Measurements of particle number concentrations in different vehicles (tram, Mass Transit Railway, and air-conditioned and non air-conditioned franchised buses) were carried out from July 1 to November 1, 1994, in Kong Kong. A total of 320 samples in 14 days were collected for four count median diameters (0.3, 0.5, 5.0, and 10.0 mu m). The total particle number concentrations of airborne particulate matter exposed to commuters varied from 3.61E6 to 7.68E7 ft(-3) for particle diameter 0.3 mu m or greater. The concentrations were much higher, from 4 to 16 times, than those (8.02E5 to 4.60E6 ft(-3)) in rural areas (at the seaside near Hong Kong University of Science Technology), but the concentrations in non air-conditioned transportation (trams and franchised buses) were closer to those in the streets. It was found that the number concentrations of fine particles (0.3 to 5 mm) were relatively constant, and the fine particles could not be filtered by the air-conditioners used.