standard containing known amounts of drug ester inhalable aerosol particle density is determined for example by delivering aerosol phase drug ester into a confined chamber via an inhalation device and measuring the number of particles of given size collected in the chamber the number of particles of a given size may be directly measured based on the light scattering properties of the particles alternatively the number of particles of a given size is determined by measuring the mass of particles within the given size range and calculating the number of particles based on the mass as follows total number of particles sum from size range 1 to size range n of number of particles in each size range number of particles in a given size range mass in the size range mass of a typical particle in the size range mass of a typical particle in a given size range u03c0 d 3 u03c6 6 where d is a typical particle diameter in the size range generally the mean boundary mmads defining the size range in microns u03c6 is the particle density in g ml and mass is given in units of picograms g u2212 12 rate of inhalable aerosol particle formation is determined for example by delivering aerosol phase drug ester into a confined chamber via an inhalation device the delivery is for a set period of time e g 3 s and the number of particles of a given size collected in the chamber is determined as outlined above the rate of particle formation is equal to the number of 100 nm to 5 micron particles collected divided by the duration of the collection time rate of aerosol.