Where are the world's highest cities?

Despite the lack of oxygen and health risks, high-altitude locations are home to at least 140 million people around the world. From Bolivia's El Alto to Lhasa in Tibet, what's urban life like at such dizzying elevations?

At 3,640 metres above sea level, the city of La Paz sits in a canyon resembling something of a bowl within the Bolivian altiplano - a high altitude, windswept plain that dominates the southern and western territory of the country. It's the world's highest administrative capital, yet more of the metropolitan population lives in the even higher city of El Alto at 4,150 metres, on the rim of the canyon.

El Alto was uninhabited at the start of the 20th century, but as land became more expensive in neighbouring La Paz, the city grew: for the last 50 years, new development has spiralled out of control into a chaotic mix of winding streets through which water and sewer services struggle to extend.