Air pollution

Problems from air pollution have existed ever since the human race started to use fire. Anyone who has lit a bonfire or who has a coal fire at home will be well aware of the amount of smoke that is generated when they are first alight. Multiply the individual fires by the number of homes using them and it's easy to see why our towns and cities were such unhealthy places in the past.

The first documented complaints about air pollution can be traced back to 1257 when the wife of Henry III, Queen Eleanor, refused to stay in Nottingham Castle because of the choking air sent up to the royal chambers from the coal fires in the surrounding houses below the castle.

Hundreds of years passed before Parliament decided to act to improve air quality. Before then, the smoke and muck emanating from factories were signs of industrial progress. 'Where there's muck there's money' was an expression of the time. However, although a committee was appointed by Parliament in 1819 to investigate whether engines and furnaces could be operated without causing harm to health and comfort, no action was taken on its findings. Much later, the Public Health Act 1936 allowed local authorities to carry out inspections to detect emissions which were regarded as harmful to health, but again this measure was ineffective because the imminence of war required the maximum production of goods.

The milestone in air pollution prevention in the UK arose from the terrible smog which lasted for five days in London in December 1952. It occurred because the weather was particularly cold and still over the city which is situated in a river valley. A 'temperature inversion' took place which resulted in the warm air and fumes over the city being trapped beneath cold and denser air above it. The combination of dust, smoke and moisture resulted in a thick smog (the name was derived from a combination of 'smoke' and 'fog') which blanketed the area. Conditions were so bad that ducks from the parks crash-landed in the streets, buses were preceded along the streets by men with torches, and performances in the Sadlers Wells theatre were cancelled because the audience couldn't see the stage. These problems were trivial though when compared with the impact of the pollution on human health. Thousands of people suffered from breathing difficulties and the hospitals were full of casualties from the bad air. By the end of the event, it was estimated that 4,000 people had died as a result of the smog.

This episode spurred government action and a committee was appointed to investigate the causes and recommend action to prevent its repetition. The committee found that the major pollutants were particu-lates and gases, and proposed that new laws should be established to reduce the emissions of smoke grit and dust. An important part of the legislation was that it was be applied to domestic fires. The Clean Air Act of 1956 introduced controls over the height of chimneys and a new concept of 'smoke control areas'. In these areas, set up in towns, households were restricted to burning only fuel that did not emit smoke when lit, and people were given grants to help them convert their fireplaces to burn the 'smokeless' fuel.

The impact of this legislation was profound. For example, in Sheffield, an industrial city, the annual average amount of smoke and sulphur dioxide emitted today is only about one-tenth of the level in the early 1960s. Other changes have taken place over the past 40 years. There has been a large shift from coal to natural gas as a domestic fuel, and most houses have central heating or are warmed by electricity. Most of the electricity is now generated in large power stations situated well outside built-up areas and these are fitted with high stacks to allow maximum dispersion of the emissions. The modern power stations burn fuel more efficiently with the result that the amount of SO2 emitted into the atmosphere has decreased. In 1979, an estimated 6.3 million tonnes entered the air from the UK's emissions but by 1990 this had fallen to 3.7 million tonnes. This reduction has led to an improvement in the quality of rainwater so that it is not as acidic as it used to be (as described in Chapter 8).1

Despite these overall improvements in smoke emissions, there is still public concern about urban air quality because of the increase in road traffic. Figure 29 shows the increase in the number of vehicles on Britain's roads, with the greatest use being on roads in built-up areas. For example, there are more households in the Greater London area that have two or three cars than anywhere else in the UK, but this is the most built-up

Figure 29. Increase in the number of vehicles in the UK, 1970-94 Source: The Digest of Environmental Statistics No 19. 1997. The Environment in Your Pocket. Crown Copyright is reproduced with the permission of the Controller of Her Majesty's

Stationery Office

Figure 29. Increase in the number of vehicles in the UK, 1970-94 Source: The Digest of Environmental Statistics No 19. 1997. The Environment in Your Pocket. Crown Copyright is reproduced with the permission of the Controller of Her Majesty's

Stationery Office area in the country. So these cars spend most of their time in the city streets, often proceeding at little more than running speed! By 1990 there were over 25 million vehicles on Britain's roads - a 28 per cent increase from 1980; 20 million of these vehicles are private cars, and their numbers are still growing rapidly (Cover Illustration 11).

The emissions from vehicles contain a complex mixture of pollutants and these vary according to the type of fuel. There has been a significant increase in the proportion of cars that have diesel engines. Although these are 'cleaner' because they have a better fuel consumption (more kilometres to the litre) and don't use petrol with lead in it, they emit more particles into the air. You are more likely to see black smoke coming from a diesel vehicle than from a petrol engine one. Between 1980 and 1990, there was a 75 per cent increase in the amount of black smoke emitted from road traffic whilst other pollutants, such as carbon monoxide, oxides of nitrogen and carbon dioxide, also increased substantially.

