Air Pollution Guide » What is air pollution?

Pollution GuideFind out about the following species:
Key Points:
  • There are seven main pollutants of concern - carbon monoxide (CO), nitrogen dioxide (NO2), ground level ozone (O3), particulate matter (PM10 and PM2.5, sulphur dioxide (SO2), hydrocarbons and lead).
  • Each pollutant has different sources, effects and behaviour.
  • Concentrations of carbon monoxide, nitrogen dioxide and particulate matter will generally be highest close to busy roads.
  • Due to complex atmospheric chemistry, ozone levels will generally be highest in rural areas during the summer months.
  • Sulphur dioxide concentrations are highest in the vicinity of large industrial combustion processes.
  • Hydrocarbons are a group of chemicals, which contribute to the formation of ground level ozone. Benzene and 1,3 butadiene are part of this group and known carcinogens.
  • Concentrations of these hydrocarbons are generally highest close to busy roads and in the vicinity of petrol filling stations.
Air pollution is made up of a mixture of gases and particles that have been released into the atmosphere by man-made processes. Such emissions are typically from the combustion of fossil fuels such as coal, oil, petrol or diesel.
The sources, health effects and chemical behaviour of each separate pollutant are different, making the task of understanding and controlling air pollution as a whole very complex. The following paragraphs explain each of the major pollutants, their sources, health effects and upward or downward trends.
The pollution we are measuring is principally man-made e.g. transport and industry, but natural sources can also cause exceedences e.g. desert sand storms etc. It is primarily the weather that dictates what will happen once pollution is released into the air.
During wet or windy conditions pollution levels remain low, either blown away and dispersed to harmless levels, or removed from the air by rain. During certain conditions pollution levels are able to build up to harmful levels leading to pollution 'epiodes'.
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Carbon Monoxide (CO)
Carbon Monoxide (CO) is a colourless, odourless poisonous gas produced by incomplete, or inefficient, combustion of fuel including 'cold' or badly tuned engines. It is estimated that road transport is responsible for almost 90% of all carbon monoxide emissions in the UK. Badly ventilated domestic fuel appliances (gas, oil or solid fuel) can cause high levels indoors, as can smoking.
The gas affects the transport of oxygen around the body by the blood. At very high levels, this can lead to a significant reduction in the supply of oxygen to the heart, particularly in people suffering from heart disease.
As traffic is a major source of carbon monoxide, ambient concentrations will generally be highest close to busy roads. Monitoring data suggests that annual average CO levels have been decreasing over the last few years. This is probably due to improved vehicle engine efficiency and the introduction of catalytic converters. The effect of technological improvements has been cancelled out to some degree by an increase in traffic levels.
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Carbon Dioxide (CO2)
As opposed to carbon monoxide, carbon dioxide (CO2) is produced by 'complete' fossil fuel combustion. While ambient levels do not have any direct health effects, it is an important 'greenhouse gas' which contributes to global warming. Its major anthropogenic (i.e. man-made) sources are road transport, power stations and other industrial combustion processes and domestic heating. As carbon dioxide emissions lead to global environmental problems, efforts to reduce levels have to be co-ordinated across the world. The UK Government, along with many other countries, has agreed a commitment to progressively reduce emissions in future years.
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Nitrogen Dioxide (NO2)
Nitrogen dioxide ('NO2') is one of a group of gases called nitrogen oxides ('NOx') formed in the combustion of fossil fuels. The majority of nitrogen oxides emitted from a vehicle exhaust are in the form of nitric oxide ('NO'), which is not considered harmful to health. However, this gas can react with other gases present both in the exhaust and the atmosphere, to form nitrogen dioxide. Nitrogen dioxide is harmful to health and is also an important component in the formation of ozone.
Road transport is estimated to be responsible for about 50% of total emissions of nitrogen oxides (NOx), power stations contributing another 25%. The calculation of how much of this NO is converted to NO2 is an important, but extremely difficult factor in the reduction of pollution levels. Because of the domination of traffic sources, mean nitrogen dioxide levels are highest close to busy roads and in large urban areas.
At very high levels, nitrogen dioxide gas irritates and inflames the airways of the lungs. This irritation causes a worsening of symptoms of those with lung or respiratory diseases.
A shift from coal to gas-turbine power stations and the increased use of catalytic converters during the 1990s should have lead to a decrease in nitrogen dioxide levels over recent years. It is possible that increases in traffic volume have cancelled out these improvements. The situation is further complicated by complex reactions with other pollutants. A long-term trend is not clearly identifiable from monitoring data.
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Ground Level Ozone (O3)
While naturally occurring ozone in the upper atmosphere, 'the ozone layer', protects the Earth, ground level ozone ('O3') is harmful to health. Ozone is not directly emitted, but is formed by a complex set of reactions involving nitrogen oxides and hydrocarbons (described below) in the presence of sunlight. In natural, unpolluted conditions, a cyclic balance is reached where sunlight breaks down nitrogen dioxide to form ozone, which then reacts with nitric oxide to reform nitrogen dioxide. At night, when there is no sunlight to drive the first part of the cycle, ozone is destroyed but not replaced.
