The reasons for fog in radiographic films are reviewed. Fog can be caused by chemical reactions forming catalytic development centers in the emulsion layer, by unwanted exposure to radiation, or by the attack of the developer on silver halide crystals lacking catalytic development centers.
M. Dienst, M. Dienst,
"The Causes Of Fog In Radiographic Film", Proc. SPIE 0070, Application of Optical Instrumentation in Medicine IV, (25 March 1976); doi: 10.1117/12.954567; https://doi.org/10.1117/12.954567