What metric is used to determine ventilation needs in a commercial greenhouse?

• A. Air changes per hour (ACH)
• B. Soil moisture content
• C. Leaf wetness duration
• D. Relative humidity

Answer: (A)

Ventilation needs are driven by how quickly the air inside a greenhouse is replaced with outside air. This rate is measured as air changes per hour (ACH). ACH is the best metric because it directly quantifies the turnover of the greenhouse air, which controls key conditions like temperature, humidity, and CO2 levels, and helps prevent heat buildup or excessive moisture. A higher ACH means air is refreshed more often, while a lower ACH means slower replacement.

The other metrics measure different things and don’t specify how much air exchange is needed: soil moisture content tracks the water status of the growing medium, leaf wetness duration relates to moisture on leaf surfaces and disease risk, and relative humidity shows the current humidity level but doesn’t by itself define how much air should be exchanged. In practice, you set an ACH target based on climate, crop requirements, and greenhouse design to size ventilation equipment appropriately.