Commuting by subway? What you need to know about air quality

Story highlightsInternationally, more than 120 million people commute by them every day.A study has shown that air pollution underground can differ vastly owing to station design.On the other side of the Himalayas, Shanghai is building its 15th subway line, set to open in 2020, adding 38.5 km and 32 stations to the world's largest subway network.And New Yorkers can finally enjoy their Second Avenue Subway line after waiting for almost 100 years for it to arrive.In Europe alone, commuters in more than 60 cities use rail subways. Internationally, more than 120 million people commute by them every day. We count around 4.8 million riders per day in London, 5.3 million in Paris, 6.8 million in Tokyo, 9.7 million in Moscow and 10 million in Beijing.A Metro train travels between Varsova and Ghatkopar stations in Mumbai.Subways are vital for commuting in crowded cities, something t ………………………………