(CNN) -- Four more major Indian cities will soon have their own metro lines, the country's government has announced.
On the other side of the Himalayas, Shanghai is building its 14th 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.
Subways are vital for commuting in crowded cities, something that will become more and more important over time -- according to a United Nations 2014 report, half of the world's population is now urban.
They can also play a part in reducing outdoor air pollution in large metropolises by helping to reduce motor-vehicle use.
Large amounts of breathable particles (particulate matter, or PM) and nitrogen dioxide (NO2), produced in part by industrial emissions and road traffic, are responsible for shortening the lifespans of city dwellers. Public transportation systems such as subways have thus seemed like a solution to reduce air pollution in the urban environment.
But what is the air like that we breathe underground, on the rail platforms and inside trains?
Mixed air quality
Over the past decade, several pioneering studies have monitored subway air quality across a range of cities in Europe, Asia and the Americas. The database is incomplete, but is growing and is already valuable.
For example, comparing air quality on subway, bus, tram and walking journeys from the same origin to the same destination in Barcelona, revealed that subway air had higher levels of air pollution than in trams or walking in the street, but slightly lower than those in buses.
Similar lower values for subway environments compared to other public transport modes have been demonstrated by studies in Hong Kong, Mexico City, Istanbul and Santiago de Chile.
Of wheels and brakes
Such differences have been attributed to different wheel materials and braking mechanisms, as well as to variations in ventilation and air conditioning systems, but may also relate to differences in measurement campaign protocols and choice of sampling sites.
Key factors influencing subway air pollution will include station depth, date of construction, type of ventilation (natural/air conditioning), types of brakes (electromagnetic or conventional brake pads) and wheels (rubber or steel) used on the trains, train frequency and more recently the presence or absence of platform screen-door systems.
In particular, much subway particulate matter is sourced from moving train parts such as wheels and brake pads, as well as from the steel rails and power-supply materials, making the particles dominantly iron-containing.
To date, there is no clear epidemiological indication of abnormal health effects on underground workers and commuters. New York subway workers have been exposed to such air without significant observed impacts on their health, and no increased risk of lung cancer was found among subway train drivers in the Stockholm subway system.