6/13/2016 The New York Times
MEXICO CITY — Back in the late 1980s, the air got so dirty in Mexico City that birds would emit a final chirp before they tumbled from the trees onto the sidewalk, their small, still bodies a sad testament to some of the world’s worst air pollution.
I heard this story a lot when I first arrived in the city a few years later. Nobody had actually seen it happen, of course, but the urban legend’s black humor revealed a perverse pride in survival.
Then the smog began to lift. The government shut down a big oil refinery and pushed heavy industry out of the city. Regulations stripped the lead from gasoline and created incentives for people to buy cleaner new cars.
And the birdsong resumed.
Until now. The city has registered only 20 “clean” days so far this year. On all the other days, particulates and ozone rose above the government’s limit.