The story is the stuff of urban legend: sometime at the end of the 1980s, the air quality in Mexico City got so bad that birds began falling from the trees. Nobody ever studied their tiny corpses to determine what it was that actually killed them, but it did not matter. Everybody believed that it was the city’s polluted air. A cartoonist in a Mexico City paper drew dead birds as a metric to report how bad the air was. A lot has changed since those murky days. Two decades of evolving anti-pollution measures have succeeded in lowering the emissions of important contaminants, particularly lead, sulfur dioxide and carbon monoxide, to levels that are well within international norms. But the early progress made in... [Log in to read more]