Relative magnitudes of the effects of electrostatic image and thermophoretic forces on particles in the respiratory tract

The National Radiological Protection Board's Advisory Group on Non-ionising Radiation has recommended research into the deposition, in the lung, of charged particles in the size range 0.005-1 μ m. In vivo measurements of the temperature distribution in the respiratory tract have been used to estimate the temperature gradients in the generations up to the segmental bronchus. These gradients define the thermophoretic velocities, which oppose deposition during inhalation and assist it during exhalation. The thermophoretic forces are effective over a longer range than those due to the electrostatic image of a single charge; and, at distances greater than a few microns from the airway wall, the thermophoretic velocities of 0.02 and 0.1 μ m particles are greater than those due to electrostatic drift. It is concluded that models describing the effects of electric charge on the deposition of particles with diameters of order 0.1 μ m need to take account of the thermal conditions in the respiratory tract.