The mass percentage of different diameters of urban dust grains is as follows: there are no dust grains larger than 20 M; dust grains 20~30 M account for 13.54%; 30~50 M 23.00%; 50~80 M 14.04%; 80~100 M 10.40%; 100~120 M 5.52%; 120~140 M 3.02%; 140~160 M 7.90%; 160~180 M 1.74%; 180~200 M 7.52%; smaller than 200 M 11.98%. Generally the diameters of dust grains in Xi' an area are smaller than one millimeter, and hence they tend to form a repeated cycle of rising and sinking under the action of ex- ternal forces, and develop into particles that are likely to be breathed by human bodies, thus causing great harm to human health and environment.