Volatile Organic Compounds (VOCS), as one of the major sources of air contaminations, can not only cause directpollution, but also lead to secondary pollution, such as photochemistry smog.
Well, direct ones are pollution directly from factories, refineries, waste treatment plants etc, aren't they?
额，直接污染源是那些从工厂、精炼厂、污水处理厂排放出来的污染，是吗？
3
Air pollution hasn't gone away, but these PBX RIGS have been replaced just about everywhere by direct-dial phone lines and computerized call-routing systems.