Air quality is not easily defined. It is related to ventilation and the absence (or presence) of contaminants in the air. For animals, good air quality generally implies that the ambient air causes no harmful effects on the animals in the space. Ambient air contains varying amounts of water vapor. Moisture in the air becomes a problem when it exceeds the relative humidity range of 60%–75% preferred for animals; above this level, it is considered to be an air contaminant. Other contaminants may include pathogens, harmful gases, dust, and undesirable odors. Concern arises from the concentration of a contaminant above some predetermined level rather than the mere presence of the contaminant itself.
Numerous viral pathogens known to survive in aerosols are apparently spread by the airborne route. Two factors can be significant in the relationship between microbial aerosols and disease incidence: 1) the survival time of the aerosolized pathogen, and 2) the total number of pathogens per volume of air (ie, concentration). Survival is influenced by conditions of the ambient air; conditions of the ambient air and concentration are related to ventilating air.
Relative humidity is the most important factor influencing pathogen survival and concentration, but its effects vary greatly between pathogens—some survive best in humid conditions, others survive best in dry air. Maintaining a relative humidity in the range of 60%–75% apparently results in the shortest survival time for the greatest number of potential pathogens. Ventilation is used to remove moisture from an animal space with the intention of maintaining relative humidity in this range. Inside air is diluted with outside air, reducing moisture levels. Continuous replacement of contaminated air with fresh, outside air is also the most effective way to reduce the concentration of aerosol pathogens.
Most likely, reducing the concentration of any air contaminant, including gases and dust, is important to reducing its detrimental effect. For example, ammonia is produced by the decay of feces and urine and is probably the most significant air pollutant in cattle barns. Allowed to accumulate, ammonia’s irritating effects on the respiratory epithelium appear to directly reduce the number of ciliated cells and thus decrease the efficiency of mucociliary transport.