Almost half of the world’s population presently lives in urban areas (UNPD 2004). Energy is needed in cities for lighting, transportation, industrial processes, and various household uses (such as cooking, heating, cooling and ventilation). Along with
urbanization comes an increase in the concentration of economic activities and, since these activities rely heavily on the
combustion of fossil fuels, associated increase in outdoor air pollution and its negative impacts (Figure 5). In many regions
of the world, outdoor air pollution problems are aggravated by rapid economic development and industrialization, and a lag in adopting pollution control strategies. Of the 800 000 premature deaths attributed to urban air pollution every year,
about 65 per cent occur in the developing countries of Asia (Cohen and others 2004) (Box 2). Air pollution from transport and
industry contributes to an increased risk of death from cardiopulmonary causes; increased risk of respiratory symptoms;
an increased incidence of lung cancer in people with long-term exposure; and adverse outcomes in pregnancy, such as premature birth and low birth weight (Krzyzanowski and others 2005). As with indoor air pollution, the type of pollutant, intensity of exposure and the age and health of the individual exposed determine the severity of the impacts.
A growing body of evidence indicates that
small particulates are associated with an increased mortality risk. Accordingly, attention has focused on exhaust from diesel engines, which contains finer particulates than gasoline. Moreover, these particulates contain polyaromatic hydrocarbons (PAHs), which are potent carcinogens and mutagens. Although the impact increases with the exposure level, WHO guidelines have been revised to reflect that there is no safe level of particulates – they have negative health impacts on humans no matter how low the concentration in the atmosphere (WHO 2000). Trends towards higher levels of urban air pollution around the world have been
addressed in the more developed economies by more efficient technology and pollution control policies.
The level of development also influences the sources of pollution, the ambient levels and the ‘pollution mix’ in a particular urban area. For example, in higher- income cities, air pollution is dominated by finer particles and photochemical smog – mainly from the transport sector. In lower income cities particulate matter, sulphur dioxide from coal and other fossil fuel burning, and suspended dust from disturbed land, unpaved roads and construction are more common (Molina and others 2004).
The level of development also determinesthe transport mix in a city. Poorer countries have fewer cars, but more two- and threewheeled vehicles with dirtier two-stroke engines, and older fleets of vehicles with inefficient or badly maintained engines, and no or poorly functioning emission control devices. In richer countries, vehicle technologies are more efficient and less polluting, but the number of vehicles is higher. Urban planning – or lack thereof – can determine how well a city manages rising energy demands from transport and industry. Some cities are more services-oriented, while others may have a concentration of industries nearby, contributing to air pollution. Historic and social-cultural reasons often explain the differences – and often also determine factors such as the early creation of an underground transport system, or bicycle lanes to encourage more sustainable forms of transport.
Investments in reliable public transport can help rein in urban pollution levels. Geographical and demographic factors have an important role to play. For instance, the location of a city is an important factor (enclosed basin versus open plain; sea level versus high altitude where the lower level of oxygen affects combustion; latitude with corresponding differences in radiation levels for photochemical pollution). The size and shape of a city; meteorological and climate conditions (for instance, temperature, wind speed and the existence of thermal inversions); number of inhabitants per square kilometre (hence emission density); and the seasonal distribution of the emissions are also important. For instance, in China, cities often suffer from lack of sufficient wind for the quick dispersion of air pollution. Mexico City, Athens, Los Angeles and Kathmandu are disadvantaged because of local topographical conditions.
Economic policies, particularly those relating to energy, can have a significant impact on the level of outdoor air pollution. Subsidizing the price of diesel fuel relative to gasoline stimulates the use of more diesel vehicles, resulting in a rise in particulate levels. The relative price of cleaner options like natural gas influence choices made by industries (Box 3).
Economic policies that
force users to internalize the costs of air pollution can often provide important incentives for good practice. In the case of industrial pollution, end-ofpipe technologies like scrubbers and filters have helped reduce emissions of certain pollutants. However, the only long-term solution is to attack the problem at its root by improving energy and materials efficiency and cycles and switching to sustainable energy sources.