Glossary
ACH
Plain-Language Definition
Air Changes per Hour — the number of times the total volume of air in a room is replaced in one hour. A measure of ventilation rate.
Technical Context
ACH requirements vary by space type: office 4-6 ACH, hospital operating room 15-20 ACH, cleanroom 20-600+ ACH, toilet exhaust 10+ ACH. ACH is calculated as: airflow (m³/hr) ÷ room volume (m³). Higher ACH means more fresh air but higher energy cost. ASHRAE 62.1 specifies minimum outdoor air rates rather than total ACH.
Why It Matters
Adequate ACH prevents CO₂ build-up, removes pollutants, and controls moisture. During COVID-19, ACH became a key metric for infection control. Insufficient ACH causes stuffiness, headaches, and poor cognitive performance.
Related Terms
Related Articles
Explore in-depth articles related to this topic in the Knowledge Library.
Search Knowledge Library →