What does air changes per hour (ACH) indicate?

Air changes per hour (ACH) is a critical measure in evaluating indoor air quality as it indicates the number of times that the air within a defined space (such as a room or building) is replaced with outside air in the span of one hour. This metric is significant because it provides insight into the ventilation effectiveness within a space. A higher ACH signifies better air exchange, which can contribute to improved air quality by diluting indoor pollutants, controlling humidity, and ensuring adequate oxygen levels.

In practical terms, ACH helps in assessing whether a space is adequately ventilated for comfort and health. For example, spaces where people congregate, like offices or schools, typically require a higher ACH to maintain air quality standards, while smaller or less-occupied areas may require less frequent air replacement.

The other options do not accurately represent what ACH measures. The speed of air movement pertains to airflow dynamics, but does not directly equate to the frequency of air exchange. Humidity levels pertain to moisture content in the air, which is a separate consideration from air changes. Finally, the efficiency of heating systems relates to energy consumption and thermal comfort, not specifically to air exchange rates. Thus, understanding ACH as a measure of air replacement is essential for anyone looking to ensure healthy