scientific american: One of those hurdles is radiation. For reasons unclear to me, this tends to get pushed aside compared to other questions to do with Mars's atmosphere (akin to sitting 30km above Earth with no oxygen), temperatures, natural resources (water), nasty surface chemistry (perchlorates), and lower surface gravitational acceleration (1/3rd that on Earth). By Caleb A. Scharf
'But we actually have rather good data on the radiation situation on Mars (and in transit to Mars) from the Radiation Assessment Detector (RAD) that has been riding along with the Curiosity rover since its launch from Earth.
'The bottom line is that the extremely thin atmosphere on Mars, and the absence of a strong global magnetic field, result in a complex and potent particle radiation environment. There are lower energy solar wind particles (like protons and helium nuclei) and much higher energy cosmic ray particles crashing into Mars all the time. The cosmic rays, for example, also generate substantial secondary radiation - crunching into martian regolith to a depth of several meters before hitting an atomic nucleus in the soil and producing gamma-rays and neutron radation.
'An analysis by Hassler and colleagues, published in 2014 in Science, noted that a human expedition with 360 days total in interplanetary space, plus 500 days on Mars itself, would expose astronauts to just over 1 sievert of radiation. Now statistically that's not too awful. It would increase the odds of you getting fatal cancer by some 5% over your lifetime. However, if we consider just the dose on Mars, the rate of exposure averaged over one Earth year is just over 20 times that of the maximum allowed for a Department of Energy radiation worker in the US (based off of annual exposure).'