It’s not just the radiation, it’s also the isotopes that emit the radiation that need to be kept in mind. Fallout.
Radiation comes in the form of alpha, beta, and gamma rays, which are easy to measure using radiation detectors. It’s a bit more of a laboratory chore to measure levels of individual elements that emit the radiation (aka radioactive isotopes). But that has to be done.
For example, concerns over radioactive iodine and cesium are important here, because these elements can be ingested, causing damage as they emit radiation in the body. I believe that potassium iodide pills are being distributed for people in Japan to eat, to flood their systems with non-radioactive iodine so that their thyroids won’t take up much of the radioactive type.