You can use contributor to refer to one of the causes of an event or situation, especially if that event or situation is an unpleasant one. 造成 (不良后果的) 因素
noun [专属名词]捐赠者,作出贡献者;促成因素;撰稿人,投稿人
Old buses are major contributors to pollution in cities. 旧公交车是造成城市污染的主要原因。