The present note describes new advancements to a valued tool in retrospective biodosimetry that lower the threshold of detection of radiation dose to 29 mGy. The advanced method is based on the electron paramagnetic resonance measurements of stable radiation-induced radicals in tooth enamel. Earlier this method had been used only for reconstruction of high radiation doses obtained accidentally. New opportunity of reconstruction of doses lower than 100 mGy opens a new realm of possibilities for assessing the health effects of ionizing radiation at the natural radiation background level.