Commuting by subway? What you need to know about air quality
http://edition.cnn.com/2017/09/25/world/commuting-by-subway-one-square-meter/index.html
There are many factors affecting air quality.
For example, in China's northern cities, air pollution by vehicles and factories would be the main reason for deteriorating air quality.
But, in cities where air is kept clean, ventilation of each transportation would be the main one for it.
As a result, transportations in clean cities should think about how to ventilate more efficiently, while in air-polluted cities, they should think about how to reduce exhaust gas.
