Changes in the physical environment are frequently difficult to value. Such changes may affect human welfare through modification of factors that affect human health or the utility derived from various activities. Improved environmental quality may create opportunities for additional outdoor recreational activities, or may increase enjoyment of current activities by providing better visibility, reduced noise, cleaner water, or other amenities that affect enjoyment of these activities. In addition, people may simply value the existence of certain environmental conditions for philosophical, religious, or other reasons.