The hourly air change reflects how many times per hour the air in a room is changed by normal circulation. Clean air is critical for health and comfort as well as structural integrity; Stale, moist air can contribute to the growth of microorganisms that can damage structural components such as beams and flooring. Many building codes make recommendations for air changes per hour, and in some cases a certain number is required by law for safety.
Improvements in building techniques make hourly air changes particularly important. In older structures, there is a certain amount of porosity and air moves through cracks and jaws, creating some circulation even without fans, windows, and other ventilation options. New structures are more tightly sealed, especially when they are designed to be clean and sterile. Laboratories and operating rooms, for example, use heavy mechanical ventilation for safety.
In an area with little activity, air changes per hour may be low. Only two to four changes are required to ensure proper circulation in a place such as a warehouse. Workers will be able to breathe freely, carbon dioxide will be eliminated, and the materials in the warehouse will be protected from mould and mildew. In environments where hazards exist, more circulation is required. Gases and particulates need to be ventilated through both special hoods and ventilation systems, and some environments can be closed and uncomfortable due to poor ventilation.
Laboratories and other facilities where workers handle potentially hazardous materials may need 15 to 20 air changes per hour for safety. In a place like a commercial kitchen, as many as 60 may be required. This environment can be hot, closed, and difficult to work with, due to fumes from food, oxygen consumption by the flames in stoves, and heat from freezers and refrigerators used to keep food at safe temperatures.
Contractors and engineers will consider air changes per hour in the design of a site. They can install ventilation systems to meet the need and provide flexibility so that the system can easily increase ventilation as needed. If the code requires a certain number of air changes per hour, it should also be integrated into the design, showing building inspectors’ evidence that the area will be safe for use. Personnel is responsible for maintaining ventilation systems while a structure is in use, including changing filters and keeping vents clean, so air can flow freely. Obstacles can limit air exchange and create hazards such as dangerous gas build-up.
What is ACH (Air Change per Hour)?
Air change per hour (ACH or ACPH) is a measurement of air volume that is added to a room divided by the total volume of the room. Put simply, it measures how many times the air in the room is replaced. Higher ACH values mean better ventilation. The formula is as follows:
ACPH = Q / Vol
This table shows us the reference values of air volume by room usage according to DIN 1946.
If you’re not sure how to determine your room value or choose the correct air purifier, you can contact us directly.
Minimum Outside Air
Recommended Number of
4 (private) 10 (public)
6 (private) 10 (public)
Swimming Pools (private)