An important element of aerosol technology is the production of test aerosols for calibrating instruments, conducting aerosol research, developing and testing air cleaning and air sampling equipment. Monodisperse aerosols are used to calibrate particle size measuring instruments and to determine the effect of particle size on sampling devices. A monodisperse aerosol is usually defined as an aerosol that has a geometric deviation of less than 1.2. In aerosol research, monodisperse test aerosols of known size, shape and density are highly desirable because most aerosol properties depend strongly on particle size, which is best controlled by using monodisperse aerosols. Tests made with a series of monodisperse aerosols, each having a different particle size, permit the evaluation of the effect of particle size on aerosol properties or the performance of instruments like impactor. Test aerosols also used for various types of inhalation studies like animal exposure to toxic substances, respiratory deposition in humans or animals and the administration of therapeutic aerosols.