# Definitions

**Mass Median Aerodynamic Diameter (MMAD)** is defined as the diameter at which 50% of the particles by mass are larger and 50% are smaller. USP <601> calls for determining the MMAD by plotting, on log probability paper, the percentages of mass less than the stated aerodynamic diameters versus the aerodynamic diameters. The MMAD is taken as the intersection of the line with the 50% cumulative percent. Computational methods can also be applied.

**Geometric Standard Deviation (GSD)** is a measure of the spread of an aerodynamic particle size distribution. Typically calculated as follows:

GSD = (d_{84}/d_{16})^{1/2}

where d_{84} and d_{16} represent the diameters at which 84% and 16% of the aerosol mass are contained, respectively, in diameters less than these diameters.

# Methodology Applied on MMADcalculator.com

**In plotting the cumulative masses versus the aerodyanamic diameters as described above, one must decide which points on the curve to use for determining the MMAD. Performing linear regression on all the data points is often not appropriate, as certain points (especially those toward the edges of the distribution) can deviate from linearity.
This website uses the approach recommended by ISO 27427 (Nebulizing Systems and Components), which is to use the two values, one above and one below the 50% cumulative collection efficiency value, to plot the line representing the size distribution. The intersection of this line with the 50% cumulative efficiency value is taken as the MMAD. The ISO standard suggests that plotting more points could result in the value being determined with less accuracy. It is argued that points further from the 50% cumulative value have less accuracy because they represent a smaller fraction of the collected mass.**

References:

1. Alternative Approaches to MMAD Determination, Christopher et al.

2. ISO 27427 (Nebulizing Systems and Components), 2009