The right to drive or the right to breathe?

Politicians have taken some steps to cut deaths from dirty air, but more are needed.

In 1554, a band of wandering Jesuits, after sweating through southern Brazil’s forested coastal hills, stopped by a river on the high Piratininga plateau and, delighted at its fresh, cool air, founded the city of São Paulo. Were they to return now, for much of the year they would find a grey-brown smog shrouding a metropolis of 18m people and 6m vehicles. The foul air kills thousands of people a year and inflicts chronic illness on countless others. Mexico city has long been notorious for its polluted air. Fuel burns less efficiently at high altitudes, and thermal inversions mean that the surrounding mountains trap a layer of cold air above the city, preventing the dispersal of fumes.

But the surge in car ownership throughout Latin America since the 1970s means that São Paulo and other Latin American capitals are no longer far behind. Though at lower altitudes, both São Paulo and Santiago suffer from thermal inversions, too.

The pollution in São Paulo has worsened because of: