Each episode of illness caused by or made worse by air has direct and indirect economic consequences for individuals and society. They range from the costs of medical care through costs due to absence from work, to the loss of healthy years of life. For more than a century, severe air pollution incidents in cities such as London have shown that breathing dirty air can be dangerous and, at times, deadly. In 1880, 2,200 Londoners died in one such incident when coal smoke from home heating and industry combined to form a toxic smog of sulfur dioxide gas and airborne combustion particles. But concern about the health effects of outdoor air pollution did not effectively coalesce until the late 1940s and early 1950s, when air pollution disasters on two continents raised an alarm. Both the 1948 "killer fog" in the small town of Denora, Pennsylvania, that killed 50, and the particularly virulent London "fog" of 1952, in which some 4,000 died, were associated with widespread use of dirty fuels and were catalysts for government efforts to tackle urban air pollution.
Since then, many nations have adopted ambient air quality standards to safeguard the public against the most common and damaging pollutants. These include sulfur dioxide, suspended particulate matter, ground-level ozone, nitrogen dioxide, carbon monoxide, and lead all of which are tied directly or indirectly to the combustion of fossil fuels. Although substantial investments in pollution control in some industrialized countries have lowered the levels of these pollutants in many cities, poor air quality is still a major concern throughout the industrialized world. A recent assessment by the European Environment Agency found that 70 to 80 percent of 105 European cities surveyed exceeded World Health Organization (WHO) air quality standards for at least one pollutant. In the United States, an estimated 80 million people live in areas that do not meet U.S. air quality standards, which are roughly similar to WHO standards (reference).
Meanwhile, urban air pollution has worsened in most large cities in the developing world, a situation driven by population growth, industrialization, and increased vehicle use. Despite pollution control effects, air quality has approached the dangerous levels recorded in London in the 1950s in a number of megacities, such as Beijing, Delhi, Jakarta, and Mexico City. In these cities, pollutant levels sometimes exceed WHO air quality standards by a factor of three or more. In some of China's major cities, particulate levels are as much as six times the WHO guidelines. Worldwide, WHO estimates that as many as 1.4 billion urban residents breathe air exceeding the WHO air guidelines.