Fine particles of pollution lodge deep in the lungs, where they cause serious health problems. Scientists are working to tally the global consequences.
Simulations of air pollution from the GOCART model help show how much skies have cleared over Europe and North America even as they have darkened over Asia in the last decade.
Thick haze hung over northern India and Bangladesh in January 2013. Over the past decade, large cities in India have seen some of the world's fastest-rising levels of air pollution.