In order to estimate the growth and deposition of hygroscopic aerosol particles during respiration the relative humidity (RH) of the air in the airways must be known. The RH has been calculated with a transport theory for the heat and water vapour in a tube using a numerical method, Cartesian coordinates, and nasal respiration [Ferron et al., J. Aerosol Sci. (1983) 14, 196; Ferron et al., Bull. math. Biol. (1985) 47, 565]. A similar theory is described here to calculate the transport for cylindrical coordinates, both for nasal and oral respiration. The method has the advantage of a reduced computational time compared with the method used before. Calculations are carried out both for nasal and oral inhalation and exhalation. The values of several parameters of the theory, the thickness of the boundary layer, the additional diffusivity by airflow instabilities, the profiles for the temperature and RH at the airway wall, are chosen to fit experimental data on the mean air temperature and RH in the human airways. Since the experimental data are scattered, maximum and minimum curves for the experimental data are derived. It was found mathematically that the RH of the air is more strongly dependent on the RH at the airway wall and less on its temperature. However due to the large uncertainties of the experimental data on the RH no firm conclusion can be drawn.