The effect of natural background radiation is one of the most important determining factors for shelf life of high speed color negative films. Namely, high speed color negative films are inclined to suffer from increase in fog, decrease in speed and deterioration in graininess during their preservation. This damage is caused mostly by natural background radiation. We divided natural background radiation into three components: cosmic rays, environmental radiation, and self-contamination, and determined the fraction of the contribution of each component to this damage. As a model experiment of the above phenomena, we proved that the sensitivity of a silver halide photographic emulsion to gamma rays was in proportion to its intrinsic sensitivity to blue light and to the amount of gamma rays absorbed by the emulsion grains.