Absorption of ultraviolet radiation in bacterial spores in clusters in air and on surfaces: Model calculations using the multi-sphere T-Matrix method

•Bacterial spores in clusters and/or host spheres on surfaces can be shielded from UV.•Multi-Sphere T-Matrix method was used to model UV absorption by spores on surfaces.•Spores are more shielded in larger and more-absorbing host spheres/clusters.•Sizes and compositions of disease transmitting particles are inadequately known.•Test particles are needed for designing and verifying UVGI decontamination systems.