How Much Radiation-Contaminated Water Will Kill You?

Radioactive substances are leaching into the water supply near and far from the quake-damaged nuclear reactors in Fukushima, Japan. After an abnormally high level of iodine 131 was detected in the water in Tokyo on March 23, a media frenzy ensued, residents were told not to let their infants drink the city supply, and stores quickly sold out of bottled water.

Are fears of contaminated water justified in Japan? Is the level of contamination actually dangerous?

A 1-sievert (Sv) dose of radiation increases a person's lifetime cancer risk by 4 percent, according to health physicist and radiation safety expert Peter Caracappa of the Renssealaer Polytechnic Institute. To put that in real terms, if 1,000 people are exposed to 1 Sv of radiation, 40 more of them will develop cancer in their lifetimes than would otherwise.

A person would have to ingest 77 million becquerels of radioactive iodine in order to receive a 1 Sv radiation dose, Caracappa told Life's Little Mysteries.

At its highest level of contamination (recorded on March 23), Tokyo water contained 210 becquerels of radioactive iodine per liter. A simple calculation shows that a person would have to drink about 370,000 liters (97,000 gallons) of that water to expose himself to 1 Sv of radiation, and thus increase his lifetime cancer risk by 4 percent.

At the recommended rate of eight glasses of water a day, it would take someone about 530 years to consume that much water. Besides the obvious fact that no one lives that long anyway, iodine 131 also radioactively decays within days, so the Tokyo water supply will not remain contaminated for nearly that long. On March 24, a day after the high reading, the radioactive iodine level had already fallen to 79 becquerels per liter.

"My opinion is that we're unlikely to see an increase in cancer deaths as a result of the nuclear accident in Japan, given the information I have access to about the levels of dose that the population is being exposed to," Caracappa said.

• Is Radiation Exposure Contagious?
• Can Your Tap Water Kill You?
• What Everyday Things Around Us Are Radioactive?

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