The Apollo astronauts narrowly avoided serious health problems, and even death due to exposure to radiation from the Sun. Here’s a plot of the approximate Solar proton flux during the Apollo era:

Here’s the same but with the missions and health warnings labelled:

Scary stuff! I made these images for a talk I gave a while ago, inspired by @DrChrisDavis and using data from WOTM. Just found them and thought I might as well post them up.