23/03 How much radiation is too much radiation?

The Yomiuri Shimbun

There is a lot of erroneous information floating around about radiation contamination, making it important to brush up on the basics so you can know what is really happening.

The government has placed restrictions on some products, including spinach from certain areas, after radioactive isotopes were detected on the produce and in the water supply after a series of accidents at the Fukushima No. 1 nuclear power plant.

It is important to remember that "radiation" refers to a wide variety of radioactive elements.

What effects do these elements have on the human body? How can one prevent exposure and minimize the health risks?

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It's in the food--can I eat it?

On Monday, government inspectors found levels of radioactive iodine and cesium that exceeded legal limits in spinach, milk and other produce from Tohoku and Kanto.

The radioactive material was leaked from the power plant and scattered by the wind, which carried it to nearby farms, where it adhered to the surface of spinach and pastures where cows were grazing.

Ingesting radioactive isotopes leads to internal exposure to radiation.

The Nuclear Safety Commission has set standards for acceptable levels of radiation in water, milk and produce--excluding root vegetables, such as yams.

The decision to restrict shipments of these items was based on these numerical standards.

For dairy products such as milk, the NSC has determined 300 becquerel of radioactive iodine per kilogram and 200 becquerel of radioactive cesium per kilogram are safe for consumption.

For vegetables, acceptable levels increase to 2,000 becquerel and 500 becquerel, respectively.

Following the detection of high levels of radioactive substances in some agricultural products Monday, the government ordered Fukushima, Ibaraki, Tochigi and Gunma prefectures to halt shipments of spinach and kakina greens. It also ordered Fukushima Prefecture to halt milk shipments for the time being.

"Even if people eat these products, there will be no immediate effect," Chief Cabinet Secretary Yukio Edano said. "We have imposed the restrictions in case the radiation leak becomes protracted."

Food safety expert Hideaki Karaki, professor emeritus at the University of Tokyo, said: "As it stands, the radiation levels that have been detected in food present no danger. But in all likelihood, many consumers would refuse to buy the products even if they were shipped, so factoring in distribution costs, this restriction was unavoidable."

"Moving forward, similar incidents and harmful rumors need to be nipped in the bud by disclosing correct information, and farmers need to be properly compensated for their losses," he said.

In Iitatemura, Fukushima Prefecture, radioactive iodine was detected in the water supply at levels three times higher the acceptable limit of 300 becquerel per kilogram. Nuclear experts believe airborne isotopes entered the water supply via the reservoirs and rivers from which the water is pumped.

The Health, Labor and Welfare Ministry instructed the village's residents to refrain from drinking the water, but said they could still use the water for bathing and washing.

The numerical standards for food and water are based on the premise of the radioactive material being ingested for a year.

Based on this, the experts then calculate how much of the radioactive substance would be absorbed into the body by both adults and children, and to what extent their health would be affected. The government's limits are based on the results of these calculations and set at a level that avoids health risks.

At the levels that have been detected so far, people can still safely ingest the food or water until safer alternatives are secured without facing health risks, according to the ministry.

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Low doses may not cause cancer

Past radiation leaks have shown that people exposed to 3,000 millisieverts of radiation per hour have a nearly 50 percent chance of dying within 60 days if they do not receive medical treatment.

However, the only precise data illustrating the long-term effects of radiation are found in studies of A-bomb survivors from Hiroshima and Nagasaki.

It shows that exposure to 1,000 millisieverts of radiation per hour increases the chances of developing cancer by 150 percent. The lower the exposure, the lower the risk of cancer.

About 28,000 people who were exposed to levels of up to 100 millisieverts were monitored over a 40-year period. About 4,400 of them developed cancer. This figure, however, is less than 2 percent--81 people--higher than the average for people who have not been exposed.

There is almost no difference. Even nuclear experts are divided over the correlation between exposure to less than 100 millisieverts of radiation and higher risks of cancer.

Because of this, the risk line is set at 100 millisieverts.

In day-to-day life, people are exposed to about 2.4 millisieverts of naturally occurring radiation every year.

No upper limits are set on exposure to radiation for medical treatments, such as CT scans, because benefits outweigh the risks.

However, as long as there is uncertainty about the effects, people should avoid unnecessary exposure to radiation.

Government regulations require nuclear power plants to be built in a way that limits exposure of local residents to one millisievert per year.

The government is currently monitoring radiation levels across the country. From 4 p.m. to 5 p.m. Monday in Shinjuku Ward, Tokyo, the figure was 0.125 microsieverts, a level at which medical experts say there is no immediate effect on human health.

(Mar. 23, 2011)