The main air pollutants in urban areas are: carbon monoxide (CO), oxides of nitrogen (NOx), hydrocarbons (HCs), sulphur dioxide (SO2) and particulate matter - especially small particles which are sometimes called PM10s because they are smaller than 10 pm. On sunny days, especially in summer, these gases interact with one another, particularly the nitrogen oxides and the hydrocarbons, to form ozone. This gas is harmful to human health and damages plants. High levels of ozone can cause breathing difficulties and are alleged to set off asthmatic attacks in vulnerable people. In the past ten years there has been a fourfold increase in the number of people suffering from asthmatic attacks in our cities. The Department of Health also estimates that pollutants in the air account for up to 20,000 hospital admissions and several thousand premature deaths each year.

Ozone is a very chemically active gas and reacts with other pollutants, such as the hydrocarbons, to form many so-called secondary air pollutants. One group of these secondary pollutants are known as the peroxy acetyl nitrates (PANs) which are particularly harmful to plants and humans.

The chemical reactions that take place in urban air on sunny days are complex and are still being studied. Some of the chemicals formed in the ultra-violet light from the sun are called free radicals and they are very reactive. PAN is formed by the reaction of the hydroxide radical on acetaldehyde which is present in vehicle exhausts:

OH + CH3CHO ^ H2O + CH3CO (acetyl radical) CH3CO + O2 ^ CH3COO2 (peroxy acetyl radical) CH3COO2 + NO2 ^ CH3COO2NO2 (peroxy acetyl nitrate (PAN))

In August 1996, the UK Government decided to take action against air pollution, particularly that caused by motor traffic. It established targets for air quality that have to be achieved by the year 2005. These are shown in Table 20.

The quality of the air can be assessed in many ways. Sometimes, there are warnings about poor air quality on the weather forecasts on radio and TV. These warnings usually relate to the formation of ozone in built-up areas on calm sunny days. In Chapter 12, there are descriptions of simple methods of measuring air quality using filters or by measuring the yeasts

Table 20. Air quality targets for 2005


Target concentration

Benzene 1,3-Butadiene Carbon monoxide Lead

Nitrogen dioxide Ozone

PM 10 particulates Sulphur dioxide

5 parts per million 1 part per million 10 parts per million 0.5 micrograms per cubic metre 104.6 parts per billion 50 parts per billion 50 micrograms per cubic metre 100 parts per billion that live on leaf surfaces. These yeasts are sensitive to air quality and are killed off by pollutants. By carrying out a survey of the yeasts living on the leaves of trees in your local neighbourhood you should be able to find the most polluted areas and track down where the pollution is coming from.

There have been differing trends in air quality in our towns, cities and the countryside: some pollutants have increased whilst others have declined. For example, the amount of black smoke emitted from houses in urban areas is much less than 40 years ago because fewer people heat their homes with coal fires, and, for those who do, there are restrictions on the type of coal that can be burned. However, as mentioned earlier, the amount of black smoke from vehicles has increased greatly.

The amount of lead in the atmosphere has declined in recent years because many more vehicles use lead-free petrol and diesel for fuel, mainly because it's cheaper. This is a very good example of how the pricing of goods can influence people's buying habits with a benefit to the environment.

It was realized a few years ago that the lead in the atmosphere was increasing as a result of the rising numbers of vehicles on the roads. At that time, in order to make engines burn petrol more efficiently, a substance called tetraethyl lead (TEL) was added to petrol. TEL was an 'anti-knock' agent; in other words, it stopped the petrol igniting too early in the compression stage of the engine cycle. The lead from all the emissions from vehicles stayed in the atmosphere for a long time and spread slowly around the globe. It even contaminated the snow falling on Greenland: scientists who collected samples of snow from different layers in the glaciers there were able to show the increase in the amount of lead as a result of human activities.2 Figure 30 shows the concentration of lead

Figure 30. Increase in the concentration of lead in Greenland snow,

Reprinted from Geochim. Cosmochim. Acta 33, 1969 with permission from Elsevier Science

Figure 30. Increase in the concentration of lead in Greenland snow,

Reprinted from Geochim. Cosmochim. Acta 33, 1969 with permission from Elsevier Science at different depths (i.e. in earlier and later layers). Once the ages of the layers have been determined, the increases in lead concentration can be attributed firstly to the Industrial Revolution, when lead was smelted for a variety of uses such as lead pipes, and later to its use in the internal combustion engine.

This investigation was carried out in the late 1960s. Since then there has been a marked change in our use of leaded petrol. In the USA, it was banned, whilst in the UK, the price of leaded petrol was made more expensive than unleaded to persuade motorists to switch to the more ' environmentally friendly' fuel. Similar measures have been taken in other countries so that most cars now are using unleaded petrol. In the UK, the number of vehicles using unleaded petrol has increased greatly since it was first introduced in 1987, and this has resulted in a reduction in the amount of lead emitted (see Table 21). The effect of these changes has been remarkable. The air in our cities is less polluted by lead compounds, as shown in Figure 31 for two Scottish towns.

It is not only the local air that has benefited from these measures.

