The ratios of short-lived radon (222Rn) daughter product activity to long-lived 210Pd in surface air at a few stations in India are compared with similar ratios at a number of other locations. It is noted that where systematic measurements are available, the seasonal changes in the activity ratios are within a factor of 2. Also, all the available average ratios from the various locations fall within the limits 6000¿5000. It is suggested that with properly averaged ratios the aerosol residence time can be calculated correct to possibly a factor of 2 if vertical diffusion is also taken into account. Some advantages of using the ratios of Rn/210Pb compared with other pairs of isotopes for calculating aerosol residence times are pointed out.