Natural wool was irradiated with ultraviolet and visible light, and the effects of dose, time, and atmosphere on the electron paramagnetic resonance spectra were studied. The results support the view that a complex radical population is created by low energy radiation, and that both during the irradiation and after its cessation the total concentration of the radicals and the ratios of the different species are changing with time. It was found that at sufficiently low intensities the signal associated with sulfur did not appear even after prolonged times of exposure, while the development of the signal not associated with sulfur was similar to that found under stronger illumination. The dependence of the above-mentioned two types of signal on time of high intensity irradiation was studied over prolonged periods. It was found that while the sulfur signal appears to approach saturation, the other signal passes through a maximum. Significantly different effects were observed when the experiments were conducted in atmospheres of nitrogen, oxygen, or pure water vapor.