New Delhi, July 20 -- US President Donald Trump has spotlighted how unacceptable polluted air is. Yesterday, as Canadian wildfires affected New York, he held Canada responsible for the poor air quality under which the World Cup final was being played. His reaction to air pollution offers an opportunity to reflect on air quality for three reasons. First, Trump's anger about the situation is justified. By condemning the air quality, he reminds the world not to normalise it. Second, by blaming Canada, he raises a more complex issue: the commons belong to everyone. Canada cannot control climate change and the resulting forest fires, much less the direction of the wind. But what happens when one country suffers because of activities in another? ...