20030638 Du Peixuan (College of Geosciences and Territory Resources, Chang’an University, Xi’an, Shaanxi); Tian Hui The Basic Types of Time Bomb of Calamity Chemistry in Urban Environment (Geology of Shaanxi, ISSN1001-6996, CN61-1150/P, 20(1), 2002, p. 60-65, 2 tables, 11 refs. , with English abstract)key words: environmental degradation, classification