Try this thought experiment...
Take a ping pong ball, and put it in a gun that imparts an enormous velocity to it. This is a radioactive molecule, released from a power plant, or a waste site.
Now, take a room fifty miles long, and hang ping pong balls from strings all along its length, about one per cubic foot of the entire space. This is the atmosphere above all the radiation sources.
Fire the ping pong ball into one end.
How many ping pong balls come out the other side with that same velocity?