Chile's capital, a metropolis of 7.2 million people that has long suffered from dangerous levels of air pollution during the winter months, faces a future of thickening smog amid declining rainfall and an inexorable growth in the number of vehicles on its streets.

By 2020, according to projections, Santiago will have more than 7 million vehicles.

The capital, ringed by mountains, is affected during the Southern Hemisphere fall and winter by a phenomenon known as thermal inversion, in which hot air is concentrated in the upper layers of the atmosphere.

That, combined with the almost complete absence of wind, impedes the dispersion of particulates, and a reduction in precipitation only amplifies the effect.

Measures to alleviate the smog, such as restrictions on driving and industrial activity, are a regular feature of life in Santiago.

So far this year, district governor Karla Rubilar has declared a a score of pollution alerts and imposed a blanket prohibition on burning wood for heating, though smoke can be seen pouring out of dozens of chimneys.

"Santiago is a very polluted city. You can see that there's a lot of smoke and that harms us in our lungs," cyclist Joaquin de Tezanos told EFE atop San Cristobal Mountain, whose peak juts above the smog.

In the winter, the city's hospital emergency rooms are typically filled with children and elderly people suffering from pollution-related respiratory problems.