The breathing zone is the specific volume of air immediately surrounding a person’s nose and mouth from which they inhale, typically a hemisphere extending 0.25-0.3 meters (10-12 inches) from the face. This zone represents the most critical air quality area in ventilation design. Contaminant concentrations measured in the breathing zone determine whether indoor air quality meets health standards.
Technical Significance
Breathing zone air quality directly impacts occupant health, comfort, and productivity. Carbon dioxide levels in the breathing zone should remain below 1,000 ppm per ASHRAE 62.1, though 800 ppm is preferable. Supply air velocity entering the breathing zone must stay below 0.4 m/s to avoid drafts and discomfort. Contaminant removal efficiency in the breathing zone depends on ventilation effectiveness, air distribution patterns, and source location relative to the occupant. Measurements using tracer gas or thermal anemometers quantify actual breathing zone conditions versus design assumptions.
Design Implications
Breathing zone protection requires placing supply registers away from direct face exposure while maintaining adequate air circulation. Source control (removing pollutants at origin) protects breathing zone quality more effectively than relying solely on dilution ventilation. Ceiling-mounted supplies with low velocity achieve breathing zone protection; direct high-velocity supplies can create discomfort or push contaminants into the breathing zone. Proper breathing zone design reduces respiratory complaints and improves occupant satisfaction by 15-25%.