Home

Personnel

Publications

Newsletters

Glossary

Bibliography

Calendar

Services

Products

Instructions,
Descriptions,
Manuals

Links

THE EFFECTS OF RELATIVE HUMIDITY ON THE SIZE DISTRIBUTION OF DRY POWDER PARTICLES 

Of all the physical parameters that influence the delivery of dry powder medicine to the lung, particle aerodynamic diameter is the most critical. Only particles within the narrow size range of 0.5 to 5µm can reach the deep lung to medicate this region (Gonda, I. J. Pharm. Pharmacol. 33:692, 1981); thus, all dry-powder inhalers attempt to present drug particles in this size range. Some dry powder particles absorb moisture and grow if exposed to a humid environment. This can change the location of deposition in the respiratory tract and the efficacy of drug delivery by inhalation. The purpose of this study was to investigate the size distribution of dry powder particles in a range of humid environments that might occur as particle enters the human respiratory tract. The knowledge derived could help us to understand the effect of humidity on the delivery of dry powder to the lung and suggest ways to reduce adverse effects.

The built-in powder disperser module of the Aerosizer was used to suspend the powder particles in a stream of carrier air of known relative humidity. The measurement began by loading approximately 0.01 cm³ of dry powder into the disperser cup. A jet of carrier air from the disperser nozzle caused the powder to become airborne. Relative humidity of the carrier air was controlled by adding an appropriate volumetric flow of humid air to a known flow of dry air. The duration of each test, from start of powder dispersion to completion of powder size measurement, was 3 min. Essentially all of the powder sample was dispersed from the cup during each test. The duration of powder dispersion was deliberately kept short to evaluate the short-term influence of relative humidity on particle size. Such a condition is representative of what may occur when dry powder is dispersed into a humid environment such as the human respiratory tract. Different results might have been obtained if the powder had been allowed to equilibrate in the humid environment prior to dispersion.

The changes in particle size as a function of relative humidity are represented in Figure 1 as mean geometric diameters for the three kinds of dry powders. The mean geometric diameter of three powders increased with increasing relative humidity. As the dry powders were dispersed, they apparently absorbed moisture and either grew or agglomerated.

To assist in quantifying the susceptibility of the different powders to hygroscopic growth, we defined a simple dimensionless growth parameter, R = D_90%/D_10%, where D_90% was the mean geometric diameter at 90% relative humidity, and D_10% was the mean geometric diameter at 10% relative humidity. As the relative humidity of the air changed from 10% to 90%, it was found the TiO₂ was more susceptible to particle growth, with a growth parameter of R = 1.3. Cobalt was somewhat less susceptible with R = 1.2, and Bricanyle was even less susceptible with R = 1.1. Note, that particle deposition within the human respiratory tract is likely to depend on both the initial particle size of the powder and the magnitude of the growth parameter R after the aerosol enters the nose or mouth.

Figure 1. Influence of the relative humidity of the dispersion airjet on the mean geometric particle diameter of three powders resuspended by the Aerosizer aerodynamic particle sizer.

______________________