Radiation is an insidious thing. Take a massive dose and it’s very immediately lethal, and dangerous, and should be avoided. Take a series of smaller doses, however, and the consequences are less obvious. You might come out of it unscathed. Your cells might be able to repair the damage done by radiation particles slicing off bits of your genetic structure. Even if you get hit by an electron in just the wrong place the cancer this will cause won’t become apparent for a decade or two. This meant that it was quite a while before anyone twigged that radiation was actually dangerous, and this led to people doing some really, really stupid things with radioactive substances.