THEORETICAL INVESTIGATION OF THE EFFECTS OF RELATIVE HUMIDITY ON AEROSOL RESPIRABLE FRACTION

The possibility is investigated that high relative humidities, such as the humidity of the upper respiratory tract, may cause hygroscopic particles to grow enough to invalidate the results of respirable mass determinations at lower humidities. The penetration-versus-particle-size characteristics of cyclone-type respirable samplers are approximated with a cumulative log-normal curve. For hypothetical aerosols having log-normal distributions of aerodynamic diameter, this allows the use of a special property of the log-normal distribution to calculate conveniently the penetration of the cyclones, the aerosol respirable mass fraction. Growth in particle diameter is calculated for several humidities, for NaCl and H2SO4 aerosols, then used to predict respirable mass fractions. The ratio of the penetration of the humidified particles to that of the dry particles is found to be as low as 30% for particles of 1 μm and as low as 10% for 3.5 μm diameter particles. The calculations show that aerosols may produce substantially different measured mass respirable fractions as they approach equilibrium under different humidity conditions. © 1979.