Theoretical quantification of the entry of coarse particles to the human mouth or nose under different inhalation rates and ambient wind velocities is an important topic in understanding inhalability, in extrapolating empirical results and designing inhalable aerosol samplers. Using an idealized human head model facing the wind and potential flow approximation for the velocity fields, coarse particle aspiration efficiencies were calculated for a set of inhalation and external fluid motion parameters, The inhalation parameters are orifice size, the frequency and the tidal volume of breathing which depend upon the level of exertion assumed as well as interpersonal variabilities. The external fluid motion parameters are introduced by the fluid motion external to the head and the orientation of the orifice axis with respect to the flow axis. The total velocity field was modeled by superimposing the velocity field generated by breathing onto the external velocity field generated by an ambient wind. In this three-dimensional velocity field around the idealized human head figure, the particle equations of motion for coarse particles result in a set of coupled ordinary differential equations. The numerical solution of this set can be used to obtain particle trajectories and particle aspiration efficiency. The aspiration efficiency calculation for 25-200 mu m particle aerodynamic equivalent diameters, for tidal volumes of 750, 1450, 2250 cm(3) and for 0.5-9 m/s ambient wind speed conform to the main trends observed in the experimental studies. Thus they may be used to extrapolate the experimental findings. Most importantly, the calculations did not suggest an evidence of zero aspiration efficiency (cutoff) for aerodynamic equivalent diameters up to 200 mu m for the range of inhalation and external fluid motion parameters examined.