Since the beginning of the industrial revolution, people have increasingly congregated in urban areas so that as of 2005, more than half of us lived in cities [Cohen, 2006], There are about 28 megacities, defined by populations greater than 10 million. Most projected future growth in the world’s population will occur in urban areas [United Nations, 2014].
Air pollution often plagues industrialized cities, particularly during their early development. Episodes of high levels of sulfurous smog killed or sickened thousands in Donora, Penn., in 1948, as well as in London in 1952 [Bell and Davis, 2001; Helfand et al., 2001]. Other cities—primarily in the industrialized regions of the United States and Europe—also suffered from notoriously bad air quality. These events were the result of very high emissions of sulfur dioxide, smoke, and other particles during stagnant, foggy weather conditions.
As governments controlled more traditional pollution sources and urban vehicle fleets grew, a different type of air pollution also arose. Photochemical air pollution—a new phenomenon, distinct from sulfurous smog—clouded the skies over Los Angeles, first recognized in the 1950s. This type of air pollution results from photochemical reactions involving nitrogen oxides (NOx) and volatile organic compounds (VOCs; e.g., ethylene and benzene) that produce ozone (O3) and particulate matter. Both cause lung problems, among other deleterious effects, and particulate matter reduces visibility.
In North America and Europe, the coupling of industrialization and air pollution required the creation of air quality standards and regulations for emission sources such as vehicles, electrical power generation, and industrial facilities. The success of these efforts has caused the most severe air pollution episodes to be distant memories in those regions. However, as industrialization spread, air quality concerns also spread to other areas of the globe. For example, recent news headlines warn of extreme air pollution episodes in many Chinese cities, including Beijing, Shanghai, and Guangzhou (Figure 1), showing that urban air pollution remains a major world health issue [Chen et al., 2013; Grahame et al., 2014].
Cleaning up the world’s air is a daunting task. However, a broad review of about 6 decades of efforts in Los Angeles, including how scientists overcame societal and technical challenges, demonstrates that air quality in megacities can, in fact, be greatly improved. Several questions remain: Looking forward, what new challenges will megacities now developing throughout the world face? Are there limits to further improvement of air quality in more developed countries? Looking back, has the improved air quality in our cities been worth the large expense required?
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