Commentary
Definitions
Data sources
In many towns and cities exposure to air pollution is the main environmental threat to human health. Winter smogs—soot, dust, and sulfur dioxide—have long been associated with temporary spikes in the number of deaths. But long-term exposure to high levels of soot and small particles in the air provokes a wide range of chronic respiratory diseases and exacerbates heart disease and other conditions. The global burden of ill health caused by particulate pollution—on its own or in combination with sulfur dioxide—is very large. According to conservative estimates in the World Bank’s World Development Report 1992: Development and the Environment, it includes at least 500,000 premature deaths a year and 4–5 million new cases of chronic bronchitis.
Summer smogs—from small particles and ground-level ozone produced by the action of the sun on nitrogen oxides and volatile organic compounds—are a major problem in southern California, Mexico City, Tokyo, and parts of Western Europe. Exposure to ozone makes it difficult for people to breathe, causing particular problems for asthmatics. And many plants and trees are susceptible to damage from ozone exposure, which reduces yields or kills them off.
Emissions of sulfur dioxide and nitrogen oxides lead to the deposition of acidic materials as acid rain or acidic compounds over long distances from their sources—often more than 1,000 kilometers. Lakes in Scandinavia, the northeastern United States, and eastern Canada have lost fish as a result. Such deposition changes the chemical balance of soils and can lead to the leaching of trace minerals and nutrients critical to trees and plants.
The links between forest damage and acid deposition are complex. Direct exposure to high levels of sulfur dioxide or acid deposition can cause defoliation and dieback. Extensive damage to forests in Central and Eastern Europe is usually thought to be the result of large emissions of sulfur dioxide from burning poor-quality brown coals and lignites. But the effects of soil acidification vary across species and seem to depend on many factors. The working hypothesis is that acidification increases the vulnerability of trees to such other stresses as drought and insect damage.
Where coal is the primary fuel for power plants, steel mills, industrial boilers, and domestic heating, the result is usually high levels of urban air pollution—especially particulates and sometimes sulfur dioxide—and widespread acid deposition if the sulfur content of the coal is high. Countries such as China, India, Poland, and Turkey fit this pattern today, as many high-income countries once did.
A second pattern is observed when coal is not an important primary fuel or is used by plants with effective dust controls, as in Brazil, Indonesia, Mexico, Thailand, and many high-income countries. Emissions of the worst air pollutants are caused by the combustion of petroleum products—diesel oil, heating oil, and heavy fuel oil. Industrial plants and vehicles—especially those with two-stroke engines—are typically the main offending sources.
• Suspended particulate matter refers to smoke, soot, dust, and liquid droplets from combustion that are in the air. The amount indicates the quality of the air people are breathing and the state of a country’s technology and pollution control.
• Sulfur dioxide is an air pollutant produced when fossil fuels are burned. It contributes to acid rain and can affect human health.
The data in the table are drawn from the United Nations Environment Programme and World Health Organization’s Urban Air Pollution in Megacities of the World and from the World Bank’s Environmental Data Book.