposed to radiated milk products," said Shunichi Yamashita, professor of biomedical sciences at Nagasaki University.
"Just as we have become attuned to pesticide risk, we will have to learn to live with radiation risk and do what we can to avoid contaminated food and water."