This is not too bad but a little tricky, go see my solution for entering the radiation zone. The easiest way to find this is to go to the opposite area of the map, from where the radiation wind starts. At the end of the game, Get close to an exit and wait until the radiation depletes your health to about 1/4, once you feel safe to wait and extra 10 seconds to exit the game, walk to the exit and there you go.
Note, keep trying this method because it definitely works better than going around and killing everybody yourself
sometime. And I don't know how, there's people that might still be left alive somewhere but it should work 90% of the times.
Note 2 : if the furthest exit on the map is close to a signal detector and it flashes green, you can activate it when the storm is getting closer to have an idea of how many people are left and how many are inside the storm already.