In polluted conditions where the air contains hydrocarbons from fuel combustion, the balance is upset. The hydrocarbons also react to form nitrogen dioxide, which then breaks down into more ozone. As a result, both nitrogen dioxide and ozone levels increase leading, in severe cases, to summertime 'smog'.
As mentioned above, the majority of nitrogen oxides emitted from a vehicle exhaust are in the form of nitric oxide. As it is nitric oxide that destroys ozone, ozone concentrations are actually lower next to busy roads. For the same reason, ozone annual means are higher in rural locations than in cities.
Like nitrogen dioxide, high levels of ozone can irritate and inflame the lungs. It can also cause eye irritation, migraine and coughing. It is also a strong oxidising agent. This means that it can attack materials such as rubber and pigments and damage vegetation. The international costs of ozone pollution through damage to health, crops and materials are huge.
Once formed, ozone can remain in the atmosphere for many days and is often transported over long distances. It is for this reason that a reduction in ozone levels can only be achieved through European-wide action. Studies have shown that European ozone levels have increased rapidly since 1940. Monitoring data from rural sites in the UK suggest that there was a small annual increase during the 1990s.
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Particulate Matter
Particulate matter in the atmosphere can be from a whole range of sources, both natural, such as sand or sea spray, and man made, such as construction dust or soot. The amount of particulate matter (measured as 'black smoke') in the air in urban areas has decreased rapidly over the last 30 years. This is due to a decrease in coal burning, heavy industry and improved industrial pollution control measures. Attention is now focused on finer particles known as PM10. These fine particles can be breathed more deeply into the lungs and are more likely to have a toxic effect than larger particles. Increasing concern now surrounds even finer particles known as PM2.5.
Measurements of PM10 have only been carried out in this country for the last few years - too short a period to be able to identify any significant trends. PM2.5 monitoring occurs in Swansea and also at the AURN site albeit as a sub-contractor to BV. Again, it is likely that improvements brought about by a decrease in coal burning and improved technology are at least partly offset by increased numbers of vehicles on the road. The increased market share of diesel vehicles, which typically emit more PM10 particles than petrol vehicles, exaggerates this.
The smaller a particle, the longer it can remain suspended in the atmosphere. Very fine particles, made up of carbon from combustion and chemical compounds (sulphates and nitrates) can remain in the atmosphere for weeks. These particles can drift for many miles causing pollution problems across many countries. Episodes caused by this long-range transport of particulate pollution are explained below. As particulate matter is composed of such a large range of chemicals and materials from a variety of sources, the control of pollution levels is very difficult. As with ozone, local improvements will have only limited effect without international action.
The mass of sizes, shapes and chemical properties of particulate matter makes it very difficult to assess its health effects. Expert opinion is that there is no threshold concentration below which particulates have no effect on health. The aim must therefore be to reduce concentrations to a level at which minimal effects on health can be identified.
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Sulphur Dioxide (SO2)
Sulphur dioxide ('SO2') is produced when a material, or fuel, containing sulphur is burned. Globally, much of the sulphur dioxide in the atmosphere comes from natural sources, but in the UK the major contributors are power stations (65% of the total emissions). Sulphur dioxide levels in this country have dropped considerably over recent years due to cleaner power stations and a decreased use of coal.
The highest levels of sulphur dioxide are recorded in areas where coal is used extensively. Sulphur dioxide pollution episodes only generally occur where there is widespread domestic use of coal or in the vicinity of coal - or oil-fired power stations. The weather conditions that lead to an SO2 episode are explained later in this section.
Short-term exposure to high levels of sulphur dioxide may cause coughing, tightening of the chest and irritation of the lungs.
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Hydrocarbons (Including Benzene)
The term 'hydrocarbons' is often used when discussing traffic pollution. This refers to a group of chemicals of which volatile organic compounds (VOCs) are a subgroup. Volatile Organic Compounds (VOCs) comprise of a range of chemical compounds all of which contribute, to varying degrees, to the formation of ground level ozone
In addition, certain VOCs are known to cause cancer. Current attention is focused on 1,3 butadiene, primarily from vehicle exhausts, and benzene. Benzene in the atmosphere either comes from the combustion or evaporation of petrol. Levels are therefore highest close to busy roads or in the vicinity of petrol filling stations.
Long-term exposure to high levels of benzene and 1,3 butadiene has been linked to leukaemia and cancer. Health experts have concluded that the health risks from these pollutants become progressively smaller as cumulative exposure is reduced. Health standards are therefore set based on annual mean levels.
As only very small concentrations of hydrocarbons are typically found in the atosphere, the monitoring process is very complicated and expensive. As monitoring has only been carried out for the last few years, it is impossible to identify any upward or downward trend in levels. Annual results suggest the annual mean levels are well below European health standards.
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The main source of lead in the atmosphere has historically been from combustion of petrol. Since the phasing out of leaded petrol across Europe, lead levels have fallen sharply and lead monitoring is no longer considered necessary in most parts of the country.
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