Love that xkcd chart. One thing it doesn't show is that different particle species of radiation have different effects on our biology at different "potencies". Gamma rays released by an atomic weapon being worse than the x-rays utilized in CT scans. A rough guide to degree of risk per species has been given a value in the form of a quality factor (Q) or a modifying factor used to derive dose equivalent from absorbed dose for purposes of radiation protection. A scale where less dangerous alpha particles are given a value of 20 and gamma, x-ray and beta a value of 1.

Here's a relevant moment from the MIT OpenCourseware Nuclear Engineering lecture about ionizing radiation damaging DNA: https://www.youtube.com/watch?v=HfRpkTG7Iow&t=30m30s

I can't recommend that course enough if you're interested in radiation physics.