The scale and effect of radiation was not appreciated in the 40's and probably not into the 60's, given the scale of air testing. I thought much/most of the cast of a John Wayne movie (Ghengis Khan?) ended up with some kind of cancer after being downwind of a Nevada test?

Look for The Atomic Cafe on the 'Net to see what the training films from back then had to say.

From what I've read, the scientists were at least partially aware, but the secrecy of the project caused compartmentalization. Just as an indicator, only two people died of acute radiation exposure in the Manhattan Project, and both people were avoiding the established safety protocols. That seems prima facie evidence that they were aware that high-dose radiation was bad.

I think there was also pushback against those facts becoming more public after the war both due to the fact that the US used the atomic bomb, and also due to industrial uses of radioactive materials that no one wanted to pay for (a la the Radium Girls).