As the atmosphere becomes increasingly filled with greenhouse gases such as methane and carbon dioxide, the air quality declines and incidences of smog increase. Scientists are now relying more and more on pollution measurements from space to determine which parts of the globe are most affected by air pollution.
While atmospheric pollution in some places, particularly Asia, has gotten increasingly severe in recent years, it has actually been an ongoing crisis since the dawn of industrialization. In 1952, the infamous Great Smog of London killed over 8,000 people.