EDITORIAL: Radioactive iodine in water is a cause for alarm

Radioactive substances released from the quake-stricken Fukushima No. 1 nuclear power plant have been detected across many parts of eastern Japan. Radioactive iodine has been found in tap water in Tokyo. Earlier this week, iodine was found in water in Fukushima Prefecture, where the crippled plant is located.

It seems likely that radioactive substances will be detected in many other locations in coming weeks.

People have every reason to be very concerned.

One point that needs to be driven home is that infants and pregnant women are particularly vulnerable to the effects of radioactivity.

We urge adults to remain calm and put priority on protecting those most in need from the effects of radiation.

According to the Tokyo metropolitan government, radioactive iodine measuring 210 becquerels per liter has been detected in water samples at the capital's Kanamachi purification plant, which provides tap water to the city's 23 wards plus part of the Tama district.

The level is lower than the recommended limit of 300 becquerels set by the Nuclear Safety Commission. But the figure is higher than the limit of 100 becquerels for infants recommended by the Ministry of Health, Labor and Welfare.

The metropolitan government has advised residents in the affected areas to avoid providing tap water to infants.

The becquerel is a measurement unit that indicates the ability of a radioactive material to emit radiation, or the intensity of radioactivity. The sievert, in contrast, is a unit that focuses on the effects of radiation on human health.

There are regulatory limits for each radioactive material based on the average intake. These limits are designed to prevent people from being exposed to a dangerous amount of radioactivity.

Even though levels of radiation higher than these limits have been found in food and water, it doesn't constitute an immediate risk to human health.

Radioactive iodine is easily absorbed in the thyroid gland of infants and small children. The annual limit on the thyroid gland's exposure to radioactivity is set at 50 millisieverts.

If infant keeps drinking contaminated water, for example, through powdered formula, over a prolonged period, its exposure to radioactivity could exceed that limit.

It wouldn't be dangerous if the water is given to an infant for a short period. Even so, it would be better to provide bottled water in such instances so long as high levels of radioactive iodine are detected in tap water.

The problem is that bottled water has been in short supply since the Great East Japan Earthquake struck March 11. The bottles remaining on store shelves should be earmarked for small children, particularly infants, and pregnant women.

After the 1986 Chernobyl nuclear disaster in the former Soviet Union, thyroid cancer increased significantly in the affected area. More than 6,000 small children in the area who kept drinking milk contaminated with radioactive iodine later developed this type of cancer. By 2005, 15 of them had died.

But there was no noticeable increase in the number of cancer cases among adults in the area.

We need to learn from such valuable experiences and findings. If worried consumers overreact to news about radioactive contamination of water and food, serious shortages of certain products could result.

The International Commission on Radiological Protection has recommended that the limits on the intake of food contaminated with radioactivity should be kept as reasonably low as possible while taking economic and social factors into consideration.

It is important for both governments and citizens to think about the problem with cool heads.

Stopping the spread of radioactive substances requires stabilizing the situation at the crippled Fukushima nuclear power plant so that radioactive materials will not leak into the atmosphere. What is the most urgent is to speed up the on-site efforts to contain the nuclear crisis.

--The Asahi Shimbun, March 24