What does ACH stand for?

• A. Air Changes Per Hour.
• B. Acute Chemical Hazard.
• C. Ambient Condition Humidity.
• D. Active Chemical Handling.

Answer: (A)

Air changes per hour is the rate at which the air in a space is replaced with outdoor or filtered air in one hour. It’s a fundamental measure of ventilation and indoor air quality. You can think of it as how many full air exchanges happen in that room each hour.

It’s calculated by comparing how much air is moving through the space to the room’s volume: ACH equals the volumetric airflow rate (in cubic meters per hour) divided by the room volume (in cubic meters). For example, if a room is 300 m^3 and 900 m^3 of air is supplied each hour, the ACH is 3. This means the room’s air is replenished three times every hour.

Understanding ACH helps determine how effectively a space dilutes and removes airborne contaminants. Higher ACH generally means faster reduction of pollutant concentrations and better ventilation, though it also increases energy use and may cause drafts if overdone. Typical values vary by setting: general areas might be around 2–3 ACH, patient rooms higher, and cleanrooms or labs often require even higher rates.

The other phrases don’t represent this ventilation rate—they refer to hazards or conditions rather than a measure of air exchange.