This extremely small measure of radioactivity represents ten to the negative fifteenth power becquerels. It corresponds to an average of one radioactive decay event every 31.7 million years. Environmental physicists and radiochemists might encounter such small theoretical values when calculating the residual activity of highly stable isotopes or extreme dilutions.
Heard on the job
The background radiation contribution from that specific trace element drops to a few femtobecquerels per litre.