Setagaya Radiation “Hot Spot” – Not From Fukushima

Yesterday morning, there was great excitement on the internet about the discovery of a radioactive “hot spot” in Tokyo. The discovery was made by a citizens’ group that had been using their own geiger counters to search for radiation in Tokyo. The particular spot was only a couple square meters in size, located along the side of an old building:

Setagaya Ward detected radiation of 2.707 microsieverts per hour at the fence, which is in the Tsurumaki district, on Oct. 6, and measured the radiation again twice Thursday. The day’s preliminary reading was reportedly 3.35 microsieverts per hour.

A level of 2.707 microsieverts per hour would be equivalent to 14.2 millisieverts per year, while 3.35 microsieverts per hour would be equivalent to 17.6 millisieverts per year, lower than the 20 millisieverts per year at which the government is supposed to order an evacuation.

It is widely believed that a one-time exposure to 100 millisieverts of radiation may increase the risk of dying from cancer by 0.5 percent.

The contamination appears to be limited to one site, as no other hot spots were found in Setagaya.

The ward measured radiation levels at 64 local elementary schools in the summer, and the highest reading was only 0.1 microsievert an hour.

It seemed like a dream come true for anti-nuclear bloggers and conspiracy theorists. Look! The Japanese government has been lying to its citizens: Tokyo is in danger! There’s a hot spot in Tokyo that is more radioactive than some of the evacuated parts of Fukushima! The government is murdering its own people!

But, the truth came to light, and it was bad news for fear-mongers. Authorities checked the building next to the spot, and found its source. It was radium-226, an isotope not used in the Fukushima reactors:

Mayor Nobuto Hosaka said Thursday the hotspot was most likely caused by a radioactive material in several old bottles in the house. He said radioactivity from the bottles exceeded the measurable limit on a low-dose radiation counter.

Science and education ministry inspectors believe the bottles contain radium, a radioactive material used in the past as self-luminous paint for watches, Kyodo News agency reported. It said the inspectors concluded that the radiation was not related to the Fukushima disaster because no cesium was detected in the bottles. Cesium is one of the main isotopes that leaked from the tsunami-damaged nuclear plant.

Meanwhile, in Funabashi, a group of Chiba citizens discovered their own hot spot. When authorities came to check it, they found it was a false alarm:

The group reported readings of 5.82, 2.1 and 1.79 microsieverts per hour at three locations in Anderson Park, but the city measured 0.91, 1.40 and 0.79 microsierverts per hour at the same spots Thursday.

However, one should not dismiss all of the efforts that private citizens have been making. They have made real discoveries. For example, a higher than normal amount of strontium-90 was found in Yokohama:

City officials announced private tests done by an agency hired by a resident had found a concentration of 195 becquerels per kilogram of strontium in sediment on top of the apartment building. Both the Yokohama government and the Japanese government are conducting their own tests to verify the findings.

Levels of the isotope, which can lodge in bones and bone marrow and cause cancer, detected in various places in Japan prior to the Fukushima accident have been 10-20 becquerels per kilogram and are a legacy of Cold War-era nuclear weapons tests that dispersed strontium 90 throughout the globe.

It is higher than the usual 10-20 becquerels per kilogram, but it is not at a dangerous level. 195 bq/kg is very close to the 160 bq/kg safety limit for food imports to the United States. If someone were to use that dirt to grow vegetables, the vegetables would probably pass safety standards. If dirt was a food, one could even dilute it a small bit and sell it as “safe” food. Strontium is pretty scary when it’s found in large amounts. Ten or twenty times regular background levels is “high” but it is not a serious threat to health.

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