Radioactive dating

There's a question about radioactive dating.
There's a very important assumption about radioactive dating: We know what is the concentration of the radioactive element in the atmosphere
What if, during history, an event occured that artificially increased or decreased the amount of radioactive element in the atmosphere significantly. That'll make all our assumptions incorrect, won't it? We won't know of these events happening, as there may not have any signs at all.