Pajamas Media has useful primer on radiation doses. It makes a good complement to the XKCD chart.
I’m sure there will be a spike of alarmism with the detection of tiny amounts of radiation in North American rainwater. I hope this helps us remember that “measurable” is an entirely different thing from “even remotely dangerous”. The rainwater story doesn’t mention any numbers, but I’m sure they are very much smaller than the banana-equivalent dose.