Particle inhalability at low wind speeds

Accurate quantification of the dose delivered by aerosol exposures is essential for estimating the risk of potential adverse health effects. The fraction of airborne particles that can enter the nose or mouth during inhalation is referred to as the inspirable particulate mass fraction. This inhalable fraction is equivalent to delivered dose for particles greater than approximately 25 mu m (aerodynamic particle diameter, d(ae)), which deposit completely and almost exclusively in the extrathoracic airways. Particle inhalability at high wind speeds (1-9 m/s) has been well characterized. However, there is a paucity of data describing the inhalability of particles at low wind speeds (<= 0.3 m/s), which are typical of indoor environments. High-wind-speed criteria poorly describe inhalability at low wind speeds. Based on the aspiration efficiencies of blunt and sharp-edged inlets, a function was developed for oral inhalability, P(I-O), of particles at low wind speeds. This function predicts a slow decline in P(I-O) from 0.95 at d(ae) = 8 mu m, to 0.5 at d(ae) = 74 mu m, and 0.1 at d(ae) = 175 mu m. Data available from the literature for inhalability at relatively low wind speeds during oral breathing are well described by this logistic function (r(2) = 0.69).