Before about 1970, visibility was meant only to denote the human capability to detect, recognize, and identify objects by means of the human visual mechanism. During the 1970s, as energy development in the form of strip mines and coal-fired power plants with associated emissions spread across the United States, the ability to see and appreciate scenic vistas evolved as an environmental value worth preserving. Today it is recognized that emissions from other sources also can have an adverse effect on visibility. Emissions from wild and prescribed fire at times degrade visibility significantly more than conventional industrial sources.