Effect of low and high relative humidity on metered-dose bronchodilator solution and powder aerosols

Physical properties of two metered-dose bronchodilator aerosols packaged as solutions and two aerosols packaged as finely ground powders were measured at low and high relative humidity. The aerodynamic size distribution and particle concentration were measured in real time using the single-particle aerodynamic relaxation time analyzer, which can measure the aerodynamic diameter of single suspended particles in the respirable size range. The count median aerodynamic diameter, the mass per dose were calculated. Significant increases were noted in the count median aerodynamic diameter, the total particles per dose, and the total aerodynamic mass median aerodynamic diameter for three aerosols and in the mass median aerodynamic diameter for two aerosols. The number of particles in the measured size range increased 3.6- and 4.1-fold for the droplet aerosols and 1.4-fold for the powder preparations. The aerodynamic mass per dose in the measured size range increased 5.7- and 11.4-fold for the droplet aerosols and 3.1- and 1.6-fold for the powder aerosols. These data indicate that all aerosols tested increased in size at high humidity and that aerosols dispensed as droplets may be more unstable than those dispensed as powders.