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This is a gamma spectrum of dust wiped from a Tokyo rooftop. The detector is located at Worcester Polytechnic Institute’s Physics Department. The results are estimates because of the low weight of dust in the sample. The test found 51 +/- 17 kBq/Kg cesium-134 and 30 +/- 10 kBq/Kg cesium-137. This result is higher than for many soil samples around Greater Tokyo. This is expected since dust samples are less diluted by existing nonradioactive soil material. Not knowing the exact amount of surface area sampled, this measurement can not be directly compared to results given as Bq/square meter.
Almost all of the radioactivity in the sample is coming from cesium isotopes. Just as you would avoid breathing in dusts that contained lead or asbestos, one should avoid breathing in dusts that contain this much radiocesium. Parents are often advised to wash childrens’ hands to avoid ingesting lead in dust. This is also a good practice around dusts that contain radiocesium.