I bought an SA and as I thought he bases most of his concern on a single study on rats.

But, more importantly it appears he is using a complete blockage of cosmic rays as his baseline.

Some cosmic rays reach us here on the surface of the Earth.

As to his use of the term "scrawny" to describe Mars atmosphere I see this as emblematic of his whole pessimistic and sensational attitude. Up to one-third of the cosmic rays are blocked by this "scrawny" Martian atmosphere! And as Geoff points out there are simple and yet logical ways to reduce your exposer. A combination of methods will be used to reduce the exposer not eliminate it.

I also find it interesting that at no point does he point out these problems apply to any human trip beyond Earth including the asteroids and the Moon.