Here's some information on radiation:
Alpha and beta radiation is more damaging to cells, but is easily blocked by normal material. Gamma radiation i very penetrating, but statistically, is likely going to pass through the material rather than tear up the atoms (it typically need 10 cm of lead to stop it). Keep in mind that once you are considering this issue, that there is a bigger issue of filtering the air in the building at all times and a requirement to have a positive air pressure on the inside. I would see that the danger of inhaling the radiative substance as much more dangerous since it stays in your body to continuously damage your cells (eg. a nuclear fallout scenario).
Here's a link on the subject:
http://www.furryelephant.com/content...mma-radiation/