Take a deep breath, then thank the subway  

By Joanne Wong•28 Feb, 2018

Here’s yet another reason for you to take the subway to work instead of drive: cleaner air around the station.

Air pollution has long been the bane of urban centers around the world, particularly cities that are experiencing rapid economic growth. In China, where industrial production and traffic congestion have contributed to persistent smog issues, the government has been putting in place drastic measures to deal with the problem, from temporarily shutting down (http://n.pr/2ClJw0J) 40 percent of its factories to re-assigning 60,000 soldiers to plant new forests (http://bit.ly/2C4XX9y) covering a landmass the size of Ireland.

 Another tactic that Chinese cities may want to implement? Build more subways. A recent study by researchers from a group of Chinese universities and MIT suggests that a new subway line can reduce air pollution (http://bit.ly/2Fc9u85) -- carbon monoxide (CO), specifically -- in areas around subway stations by nearly 20%.

The team of researchers took advantage of a natural experiment in Changsha, a metropolitan area of three million people in south central China, when the city opened its first subway line in 2014. Changsha has been ranked among the most polluted cities in the country, and is home to 1.3 million privately-owned vehicles. The researchers pulled data from the network of air pollutant monitors set up by the National Environmental Protection Agency in the city for one year before and after the opening of the subway, and used a difference-in-difference method that compares the dataset to another spatially comparable one on air quality. This method helps to account for extraneous factors that may affect both the control and treatment areas during the study period.

The study found an estimated 18% reduction in CO in areas near subway stations compared to the control group. As expected, the reduction in air pollution is the greatest during rush hour on workdays, and insignificant on days with adverse weather conditions like strong winds, heavy rains, or extreme heat, when commuters prefer driving to public transit.

Notably, there was no significant effect on other air pollutants that are not related to traffic, such as particulate matter or ozone, further validating the claim that the alleviation effect of the subway line is due to traffic reduction. The analysis also passed robustness checks that account for other factors of air pollution in the area like the construction of new shopping centers and seasonal travel patterns.

While the study acknowledges that the construction of a subway system may spur economic growth that increases traffic volume, it ultimately shows a net improvement in air pollution at least in the medium-term. Given the serious consequences of air pollution on public welfare, notes the researchers, “Despite the high cost of construction and operation, the investment in public transit could be profitable if we consider the social benefits in terms of congestion mitigation and air quality improvement.”

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