Table 21. Lead emitted by petrol engines in the United Kingdom, 1980-95

Year Lead emitted (thousands tonnes)

1980 7.5

1985 6.5

1990 2.2

1993 1.5

1994 1.3

1995 1.1

1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

Motherwell Glasgow

Figure 31. Concentration of lead in air in Motherwell and Glasgow, 1984-94 Source: State of the Environment Report 1996, Scottish Environment Protection Agency, Stirling, 1996

1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

Motherwell Glasgow

Figure 31. Concentration of lead in air in Motherwell and Glasgow, 1984-94 Source: State of the Environment Report 1996, Scottish Environment Protection Agency, Stirling, 1996

Further investigations have been carried out on the concentration of lead in the snow that has accumulated in Greenland since the 1960s.3 As Figure 32 shows, the levels have fallen steadily and are now similar to what they were at the turn of the twentieth century!

This is a very good example of how improvements to the environment can be brought about. There was a great deal of concern about the air being polluted by lead from vehicle exhausts: it was even claimed that the intelligence of children living adjacent to busy roads was being adversely affected by the metal. Governments took action, either by banning the use of TEL in petrol or by making leaded petrol more expensive. Although these measures have reduced the lead levels in urban air, there is still concern over all the other pollutants that are emitted by

Figure 32. Concentration of lead in snow in Greenland, 1965-90

Reprinted by permission from Nature 353, 1991. Copyright Macmillan Magazines Ltd

Figure 32. Concentration of lead in snow in Greenland, 1965-90

Reprinted by permission from Nature 353, 1991. Copyright Macmillan Magazines Ltd vehicles. Now, the Labour Government is hoping to improve air quality in our cities by trying to persuade us not to use cars so frequently, especially for going into towns and cities for work and shopping. The hope is that if car parking spaces are reduced and made more expensive, and the price of petrol is increased, people will find that it is cheaper and more convenient to use public transport rather than private vehicles.

Some cities in Europe and the USA have already achieved this. Trams have reappeared in Manchester, Sheffield and Paris, whilst trolley buses have been introduced in Los Angeles, Buffalo and Portland. Many cities in Holland and Germany retained their tramway systems but have recently modernized them. In Athens, a 2.5 km2 area of the city centre has been made a car-free area: streets have been made into pedestrian areas, traffic re-routed and 'no-fare' buses started.

In California, a more radical method of reducing emissions is being tried out. The state has passed a law on zero emissions vehicles which applies to manufacturers that sell more than 35,000 vehicles in the state each year. By 1998, 2 per cent of the cars sold were supposed to be zero emission vehicles, and this is set to rise to 5 per cent by 2001 and 10 per cent by 2003. This has spurred car manufacturers to produce more battery-powered vehicles and to improve the technology so that they are speedy and have sufficient battery storage to last a whole day.

The concern about air quality relates to a number of chemicals and these were listed on page 90. One of these, ozone, is a 'Jekyll and Hyde' gas. If present in the air we breathe it is a pollutant, but if it is present in the upper atmosphere it protects us from harmful ultra-violet (UV) radiation.

In the lower atmosphere - the troposphere - ozone can be damaging to human health, farm crops, forests and wild plants. It is usually formed on calm summer days because of the presence of sunlight and polluting gases, such as oxides of nitrogen and sulphur. The concentration of tropo-spheric ozone in industrial areas doubled in the period 1940 to 1970 in line with the increase in the number of vehicles but, because of chemical reactions which take place in the atmosphere, concentrations of ozone are usually highest in rural areas. This is because in towns and cities the pollutants from vehicles, such as various hydrocarbons, react with the ozone and reduce its concentration, but in adjacent country air these are absent and so ozone levels increase. From the monitoring results of air quality in different parts of England for 1997, it can be found that the health standards for ozone were exceeded on over 60 days in Lullington Heath in Sussex, on 37 days in Teddington, London, and 27 days at High Muffles in North Yorkshire. On a typical early summer day in 1998, the rural ozone concentration in south Scotland was 45 parts per billion whereas the concentration in Edinburgh was 19 parts per billion.

The combination of all the different urban pollutants can produce the problem of so-called 'photochemical smogs'. These are formed in cities because of the high numbers of vehicles emitting gases which interact to form a complex mixture. There have been a number of serious incidences of photochemical smogs in cities in the USA, particularly Los Angeles because of its sunshine and large numbers of vehicles. At one time these types of smogs were considered to be a problem only in the USA but in recent years levels which are hazardous to human health were recorded in a number of cities in Europe. There was a serious episode in Athens in August 1984 when an acid smog resulted in more than 500 people being taken to hospital with breathing difficulties. More recently, the summers of 1995 and 1996 resulted in high concentrations of ground-level ozone. According to a report from the European Topic Centre on Air Quality,4 the threshold level for protection of human health (110 pg per cubic metre as an eight-hour average) was exceeded in all EU member states in 1995 for 1-2 consecutive days. The longest episode lasted eight days. It also reported that 78 per cent of the EU urban population was exposed to levels of ozone above the threshold level for at least one day of that year and that 9 per cent suffered from excessive levels for 50 days.

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