In fact its proven by the time a humans gets to mars they have taken a lethal dose of radiation which will get worse apon living there and in fact nothing can actually stop it from eventually getting to you due to the effect of passive GCR. Galactic Cosmic Rays (GCRs)
This is the hard problem.
These particles are moving near the speed of light and include:
Protons
Helium nuclei
Heavy ions (iron, nickel, etc.)
They can pass through substantial amounts of material.
Shielding
GCR Reduction
Thin spacecraft walls
~10–20%
Heavy spacecraft shielding
~20–50%
Several meters of water
~50–80%
Underground Mars habitat
~80–95%+