Lung Humidity: Why Water Vapor Evaporates, Not Condenses, in Alveoli

Alveolar air is at body temperature (37°C) and fully saturated (100% relative humidity, ~47 mmHg water vapor pressure).

Physics

• Inspired air is warmed/humidified in upper airways.
• At 37°C, alveolar gas is at its maximum water-holding capacity.
• Net water movement: evaporation from airway lining into gas to maintain saturation, not condensation.
• Condensation would require gas cooling below dew point – doesn't happen in healthy lungs.

Clinical Relevance

• Mechanical ventilation requires heated humidifiers to prevent mucosal drying.
• Cold, dry air triggers bronchospasm in asthma (via airway cooling/drying).

The respiratory tract actively conditions air to protect delicate alveolar epithelium.