It has been found that about 90% of forest fires are due to human factors. Smokers may start a forest fire by carelessly dropping lighted matches and cigarettes in a forest. Campers and picnickers or tourists may also start a fire by throwing lighted items on the ground or into the bushes. Some times sparks from locomotives powered by coal or wood may also ignite dry grass or trees beside the railway line.
Forest fires may also start spontaneously in hot dry weather. Bush-fires in the summer frequently occur in Australia. Fires lit by shifting cultivators to clear small patches of jungle land for shifting cultivation may get out of control and keep burning, especially when the ground is dry, dead-wood and other flammable vegetation. The 1994 forest fire in Sumatra and Kalimantan burnt the forests of Indonesia for days and weeks, causing acute haze in the neighbouring countries.