The photocatalytic inactivation of Bacillus subtilis spores in air was evaluated employing a fixed-bed reactor with TiO2-coated glass rings, under artificial UV-A radiation. Calculations of the radiation effectively absorbed inside the reactor were carried out by Monte Carlo simulations. The photocatalytic inactivation was assessed by analyzing the viability of the microorganisms retained by the coated glass rings inside the reactor at different irradiation periods. The initial concentration of the spores was reduced by almost 55% at the end of the experiment (12 h). Complementary assays were carried out employing Bacillus subtilis vegetative cells, obtaining a reduction of more than 96% under the same conditions. Two efficiency parameters were computed to assess the reactor performance: the photonic efficiency and the quantum efficiency of inactivation. Results of the efficiency parameters allow an objective comparison of the reactor performance under different experimental conditions and configurations.