Dilution reduces heat and concentrations of moisture, as well as concentrations of airborne disease organisms, harmful gases and dust, and undesirable odors. The dilution rate of ventilation is often expressed in air changes per unit time. For example, a ventilation rate of 4 air changes/hr implies that the entire volume of the ventilated space is replaced 4 times every hour. In fact, some of the air may bypass the occupied zone in the barn, depending on geometry of the space, design of diffusers controlling inlet air, etc. Therefore, the effectiveness of ventilation is not 100% but perhaps approaches 65%. Ventilation effectiveness becomes important to the actual dilution achieved by a particular rate of ventilation (ie, to the ability of ventilating air to reduce the concentrations of contaminants in the animal space). For a ventilation effectiveness of 1.0, one air change would achieve a complete change of air in the space, yielding a 100% reduction in contaminant levels (if the condition of the outside air is considered to be the reference standard). But if ventilation effectiveness is only 0.65, one air change will reduce contaminant levels by only 65%. As ventilation effectiveness diminishes, the ventilation rate required to achieve a certain air change rate increases.
When ventilation is reduced below recommended levels—usually in a misguided effort in cold climates to warm the barn using animal heat—less moisture is removed. Sometimes the consequences of the resulting moisture buildup and lack of proper ventilation (eg, condensation) are masked by insulating the barn, using a greenhouse effect, providing supplemental heat, or dehumidifying the inside air. For example, adding heat to the air reduces relative humidity, without the need for air exchange. It is quite possible to maintain substantial quantities of moisture in the air and, if accompanied by heating, keep the relative humidity within an acceptable range. If relative humidity is the only measure of air quality, it may be deemed to be satisfactory. However, even though excess moisture may not be apparent, the reduced dilution does result in increased concentrations of airborne disease organisms, harmful gases and dust, and undesirable odors. If these increases are ignored, animal health problems are inevitable. In addition, heating of barns is rarely economical in cold climates.