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Centre for Atmospheric Science

Urban Pollution

Motor vehicles are a major source of pollution in our cities.
Motor vehicles are a major source of pollution in our cities.

In the western world and increasingly in the developing world, many people live or work in cities, so the quality of the air we breathe in our cities is of great importance to our health. There is significant evidence of damage to human health being caused by air pollution, not just by visible pollution seen in cities in the developing world, or in the UK before the introduction of the Clean Air Act in 1956, but also caused by lower concentrations found in cities in the developed world. While the infamous London smog of December 1952 was responsible for 4000 additional deaths in just a four days, studies have also shown that exposure to lower concentrations of air pollution also contributes to respiratory and cardiovascular disease.

Since the smogs of the 1950’s much has been done to improve air quality in UK cities, with legislation preventing many types of emissions which were responsible for these smogs. Additionally, increasing use of electricity and natural gas rather than solid fuels for domestic heating has also drastically reduced quantities of smoke. Thus today there is very little visible pollution in our cities. There are also strict limits imposed in legislation on levels of invisible pollution such as oxides of nitrogen, sulphur dioxide, ozone and particulate matter. These pollutants are continuously monitored at representative sites in towns and cities across the country. Most urban pollution today comes from vehicles – engine exhaust and mechanical wear. Some also comes from domestic and industrial sources such as heating systems and industrial processes. Asthma and other respiratory and cardiovascular diseases are generally increasing, with studies showing that air pollution contributes to, or aggravates existing conditions. Thus an understanding of the type of pollution emitted and how it is distributed within a city is vital.

Complex terrain in central Manchester.
Complex terrain in central Manchester.

Cities have somewhat complicated terrain consisting of relatively narrow streets surrounded by tall buildings (sometime referred to as street canyons). This terrain significantly alters airflow within a city and hence distribution of pollutants, producing areas where pollutants become trapped, and others where it is efficiently removed. Additionally with most pollution in modern cities being emitted at street level within the canyon rather than at rooftop level as in the past, this also has a significant effect on the distribution of pollution within the city. Further, pollution may become trapped at the city scale by wider geographical and meteorological conditions and be processed photochemically in the atmosphere, to produce photochemical smog with properties rather different than the original pollutants. In these conditions pollutant levels may build up over several days, becoming significantly higher than in moderately ventilated conditions.

Effects of pollution on human health are determined by both the detailed nature of the pollutants and individual exposure to these pollutants. Individual exposure is significantly influenced by day to day activities within the city and may be very different from exposure indicated by measurements made at a monitoring site. For example walking along a busy bus route, or sitting in a traffic jam, one is likely to experience much higher levels of pollution than a monitoring station in a city centre garden, or at rooftop level. The length of time spent in such conditions is also important, so the potential implications for bus and taxi drivers or traffic wardens is likely to be much higher than for office workers.

Erosion to stonework on an old building in central Manchester, the detail on this stone carving is gradually being lost. This is the type of damage which is accelerated by elevated levels of pollution.
Erosion to stonework on an old building in central Manchester.

In addition to its potential effects on human health, pollution in our cities has other impacts on the urban environment. Regardless of hazards to health, breathing heavily polluted air is not pleasant, thus highly polluted conditions can make cities an unpleasant place to be. Pollution is deposited onto all surfaces within the city, including vegetation and buildings. This pollution may cause damage to the surfaces upon which it is deposited, or cause buildings etc to gradually take on a generally dirty appearance. Old buildings are thought to be particularly at risk from damage caused by air pollution because of the nature of the materials from which they are constructed. This is of particular concern to those responsible for caring for our built heritage.