The city’s millions of residents are crammed together, competing for space to live, play, work and rest.

The pope invoked his predecessor, Francis, for whom the environment was a core issue, but stopped short of criticizing world leaders dismissive of climate change.

States across the border issued air advisories about particulate pollution that are expected to last for several days.

He took a dry topic and made it entertaining, capturing the attention of policymakers and influencing the way cities are built.

Scientists are still studying the long-term health effects of smoke exposure.