When does 226Ra become "safe"? Hi, I was working on an assignment which deals with radioactive decay. Half life of 226Ra is 1600 years (it varies from source to source, but this is the one I chose), and the equation I got for half-life is y=Ce^(-0.000433216988t), C being the initial value and y being amount left in grams, and t being the amount of time passed in years. When does 226Ra become "safe", where the radiation from it is no longer considered to the public? I know 226Ra is a toxic element, so I'm expecting pretty low... also, if possible, give the units in grams, like x micrograms per gram, not using other units like pCi/L. Thanks!