Aerosol Particle Generator
Definition and Description of an Aerosol
An aerosol can be defined as a system of solid particles or liquid droplets that can remain dispersed in a gas, usually air. Naturally occurring aerosols, as well as those emitted by clinical aerosol generators, almost always contain a wide range of particle sizes. Because the aerodynamic behavior of an aerosolized particle is critically influenced by its mass, it is important to be able to describe precisely the size distribution of aerosolized particles. In clinical studies the mass median aerodynamic diameter (MMAD) and the geometric standard deviation (σg) are often used to characterize the dimensions of an aerosol. When the mass distribution of particles in an aerosol is fractionated and the cumulative particle distribution plotted as a lognormal distribution on probability paper, it often approximates a straight line. However, recent studies of clinical aerosols have indicated that nebulized particles are often not lognormal in distribution.5 The MMAD represents the point in the distribution above which 50% of the mass resides, expressed as the diameter of a unit density (1 g/mL) sphere having the same terminal settling velocity as the aerosol particle in question, regardless of its shape and density.
The lognormal plot is convenient because, if linear, it defines a statistically “normal” distribution and the data can be described accurately by the MMAD and the standard deviation alone. For a lognormal distribution, one standard deviation is called the “geometric standard deviation,” or σg. The σg is the ratio of the size at 84% (or 16%) to the MMAD and is an indicator of the variability in particle diameters. If the particle size varies over a wide range (σg > 1.2), it is described as having a polydisperse particle distribution; if the particles are of similar size (σg < 1.2), the particle distribution is described as monodisperse. Monodisperse aerosols are usually encountered only in research studies where specialized generators are used to create such an aerosol.6 For clinical aerosols that are not lognormal and are widely polydisperse, it is best to relate deposition studies to the entire distribution of particles and avoid focusing on simple descriptive terms like the MMAD and σg.5,7
The definition of the mass median diameter is the same as that of the MMAD except that the data are not normalized to unit density.