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Pollutant information

Information on the health effects and sources of some important air pollutants can be found below.

More detailed information can be found in the Air Quality Strategy for England, Scotland, Wales and Northern Ireland and an Addendum to the Strategy.

Benzene

1,3-Butadiene

Carbon Monoxide

Lead

Nitrogen Dioxide

Ozone

Particles (PM 10)

Polycyclic Aromatic Hydrocarbons (PAH)

Sulphur Dioxide

Benzene

Health Effects

Benzene is a recognised genotoxic human carcinogen. Studies of industrial workers exposed in the past to high levels of benzene have demonstrated an excess risk of leukaemia which increased in relation to their working lifetime exposure. Because it is a genotoxic carcinogen, no absolutely safe level can be specified for ambient air concentrations of benzene. In their 1994 report, the Expert Panel on Air Quality Standards (EPAQS) recommended an air quality standard of 16.25µg/m3 (5ppb) as a running annual mean, a level which they concluded represents an exceedingly small risk to health. The standard was included as the benzene objective in the Air Quality Strategy, to be achieved by 31 December 2003. In their report, EPAQS considered the advice of the UK Department of Health's Committee on Carcinogenicity, that exposure to benzene should be kept as low as practicable, and recommended a target of 3.25µg/m3 (1ppb), also as a running annual mean. This was adopted in 2002 in Scotland as a new long term Strategy objective to supplement the existing one, to be achieved by 31 December 2010.

Sources

Benzene is a volatile organic compound. In the UK the main atmospheric source is the combustion and distribution of petrol, of which it is a minor constituent. Benzene is also formed during the combustion process from aromatics in the petrol. Diesel fuel is a relatively small source. The amount of benzene in petrol was until the beginning of 2000 regulated to an upper limit of 5% by volume by EU legislation. In recent years it comprised on average 2% by volume in the UK. Since 1 January 2000, EU legislation has required that the amount of benzene in petrol is below 1%. The main outdoor sources of benzene remaining beyond 2005 are expected to be:

  • petrol-engined vehicle exhausts;
  • petrol refining and distribution; and
  • uncontrolled emissions from petrol station forecourts without petrol vapour recovery systems.

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1,3-Butadiene

Health effects

The health effect which is of most concern in relation to 1,3-butadiene exposure is the induction of cancers of the lymphoid system and blood-forming tissues, lymphomas and leukaemias. Like benzene, 1,3-butadiene is a genotoxic carcinogen, and so no absolutely safe level can be defined. EPAQS nevertheless believed that a standard could be set at which any risks to the health of the population are exceedingly small. In their 1994 report EPAQS recommended an air quality standard of 2.25µg/m3 (1ppb) as a running annual mean. This was subsequently adopted as the Air Quality Strategy objective, to be achieved by 31 December 2003.

Sources

1,3-butadiene is a gas at normal temperatures and pressures and trace amounts are present in the atmosphere, deriving mainly from the combustion of petrol and of other materials. Although 1,3-butadiene is used in industry, mainly in the production of synthetic rubber for tyres, motor vehicles are its dominant source.

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Carbon Monoxide

Health effects

The main threats to human health from exposure to carbon monoxide are the formation of carboxyhaemoglobin, which substantially reduces the capacity of the blood to carry oxygen and deliver it to the tissues, and blockage of important biochemical reactions in cells. People who have an existing disease which affects the delivery of oxygen to the heart or brain (eg coronary artery disease (angina)) are likely to be at particular risk if these delivery systems are further impaired by carbon monoxide. In their 1994 report EPAQS recommended an air quality standard of 11.6mg/m3 (10ppm) as a running 8 hour mean. The EPAQS recommendation is intended to limit the exposure of the population, including susceptible individuals, and specifies levels at which harm is unlikely to occur. This was adopted as the Air Quality Strategy objective, to be achieved by 31 December 2003, but has now been replaced by a tighter objective of 10mg/m3 (8.6ppm) - the second EU Air Quality Daughter Directive limit value for carbon monoxide. The achievement date remains the same.

Sources

Carbon monoxide (CO) is a gas formed by the incomplete combustion of carbon containing fuels. In general, the more efficient the combustion process, the lower the carbon monoxide emission. The main outdoor source of carbon monoxide is currently road transport, in particular petrol-engined vehicles, which in 1997 accounted for almost 75% of emissions.

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Lead

Health effects

Exposure to high levels of lead may result in toxic biochemical effects in humans which in turn cause problems in the synthesis of haemoglobin, effects on the kidneys, gastrointestinal tract, joints and reproductive system, and acute or chronic damage to the nervous system. The possible effect of lead on brain development in children, and hence their intellectual development, is the greatest cause for concern. Some studies of populations of young children suggest that there may be a loss of up to about 2 IQ points for a rise in blood lead from 10 to 20 µg/dl [Expert Panel on Air Quality Standards, Lead, 1998. µg/dl = microgrammes per decilitre (decilitre = 100 millilitres)] . The advice of the UK Department of Health's Committee on Toxicity is that it is not possible to identify a threshold for effects of lead on health. This was confirmed by a 1998 UK study which suggested a no-threshold relationship between blood lead concentration and blood pressure. Whilst the significance of the potentially very small changes in blood pressure produced by current ambient levels of lead are uncertain, this further supports the need to limit exposure to lead from all sources, including the air.

