"2011-04-16 7:22: 21 micro-Roentgens/h (~21 cpm). 0.218 micro-Sieverts/h. Location: Kita-ku, Tokyo. Weather: partly cloudy, winds from the south. Safe."
When obtaining a radiation reading, it's always helpful to consider that figure with a yearly time-frame in mind so you can properly gauge your maximum permissible dose. In the case above, it's helpful to keep in mind:
- .218μSievert per hour yields an annual dose of 191 mrem
- Normal background levels in Tokyo prior to the Daichi incident were at 0.04μSv/h, so these readings are still about 5x higher than usual
- The source contributing to these higher readings are not cosmic sources as they would be during air travel; they are from radioactive fission products that will enter the body and deposit themselves in bone, tissue
- The isotope breakdown in air is very difficult to ascertain, even when keeping track of released numbers from government agencies. For example, how much Cs-137 contributes to the 5x reading vs Cs-134 or I-131