Canadian Environmental Health Atlas

Understanding Our Environment is Key to Promoting Health and Preventing Disease

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Air Pollution

Air Pollution

Three Deadly Events
From the time human beings began cooking food over smoky fires, people have been exposed to air pollutants. Before the industrial revolution, however, air pollution occurred on a relatively small scale. With the widespread adoption of the internal combustion engine after 1850, the scale of air pollution changed. Industrial facilities and vehicles of all kinds began burning large quantities of coal, petroleum, and other fossil fuels. More smokestacks and more vehicle exhaust pipes emitted vaster quantities of a complex mix of vaporous, liquid, and solid particles. Air quality worsened over ever-larger areas. People became ill and infants, children, and adults died.

Despite some attempts in the early 20th century to control industrial emissions and improve ambient air quality in urban areas, little was accomplished until three widely publicized events.1 The first occurred in Belgium in 1930 when more than 60 people died in the Meuse Valley after pollutants from nearby steel mills, foundries, and smelters made the air toxic. The next event occurred in 1948, when 17 people died in one day in Donora, Pennsylvania, after emissions from a zinc refinery, metal works, and various coal-fired industrial operations filled the valley where the small town was located.

The most significant of these events was the London Fog of 1952 (Figure 1). For several days the city was brought to a standstill by a thick and “evil-smelling” blanket of polluted air. At least 4,000 people died because of the “smoke-like pollution” that the UK’s national weather service described decades later as toxic enough “to have choked cows to death in the fields”.2

Alarmed by these intense episodes of elevated pollution levels, public health officials and researchers turned their attention to the dangers of dirty air. The London Fog in particular led to important developments. As well as prompting Britain’s parliament to pass the Clean Air Act of 1956, the event became a catalyst for air pollution epidemiology.

Studying the Connection between Emissions and Health
In response to concerns about the effects of air pollution on human health, investigators began looking for mechanisms linking poor ambient air quality with high rates of chronic illness and death. After the dramatic events in Belgium, the US, and Britain, no one could deny air pollution was a threat to human health, but there was no agreement on the specifics of the connection between pollutants and morbidity and mortality. Many studies found connections, but were criticized for not taking the effects of other health risks, such as smoking, into account. To address this criticism, Dockery and his colleagues (1993) conducted the Harvard Six Cities Study, which controlled for individual risk factors while estimating the effects of air pollution on mortality.3

Dockery and his fellow researchers selected adults living in six eastern US cities and collected data about pollutants in each city from centrally located air-monitoring stations. Using the pollution data and death data, they evaluated the effects of air quality on mortality rates. They found a strong association between air pollution and death from lung cancer and cardiopulmonary disease, especially with fine particle pollution. The results of this landmark study led the Environmental Protection Agency (EPA) to revise its air quality standards.

  • 1. Bell ML, Davis DL. Reassessment of the lethal London fog of 1952: novel indicators of acute and chronic consequences of acute exposure to air pollution. Environ Health Perspect 2001;109(suppl3):389-394.
  • 2. Met Office Education website. The great smog of 1952. Accessed 28 February 2013. www.metoffice.gov.uk/education/teens/case-studies/great-smog.
  • 3. Dockery DW, Pope CA, Xu X, et al. An association between air pollution and mortality in six U.S. cities. N Engl J Med 1993;329:1753-1759.