Pollution is caused either by man’s release of completely new and often artificial substances into the environment, or by releasing greatly increased amounts of a natural substance, such as oil from oil tankers into the sea．
引起污染有两种途径：一是人们经常把全新的人造物质释放到环境中，或是把大量的天然物质，譬如油轮溢出的石油，释放到海里。