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In 2020 as this chapter was written, the scientific evidence is certain: ambient air pollution, that is, contamination of outdoor air consequent to human activities, is a major, global cause of morbidity (ill health) and premature mortality (early death).1 While the rise of ambient or outdoor air pollution is relatively recent in a historical context, air pollution has probably had adverse effects on human health throughout history. The use of fire for heating and cooking came with exposure to smoke outdoors and indoors, an exposure that persists today for the billions who use biomass fuels for cooking and heating. The rise of cities concentrated the emissions of pollutants from dwellings and industry and led to air pollution that received comment and was considered a danger to health. The problem of air pollution received attention centuries ago in London, polluted by widespread coal burning.2 However, regulation was resisted even then.

The Industrial Revolution created new and powerful point sources of air pollution that were unregulated and placed throughout cities and other places where people lived and, with the twentieth century, continued industrialization and also electric power generation brought new point sources of pollution into areas adjacent to where people lived and worked. During the twentieth century, fossil fuel-powered vehicles became a ubiquitous and ever-increasing pollution source in higher income countries and created a new type of pollution—photochemical pollution, or “smog”—first recognized in the Los Angeles air basin in the 1940s.3 The work of Haagen-Smidt identified the critical roles of sunlight, hydrocarbons, and nitrogen dioxides in the photochemical reactions that produced ozone and other oxidant species.4 Now, ozone has become a ubiquitous pollutant globally as vehicle fleets have grown rapidly in cities around the world.5

In recent decades, air pollution has worsened across much of the world, driven by population growth, dense urbanization, and an explosive increase in vehicle numbers. The unprecedented growth of some urban areas into “megacities,” such as Mexico City, São Paulo, London, Los Angeles, and Shanghai, has led to unrelenting and sometimes dangerously high air pollution from massive vehicle fleets and stagnant traffic and from polluting industries and coal-burning power plants. With population growth and urbanization, ever more megacities are anticipated; the current total of cities with a population over 10 million reached 33 in 2018 with a projected rise to 43 by 2030.6

While the problem of air pollution was noted centuries ago, the modern era of research on air pollution and health and evidence-driven air-quality regulation and management began at mid-twentieth century following a series of very high pollution episodes with disastrous health consequences in Europe and the United States.7,8 The most dramatic was the London Fog of 1952, which caused thousands of deaths beyond expected based on the prior weeks and prompted some of the first studies of the health effects of air pollution (Fig. 70-1).9 The ...

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