People around the world travel to cities to find jobs to stabilize their lives. At the same time, living in a city curbs the life span of billions of people. 

98% of people living in cities in low- and mid-income countries face extreme levels of air pollution. In high-income nations, that rate is 56%. 

More than half of all people live in cities. By 2050, it's estimated that two-thirds of the global population will be city dwellers. 

High levels of air pollution cause more than 3 million premature deaths per year and lead to numerous health consequences that reduce quality of life. 

Some of the health problems include: higher risk of strokes, heart disease, lung cancer and respiratory diseases.

Children, in particular, are far more likely to develop asthma and other debilitating lung conditions that haunt them for the rest of their lives. 

This is an alarming problem at face value. People shouldn't have to travel through smog to get to work and depend on expensive air purifiers for a clean breath at home. 

But what makes the problem more troubling is how it's expanding. As cities become denser, more energy is consumed and emissions increase, filling the air with more harmful particles. 

It's a growing problem that, according to the World Health Organization, is "wreaking havoc on human health." 

Zabol, Iran, had the worst air quality in the report and India contained more than half of the world's 21 most polluted cities. 

One bright spot is New Delhi, which managed to improve its air quality by 20% between 2013 to 2015 through a series of measures including "banning older cars and cargo trucks from the city, introducing steep fines for construction pollution or garbage burning, and shutting down an old coal-fired power plant."

As Yahoo News notes, "Paris came in at 1116th most polluted, London at 1,389th and the New York-Northern New Jersey-Long Island region at 2,369th."

Cities are undeniably the future of humanity. But for people everywhere to thrive, governments and businesses have to invest in clean energy and support environmental initiatives. 

Fortunately, progress is being made. 

According to WHO, "More than half of the monitored cities in high-income countries and more than one-third in low- and middle-income countries reduced their air pollution levels by more than 5 percent in five years."