Adequate expression for average particle size of microbiological aerosols.

Attention was called to the fact that mathematical expressions exist for relating the various means commonly employed to describe the average particle size of microbiological aerosols. These include the count median diameter (CMD), the mass median diameter (MMD), the petri ratio (P(a)) size, and the settling rate (V(g)) size. It was pointed out that the diameters derived from the P(a) and V(g) are mathematically identical to the average surface-diameter (SMD), because this average is determined from the square of the diameters, a relationship which also describes the settling of particles by the Stokes equation. The useful equations are: log SMD = log CMD + 2.3026 log(2)sigma(g) and log MMD = log CMD + 6.9078 log(2)sigma(g) where sigma(g) is the standard geometric deviation, and the CMD is determined on a count basis and the MMD on a weight basis. It was suggested that the SMD is the most adequate expression for the average particle size of microbiological aerosols.