What is Meant by Air Changes Per Hour?

Understanding Air Changes Per Hour: Enhancing Indoor Air Quality

Indoor air quality plays a crucial role in maintaining a healthy and comfortable environment. One significant factor that determines air quality is the concept of Air Changes Per Hour (ACH). ACH refers to the number of times the entire volume of air within a space is exchanged with fresh outdoor air in one hour. This metric is used to assess ventilation efficiency and ensure that indoor spaces are adequately supplied with clean air. In this article, we will explore what ACH means and its importance in maintaining optimal air quality.

Defining Air Changes Per Hour (ACH)

Air Changes Per Hour is a measurement that quantifies how many times the total volume of air in a given space is replaced with outdoor air within one hour. It is calculated by dividing the volume of air exchanged by the total volume of the space and expressing it as a per-hour ratio. For example, an ACH of 5 implies that the air within a room is replaced five times per hour.

Importance of Air Changes Per Hour

  1. Ventilation Efficiency: ACH serves as a benchmark to evaluate the efficiency of ventilation systems. Higher ACH values indicate better air circulation and increased removal of pollutants, odors, and contaminants. Adequate ventilation is essential to prevent the buildup of indoor air pollutants that can have adverse effects on health.
  2. Health and Comfort: Maintaining a sufficient ACH helps promote a healthy and comfortable indoor environment. Fresh air exchange reduces the concentration of airborne allergens, volatile organic compounds (VOCs), and other harmful substances. High ACH levels contribute to reducing the risk of respiratory ailments, allergies, and the spread of airborne diseases.
  3. Control of Indoor Pollutants: Efficient ventilation systems with an appropriate ACH play a vital role in controlling indoor pollutants. Pollutants such as carbon dioxide (CO2), formaldehyde, mold spores, and other volatile contaminants can accumulate in enclosed spaces. Regular air exchanges dilute these contaminants, minimizing their concentration and potential health effects.

Factors Influencing Air Changes Per Hour

Several factors impact the achievable ACH in a given space:

  1. Ventilation System Design: The design and capacity of the ventilation system greatly affect the ACH. Properly designed systems with adequate airflow capacity can achieve higher ACH values. The type of ventilation system, such as natural, mechanical, or hybrid, also plays a role in determining ACH.
  2. Occupant Density: The number of occupants in space affects the rate at which fresh air is consumed. Higher occupancy levels may require a higher ACH to maintain optimal air quality. Spaces with high occupancy, such as classrooms, offices, and public venues, typically require more frequent air exchanges.
  3. Outdoor Air Quality: The quality of outdoor air being introduced into the space impacts the overall air quality and ACH effectiveness. Areas with high levels of pollution or allergens may require additional filtration or purification systems to maintain a safe and healthy indoor environment.


Air Changes Per Hour (ACH) is a crucial metric used to measure the ventilation efficiency and air quality of indoor spaces. Achieving an appropriate ACH value is essential for providing a healthy and comfortable environment for occupants. By ensuring regular air exchanges, the concentration of pollutants and contaminants can be reduced, lowering the risk of respiratory issues and improving overall well-being. With proper ventilation system design and consideration of factors influencing ACH, building occupants can enjoy cleaner, fresher air and a higher quality of life.