The US Radiation Worker annual radiation limit is 5 rem, or 50mSv/year. Divided into the 2000 annual working hours, that’s 0.025mSv/hour.
The Mars Curiosity rover measured an average dosage on Mars of 0.67mSv/day at about -4km altitude. That’s 0.028mSv/hour.
If you worked somewhere lower altitude, like Hellas Basin (-7km or even possibly -8km) or a place like Valles Marineris at -5km and valley walls nearby, you should be able to get that down to the 0.025mSv/hour of US Radiation Worker limits.
Or work French hours. 😉
Or limit it to 1000 hours per year like commercial airline pilots, the rest indoors.
So really, at least during solar max (when GCR is at a nadir), the surface radiation levels of Mars don’t seem like any insurmountable barrier at all, provided you can adequately shield everywhere else and provided you’re okay with US radiation worker limits. For instance, 3m of polyethylene at -5km altitude (Valles Marineris) gives you 22mSv/year (although the model I use for that calculation is questionable to me… it seems there are too many low-energy neutrons). 1m of water/PE is 48mSv/year. Of course, you can also bury below a bunch of regolith or burrow to achieve arbitrarily low radiation levels.
Long-term, I suspect we’ll find drugs that are effective. Or we’ll find out if the Linear no-threshold (LNT) model is correct or not. In any case, when there are millions of people on Mars, it’ll be much easier to produce high-quality data about relative risks for low-dose radiation and also easier to develop enough statistical power to show whether or not drugs like Amifostine can protect against low-dose chronic radiation as well as the acute radiation we know it’s effective for.