Understanding ACH: Air Changes Per Hour Explained

Air Changes per Hour (ACH) is the single most important metric for evaluating air purification effectiveness. This guide explains what ACH means and why it matters.

What is ACH?

ACH measures how many times the entire volume of air in a room is filtered per hour. Higher ACH means cleaner air faster.

The Formula

ACH = (CADR × 60) / Room Volume

Where CADR is in m³/h and room volume in m³.

Target ACH Ranges

• 5-12 ACH: Recommended for homes, effective for allergens and smoke
• 6+ ACH: Healthcare settings standard
• 12+ ACH: High-risk environments (excessive for most homes)

Practical Example

Room: 5m × 4m × 2.4m = 48m³
Purifier CADR: 300 m³/h
ACH = (300 × 60) / 48 = 6.25 ACH ✓

Why It Matters

ACH directly correlates with pollutant reduction. 5 ACH removes ~63% of particles per hour, 6 ACH removes ~75%, and 12 ACH removes ~94%.