Cascade impactor method for the droplet size characterization of a metered-dose nasal spray

A relatively rapid and simple method was developed to characterize the droplet size of a metered-dose nasal spray. The study primarily concerned the measurement of the relative proportion of small to large droplets. A small droplet could potentially reach bronchi or alveoli, depending on its size, and was therefore undesirable for the topical corticosteroid therapy of rhinal disease. The nasal spray was a solution of flunisolide, a topically active anti-inflammatory corticosteroid, administered by a manually operated, metered-dose pump spray system. The method utilized a cascade impactor fitted with a glass chamber; the cascade impactor collected and sized droplets into six fractions 0.5-16 micron in diameter, while the glass chamber collected droplets greater than 16 micron in diameter as another fraction. Results showed that the majority of the spray droplets deposited in the glass chamber. Less than 0.5% by weight of the spray dose was delivered in droplets less than 8 micron aerodynamic diameter. These results are in good agreement with the droplet size distribution obtained by laser holography. The cascade impactor method showed that the number of undesirable small droplets produced by the flunisolide nasal spray unit was negligible. The method can be used with other aerosols where there is a similar concern for the inhalation of small particles.