Frequently effluents derive from many sources, none of which by themselves would produce the alleged damage. It may be impossible to prove any particular polluter responsible for the poor air or water quality.

• 常常有这样的情况，源自许多渠道的污水，没有一种能独自产生被指称的妨害，不可能证明哪种污染物应对污染的空气及水质负责。