This book identifies emerging and critical issues that directly or indirectly influence the protective performance of Respiratory Protective Devices (RPDs), along with important future research directions. The severity of the COVID-19 pandemic emphasizes the vital role of respiratory protection provided by PPE (and RPD) in novel infectious respiratory disease control. A wealth of recent research on coronavirus mitigation measures is combined with prior information on infectious diseases, RPDs, and human physiological and psychological responses to make this a fundamental resource on recent advances, innovative perspectives on respiratory protection, and new applications.