Both improper design and improper management of ventilation both may compromise animal health. Problems are most likely during winter, spring and fall, especially during rainy weather and warmer days coupled with cold nights. Viewing ventilation as a dilution process may help explain why these periods can be troublesome and why efforts to maintain warmer environments may go awry.

Cold barns, generally preferred, present less risk to animal health and are easy to design and manage. In a cold barn, sufficient ventilation maintains indoor temperatures about the same as outdoor temperatures in winter. Of course, ventilation must be adjustable to maintain this minimum temperature difference even as outside conditions change, say from a cold, blustery winter day to a winter day that is calm, sunny and mild. A major reason for air quality problems in a cold barn is adjusting ventilation for the worst case—severe winter weather—and not readjusting to allow increased ventilation when the severe weather passes and milder winter weather appears.

During ventilation, outside air is brought into a barn where it collects moisture, heat and other contaminants, all produced by the animals. Air is then exhausted to the outside. Ventilation is an air exchange process—contaminated air inside the barn is exchanged for fresh outside air. To determine ventilation rates, we focus on the moisture content of the air, measured by relative humidity. But moisture is only one aspect. Ventilation removes other undesirable contaminants as well.

Ventilation is truly a process of dilution. Air moved through a barn actually serves to dilute the inside air and, very importantly, to dilute all of its components. Dilution reduces concentrations of moisture and heat. Dilution reduces concentrations of airborne disease organisms, harmful gases and dust, and undesirable odors as well.

When ventilation is reduced below recommended levels—usually in a misguided effort to warm the barn using animal heat—less moisture is removed. Sometimes the consequences of the resulting moisture buildup are masked by: i) insulating the barn, ii) using a greenhouse effect, iii) providing supplemental heat, or iv) dehumidifying the inside air. But even though excess moisture may not be apparent, the reduced dilution factor does indeed 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.

Essential to maintaining a healthy animal is providing an environment that does not needlessly stress or challenge the animal. Maintaining good air quality is a fundamental aspect of that healthy environment with ventilation providing the key. But air quality is more than just measuring relative humidity. Through ventilation we continually dilute the air inside the barn, assuring that the air the animal breathes has low concentrations of all contaminants that threaten the animal's health