Abstract: A self-contained ventilation apparatus is disclosed for ventilating buildings, e.g., patient rooms such as hospital rooms, waiting rooms, hospital operating rooms, doctors' offices, and clinics, especially rooms contaminated with infectious microorganisms. One or more blowers is provided. When there are two blowers, a first blower is provided for discharging room air to the atmosphere and a second blower is provided for introducing fresh outside air to the room to provide a continuous supply of fresh air to the room which is free of disease organisms. Both streams of air pass through an air-to-air heat exchanger to adjust the temperature of the incoming air to approach that exhausted from the room. When filters are used, both currents of air are preferably filtered, e.g., with a high efficiency particle air (HEPA) filter. Provision is made for automatically maintaining room air pressure at the desired value (typically below that of surrounding rooms) as filters gradually become clogged with dust.