This two-year grant to Yale University provides support to Professors Bill Nazaroff and Jordan Peccia to continue their ongoing work characterizing airborne microbial populations of indoor environments. The team will study the size distributions of bioaerosols from the indoor environment under occupied and unoccupied conditions. They will examine the sources, origins, and population characteristics of airborne bacteria and fungi in indoor settings that are attributable to human occupancy and collect and analyze air and dust samples from 10 different indoor environments-all elementary schools in the U.S., Germany, and China. Collected samples will help shed light on how airborne bacteria and fungi differ from other airborne particulate matter, how internal physical processes in indoor environments shape bacterial and fungal size distributions, and the role human occupants play in shaping microbial populations in indoor air.