When the Air Quality Strategy was originally published in 1997, EPAQS had not made a recommendation on lead. The objective was derived from the World Health Organisation's revised guideline of 0.5µg/m3 as an annual average to be achieved by 31 December 2004. EPAQS published its recommendation in May 1998 for an air quality standard of 0.25µg/m3 , measured as an annual mean. This was adopted as a longer term objective in the 2000 version of the Strategy, to be achieved by 31 December 2008. At this level, the panel concluded that any effects on the health of children, the group most vulnerable to impairment of brain function, would be so small as to be undetectable. They took into account that normally only a small fraction of total lead intake occurs through inhalation. Food and water are two of the main sources for most people. Lead in air contributes to lead levels in food through the deposition of dust and rain, containing the metal, on crops and on the soil.

EPAQS and WHO agree that exposure of young children to lead can impair brain development, as reflected in a reduction in average population IQ. Despite examination in a number of studies, it has not been possible to identify a clear threshold for its effect. EPAQS has adopted a more cautious approach than WHO, arguing that it was unacceptable for exposure to lead in air to result in any detectable reduction in IQ, i.e. of one IQ point or more. They set a standard which makes a detectable reduction unlikely, based on the available evidence and on safety factors.

Sources

Lead is the most widely used non-ferrous metal and has a large number of industrial applications, both in its elemental form and in alloys and compounds. The single largest use globally is in the manufacture of batteries. As the compound tetraethyl lead, it has been used as a petrol additive to enhance the octane rating.

Most of the national airborne emissions of lead have arisen from petrol-engined vehicles. However the general sale of leaded petrol in the UK was banned from 1 January 2000, following the implementation of tighter EU fuel quality standards Industry, in particular secondary non-ferrous metal smelters, may contribute to emissions of lead in industrial areas, although in the past such emissions were generally outweighed by those from vehicles. The reduction in the lead content of leaded petrol and the increasing use of unleaded petrol have led to significant reductions in urban lead levels.

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Nitrogen Dioxide

Health effects

At relatively high concentrations, nitrogen dioxide causes inflammation of the airways. There is evidence to show that long-term exposure to nitrogen dioxide may effect lung function and that exposure to nitrogen dioxide enhances the response to allergens in sensitised individuals. In their 1996 report, EPAQS recommended a short-term air quality standard of 286µg/m3 (150ppb) measured as an hourly average. This was adopted as an objective in the 1997 Air Quality Strategy, but was replaced in the 2000 version with the EU first Air Quality Daughter Directive limit value of 200µg/m3 (105 ppb) to be achieved by 31 December 2005. In both the 1997 and 2000 versions of the Strategy an annual objective of 40µg/m3 (21 ppb) not to be exceeded more than 18 times a year was also adopted. Again, the target date is 31 December 2005. This was derived from the WHO's revised annual average guideline, which was subsequently included as the first Air Quality Daughter Directive limit value.

Sources

All combustion processes in air produce oxides of nitrogen. Nitrogen dioxide (NO 2) and nitric oxide (NO) are both oxides of nitrogen and together are referred to as NO X. It is nitrogen dioxide which is associated with adverse effects upon human health. Road transport is thought to account for about 50% of total UK emissions of nitrogen oxides, the electricity supply industry for about 20% and the industrial and commercial sectors for about 17%. In London, road transport is thought to account for over 75% of emissions. NO X is also a precursor of ozone.

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Ozone

Health effects

Exposure to high concentrations of ozone may cause slight irritation to the eyes and nose. If very high levels of exposure (1,000-2,000µg/m3 or 500-1,000ppb) are experienced over several hours, damage to the airway lining followed by inflammatory reactions may occur. There is also evidence that minor changes in the airways may occur at lower concentrations, down to about 160µg/m3 (80ppb). In their 1994 report, EPAQS recommended an air quality standard for ozone of 100µg/m3 (50ppb) as a running 8 hour mean. EPAQS noted that if their recommended standard is exceeded on less than 10 days per year, at any one site, then one would not expect, in most years, the highest 8 hour concentration to exceed 200µg/m3 (100ppb), a level at which effects in healthy individuals have been clearly demonstrated. The EPAQS standard is includede as a provisional objective in the Air Quality Strategy. However, ozone is not included in regulations for the purposes of Local Air Quality Management due to its transboundary nature. As such it is more effectively dealt with at national and international level.

