Investigations of the mass balance of dust in houses for pigs and chickens have shown that the generation of dust is related to the number and weight of animals, but is also affected by the activity in the buildings. Ventilation rate has a limited effect on the concentration of dust due to the importance of settling of the dust on different surfaces inside the buildings. Dust reducing measures such as electrostatic air cleaning, removal of dust with vacuum cleaners and ionization of the air have not realized any considerable reduction of the dust concentration. Automatic spraying of small droplets of water with two types of spraying nozzles has reduced the dust concentration. Spraying with a mixture of rape seed oil and water has also been effective with manual spraying as well as with an automatic spraying system. Investigations of ammonia in animal houses have shown that air leakage, location of air inlets and outlets, stocking rate of the animals, air flow rate and time intervals between manuring will affect the release and concentration of ammonia. Tracer gas measurements carried out in a calf stable have indicated that the location of a concentrated source of a pollutant but also the function and location of air inlets and outlets have strong influence on the spreading and concentration of pollutants in the air.