Radioactive materials from the crippled Fukushima No. 1 nuclear plant have been spreading, contaminating milk, vegetables, water and soil in Fukushima and neighboring prefectures.

On Wednesday, 210 becquerels of iodine-131 per liter of water, more than double the government-set safety level of 100 for infants, were detected in water at a purification plant in Tokyo, and 163,000 becquerels of cesium-137 per kg were detected in soil in Fukushima.

Following are questions and answers about the effects of radioactive materials, including iodine-131 and cesium-137, that have been detected in food, water and soil.

How does iodine-131 affect human health?

Once ingested, about 30 percent of iodine-131 concentrates in the thyroid gland, increasing the risk of thyroid cancer, according to Kunikazu Noguchi, a lecturer specializing in radiation safety at Nihon University School of Dentistry.

“The thyroid gland collects iodine to produce hormones. So once a body takes in iodine, the organ collects it, resulting in high concentrations of the radioactive iodine in the thyroid,” explained Noguchi.

Iodine was blamed for cancer cases following the Chernobyl nuclear accident. Will that happen in Japan?

Apart from the difference in the amount of radiation leakage in the two cases, Japanese in general consume more foods rich in iodine, including fish and seaweed, than the people around Chernobyl. Some believe as a result the presence of more nonradioactive iodide in their thyroids will reduce the intake and accumulation of toxic iodine.

Iodine not accumulated in the thyroid tends to pass into the bloodstream and is expelled in urine, Noguchi said.

What about cesium-137?

Unlike iodine-131, radioactive cesium does not concentrate in a specific organ, experts say.

It spreads almost equally, but mostly in soft tissue, such as muscles, Noguchi said. But people need to bear in mind that the half-life of cesium-137 is much longer than iodine-131.

What is a half-life?

A half-life is the time it takes for the radioactivity of a specific substance to decrease by half. Iodine-131’s half-life is about eight days, while the half-life of cesium-137 is about 30 years.

After 16 days, iodine-131 has decayed to about a quarter of its original level. Within about 90 days, it is effectively harmless, according to Noguchi.

Cesium-137, on the other hand, sticks around for about 300 years, he said.

Because soil contaminated with cesium-137 continues to emit radiation for hundreds of years, it’s important for the government to analyze the soil around the power plant and inform the public of the levels, Noguchi said.

Is there any way to remove radioactive iodine and cesium from foods?

Although complete removal is difficult, rinsing food with water removes radioactive substances from the surface.

“Think of it like a stain. It’s better to use boiling water rather than just water. And it’s even better if you use detergent, although it’s not an option,” Noguchi said.

Is it safe to shower or wash your hands with the contaminated water detected in Tokyo and Fukushima?

There is no problem using the water to shower or for other daily uses, such as laundry, according to the health ministry.

But it’s best to avoid soaking open wounds in the water because radiation can be more easily absorbed into the body through cuts, according to Noguchi.

Can iodine-131 be removed from water and air?

At its water purification plants, the Tokyo Metropolitan Government has opted to use activated carbon, which reacts with iodine-131 and, to an extent, absorbs it.

By the same token, household water purifiers that use activated carbon can filter out some radioactive iodine.

Boiling tap water can also be somewhat effective, a team of radiology experts at the University of Tokyo said via Twitter after the news of the contamination broke.

It’s harder to remove iodine from the air, according to a spokesman for Shinwa Corp., which provides air filters to nuclear power plants. Their machines use special carbon sheets infused with iodide to extract radioactive iodine from the air.

Purchasing one for home use is not practical because each machine weighs a couple of tons and costs “probably about the price of the house itself,” the spokesman said.

The spokesman also said Tokyo Electric Power Co. has asked Shinwa to keep some in stock as they may be needed.

Does iodine-131 have any medical uses?

According to the U.S. Environment Protection Agency, because radioactive iodine is taken up by the thyroid so readily, it can in some cases be given to patients with thyroid problems to aid doctors in making a diagnosis.

How can I know if I’ve been exposed to iodine-131?

According to the U.S. Environmental Protection Agency, some major medical centers are equipped to test for exposure.

But the Japan Radiological Society says those residing far from the Fukushima power plant don’t need to take the test since even people evacuated from the vicinity haven’t shown signs of serious exposure.

Why are children more vulnerable to iodine-131?

Preschoolers are about two to three times more likely to be affected by exposure to radiation than adults, according to the Japan Radiological Society.

This is because children’s thyroids are smaller, explained Noguchi of Nihon University. A Japanese adult’s thyroid weighs on average about 18 grams, whereas an infant’s is about 1.8 grams, and a 1- or 2-year-old’s is about 2.6 grams, he said.

Experts also point out that cell division is more active in children than adults.

But the special steps taken with children are only precautionary “just as every child needs extra care regarding any other issue,” the society said.

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