People who visit these places get a Geiger counter that shows the weak gamma radiation, which is existing there. This is slightly lower than that of X-rays and therefore not very dangerous. But this radiation is the proof of the existence of vast quantities of radionuclides - this means presence of radiating particles en masse.
The deadly danger there has the following cause: the incredible amounts of radionuclides that "lie around" and "fly around" and then are inhaled bring the deadly risk of cancer - just a few years later.
These radionuclides are so small (atoms) that they are not kept away by any gas mask or DIY store respirator mask.
For us as physicists it is incomprehensible how people voluntarily go there.
Apparently people have no idea of the huge difference between external radiation and the highly dangerous radiation sources (radionuclides), entering their body.
