Positive pressure maintains higher atmospheric pressure inside the cleanroom than outside, preventing contaminated air from entering. Principle: Air flows from high to low pressure. Positive pressure ensures any leakage flows outward, not inward. Typical pressure: 0.03-0.05 inches water column (7-12 Pa) higher than adjacent areas. Pressure cascade: Multiple cleanliness zones with highest pressure in cleanest areas. Air flows from clean to less clean. Implementation: Supply more air than exhaust. HVAC system maintains setpoint. Airlocks and interlocks at boundaries. Monitoring: Pressure differential sensors at zone boundaries. Alarms if pressure drops. Door management: Airlocks between zones maintain pressure during personnel transit. Interlocks prevent simultaneous door opening. Failure response: Low pressure alarm triggers investigation. May indicate filter loading, door issues, HVAC problems. Gowning rooms: Intermediate pressure between outside and cleanroom. Progressive cleanliness. Energy impact: Makeup air requires conditioning (temperature, humidity, filtration). Significant HVAC load. Critical importance: Without positive pressure, particles enter through any gap. Foundation of cleanroom contamination control.