A biological threat agent explodes out of a metal tube and travels toward a formation of warfighters. Thousands of aerosolized agents speed through the air. While some agents infect warfighters, others remain in transport or slam into rocks, trees, and the ground. Aerosolized agents that linger in the open-air environment, before decontamination efforts begin or even during that time when they are in transport, will age and decay. Until recently, because of challenges posed by outdoor testing of live biological threat agents, scientists empirically assumed that agents aged and decayed at similar rates indoors and outdoors. Yet laboratory measurements may not accurately predict – and even underestimate – the decomposition of agents released in the open air.