Cesium-137 has a radioactive half-life of about 30 years, and traces of the isotope still persist from above-ground nuclear bomb tests in the 1950s and '60s.

CNN: Fukushima tuna study finds minuscule health risks

Once read they will self-destruct in your brain faster than the half-life of radioactive cobalt.

CNN: ASIANOW - TIME Asia | Asia Buzz: Geomancy for the Millennium