Why do hospitals have negative pressure rooms?

Hospitals have negative pressure rooms to prevent airborne infectious diseases from spreading from infected patients to healthcare workers, other patients, and visitors in hallways and adjacent areas. A negative pressure room, also called an airborne infection isolation room, is engineered so that air pressure inside the room is lower than pressure in the hallway and surrounding areas, so air flows into the room and not out when doors are opened, containing contaminated air inside. The air that is exhausted from the room is passed through HEPA filters that capture 99.97 percent of infectious particles and then vented safely outside the building, away from intake vents. This is critical for diseases that spread via tiny airborne particles that can float for hours, such as tuberculosis, measles, chickenpox, disseminated shingles, and during aerosol-generating procedures for COVID-19, SARS, and MERS. Without negative pressure, opening a door or movement of staff would allow infectious aerosols to drift into corridors and infect others. Hospitals use continuous pressure monitors, 6 to 12 air changes per hour, self-closing doors, and anterooms to maintain this containment. Positive pressure rooms are used for opposite purpose, to protect immunocompromised patients like bone marrow transplant patients from outside germs. So negative pressure rooms are a key infection control engineering measure that protects the hospital community while allowing safe care of highly contagious patients.