Abstract
The bacterial content of the air and physical environment of two crated veal calf units were monitored over the growing period of 16 weeks. The rate of release of bacteria colony forming particles (BCFP) from the calves was rapid, typically 2×106 BCFP/h per unit calf area, and showed a positive correlation with absolute humidity. The concentration of airborne BCFP, arising mainly from these calves, was about 30 BCFP/l, but was not a simple function of climate. In addition to clearance by ventilation at rates ranging from 2 to 10 air changes/h, the airborne bacteria were removed by sedimentation and other physical processes, and killed by normal biological mechanisms. The combined rate for the latter two pathways of clearance was equivalent to 4–49 air changes/h, which is of a magnitude comparable with ventilation rates in naturally-ventilated livestock buildings.
At present the critical concentrations of non-pathogenic bacteria in animal houses are unknown and so a minimum ventilation rate based on this criterion cannot be calculated. However, recognition of the different routes of clearance of airborne bacteria, including ventilation, is fundamental to the design of animal houses.