A short period of high ventilation is sometimes used in piggeries to improve air quality. The effect of a 10 min period of high ventilation during the normal operation of 2 ventilation systems was studied. In a building with automatically controlled natural ventilation (ACNV) the purge produced a 60% reduction in estimated total dust concn. Most of the reduction occurred in the first 2 min. Dust concentrations increased rapidly after the purge, although not necessarily to the initial concn. The purge creates a significant reduction in air temperature for a short period. The effect of the purge in a building with an air recirculation system is not so clear. The number of airborne particles were reduced, but the benefits were partly offset by an increase in particle suspension from the ventilation ducts. The purge may be most beneficial after periods of high dust production, such as feeding or weighing.