What is more, a considerable proportion of the discharges has been treated. Lhasa is the political, economic and cultural centre of Tibet, with relatively dense population. The discharges of industrial waste gas, water and solid materials in the city account for 90 per cent of the total of Tibet. Nevertheless, Lhasa is the least polluted of China's cities, even though sometimes there is enormous amount of smoke because of religious activities and traffic noises in the daytime. Data show that in Tibet far fewer sandstones now occur than in the past.
Many noted rivers run along the Tibetan plateau.
Except in the rainy seasons when the torrential rain would wash earth and sand from the mountains, all the rivers are charmingly clear for the rest of the year. The water of Tibet's mother river, the Yarlung Zangbo River, and of its tributaries attains the first grade by state standards and is drinkable without any treatment. Even the Lhasa River, which runs past the capital of the Autonomous Region, has water of very good quality, too, for the colon bacillus averages below 500 per litre daily, the PH value is around 7, the saturation rate of dissolved oxygen is above 90 per cent and the content of cyanide, arsenic, lead and other toxic substances is all but zero. When the traditional ''Bathing Festival'' comes round in the seventh month of the Tibetan calendar, thousands upon thousands of Tibetans would bathe in the limpid ''sacred'' water most joyfully. Fish swim in its vast expanse of clean water while birds fly and skim over the surface of it.