How is air changes per hour (ACH) calculated, and why is it important for controlling contaminants?

• A. ACH = (sum of all ventilation airflow rates in CFM × 60) / room volume (ft^3).
• B. ACH = room volume / (sum of ventilation airflow rates in CFM × 60).
• C. ACH = (sum of ventilation rates in CFM) / 60.
• D. ACH equals the number of air changes per hour.

Answer: (A)

Air changes per hour measures how many times the air in a space is replaced each hour. To compute it, add up all ventilation flow rates in cubic feet per minute (CFM), convert that to cubic feet per hour by multiplying by 60, and then divide by the room’s volume in cubic feet. In formula form, ACH = (Q × 60) / V, where Q is the total ventilation rate and V is the room volume.

This matters because higher ACH means faster dilution and removal of airborne contaminants, improving indoor air quality and helping control exposures in environments like labs or healthcare settings. For example, with a total flow of 600 CFM in a 3,000 ft^3 room, ACH = (600 × 60) / 3000 = 12, meaning the room’s air is replaced about 12 times each hour.

The other approaches don’t give the right measure: dividing volume by flow gives hours per air change (not changes per hour), and dividing flow by 60 omits the room’s volume, so it wouldn’t reflect how much air can be replaced in the space. Simply stating “the number of air changes per hour” isn’t a calculation either.