It has also become a problem in heavily-trekked areas such as the Annapurna Sanctuary and the Upper Khumbu. Water contamination has also become a big problem in Kathmandu Valley due to industrial effluent, chemical pollution, and pesticides used on agricultural land. Air and noise pollution are very localised problems and refer particularly to the Kathmandu Valley. Air pollution in the Valley is mainly caused by brick kilns, cement factories, and the increasing number of vehicles. The three broad categories of environ-mental problems enumerated above owe their genesis to discrete factors.
Steep slopes are both the cause and effect of mass wasting.
Deforestation, whether in the form of depletion or outright clearance, can be attributed to human interference. The overpopulated hills are the sources of outmigrants. Some hill districts in central Nepal exceed 7.7 persons per net cultivated hectare, while the national average is 5.7 persons. This has put increasing pressure on forests for timber, fuelwood, fodder, and land for cultiva-tion. Their density per hect-are of gross cultivated land is 7.6 on grazing land and 2.1 on forest land.