Fires. While ninety percent of the forest fires in the United States are caused by human action, 'natural' forest fires most often occur due to lightening strikes in dry and windy conditions. The greatest health impact of a forest fire is, its effect on the surrounding air quality. Due to opportune local conditions, many smaller fires can contribute to poor air quality of one particular area. At the very best, the build up of smoke and pollutants in the air decreases visibility, and at the very worst it leads to new respiratory problems or exacerbates existing conditions. Burn injuries from forest fires are uncommon but not unheard of. Burn injuries are most likely associated with members of the forest fire fighting crew who are commonly called upon to be in close proximity of large and unpredictable fires. Depending on existing wind conditions, forest fires are able to spread rapidly and change direction quickly which can lead to the entrapment of fire fighters and, less commonly, private citizens. Occasionally, within the context of a large forest fire animal attacks and/or bites also increase. Depending on the proximity of a forest fire to a population center, many wild animals, who have been forced to take flight, come into greater contact with humans. This phenomenon, which is also seen in large floods, can lead not only to trauma from animal attacks, but more commonly increases the risk of zoonotic infections (infection passed from animals to humans). Great fires that rage through urban areas are less common in developed countries due to the advent of a trained and equipped fire department. However, large-scale urban fires are still possible in the developing world, as well as secondary to other natural disasters such as an earthquake. In 1906, a magnitude 8.7 earthquake in the San Francisco area triggered major fires that eventually destroyed over 25,000 buildings and left 250,000 homeless. Injuries in a large-scale urban fire are more commonly burn injuries and deaths associated with asphyxiation. Common complications of burn injuries include hypovolemic shock as well as massive infection and sepsis.