Sources

Ozone is not emitted directly from any man-made source in any significant quantities. It arises from chemical reactions in the atmosphere caused by sunlight. In the stratosphere, where ozone plays a beneficial role by shielding the earth from harmful ultra-violet radiation, ozone is produced by sunlight acting initially on oxygen molecules. The balance between ozone and oxygen in the stratosphere is currently being disturbed by migration upwards of chemicals such as chlorofluorocarbons. They remove ozone and may therefore increase the amount of ultra-violet light reaching the earth's surface.

Some ozone occasionally reaches the lower layers of the atmosphere from intrusions of air from the stratosphere. But it is primarily formed by a complicated series of chemical reactions initiated by sunlight. Oxides of nitrogen and VOCs, derived mainly from man-made sources, react to form ozone. These substances are produced by combustion, other industrial processes, and other activities such as solvent use, and petrol distribution and handling. NO x and VOCs are the most important precursors of elevated levels of ozone. Production can also be stimulated by carbon monoxide, methane, or other VOCs which arise from plants, trees and other natural sources. Ozone is also a greenhouse gas, so NO x and VOCs can be considered indirect greenhouse gases.

These chemical reactions do not take place instantaneously, but over several hours or even days depending on the VOCs, and once ozone has been produced it may persist for several days. In consequence, ozone measured at a particular location may have arisen from VOC and NO x emissions many hundreds, or even thousands, of kilometres away, and may then travel further for similar distances. Maximum concentrations, therefore, generally occur downwind of the source areas of the precursor pollutant emissions. Indeed, in urban areas, where concentrations of traffic gases may be high, nitric oxide (NO) from exhaust emissions may react with ozone to form nitrogen dioxide (NO 2) reducing ozone concentrations. However, as the air movement carries the primary pollutants away, more ozone is generated and concentrations rise in the downwind areas.

In terms of ozone measured at ground level, these photochemical episodes of high ozone concentrations are superimposed on a baseline which varies slightly throughout the year but averages around 60µg/m3 (30ppb) at UK latitudes. This is made up partly of ozone transported from the stratosphere, and some ozone produced in the troposphere (the region of the atmosphere, about 10 km deep, between the Earth's surface and the stratosphere) from naturally occurring and man-made precursors (in broadly equal proportions). There is evidence that this baseline has roughly doubled since the turn of the century, largely due to the increase in man-made NO x emissions in the whole of the northern hemisphere. The baseline is close to levels at which effects have been observed on crops and plants.

These factors, particularly the importance of sunlight in the reactions, mean that elevated ozone levels occur more frequently:

  • in summer;
  • in the southern UK more than in the north; and
  • in rural and suburban areas more than in city centres.

In northwest Europe, the time it takes for ozone to form and then be destroyed in the atmosphere, and hence the distance it can travel, makes the problem an international one.

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Particles (PM 10)

Health Effects

Particulate air pollution is associated with a range of effects on health including effects on the respiratory and cardiovascular systems, asthma and mortality. EPAQS concluded that particulate air pollution episodes are responsible for causing excess deaths among those with pre-existing lung and heart disease, and that there is a relationship between concentrations of PM 10 and health effects, such that the higher the concentration of particles, the greater the effect on health. Since the most applicable evidence related daily average concentrations of particles to effects on health, EPAQS concluded that PM 10 should be measured as a 24 hour running average. In their 1995 report, EPAQS recommended an air quality standard of 50µg/m3 as a 24 hour running mean. However, the Panel also recommended that policies should aim to ensure that there is a decline in both peak and annual average concentrations of PM 10.

There is emerging evidence to suggest that the health effects of particles are due principally to fine particles (PM 2.5) [PM 2.5 means particulate matter which passes through a size-selective inlet with a 50% efficiency cut-off at 2.5µm aerodynamic diameter]. EPAQS, in considering the most appropriate particle fraction on which to develop air quality standards, has reaffirmed its view that the PM 10 standard recommended in 1995 provides an appropriate level of protection for public health. The Panel has, however, recognised that PM 2.5 might better represent the toxic fraction of particulate air pollution, and that a PM 2.5 standard may be a desirable objective. The Panel's current view is that the evidence on which to base such a standard is at present insufficient. It is continuing to review the emerging evidence and will give further advice as necessary. The existing PM 10 objectives of 40µg/m3 (annual mean) and 50µg/m3 not to be exceeded more than 35 times a year (24 hour mean), to be achieved by 31 December 2004, were supplemented in Scotland in 2002 by new long term objectives of 20µg/m3 (annual mean) and 50µg/m3 not to be exceeded more than seven times a year (24 hour mean), to be achieved by 31 December 2010. Less stringent long term annual objectives were adopted in other parts of the UK.

Sources

Unlike the individual gaseous pollutants which are single, well-defined substances, particles (PM 10) in the atmosphere are composed of a wide range of materials arising from a variety of sources. The report from the Airborne Particles Expert Group (APEG) [Source Apportionment of Airborne Particulate Matter in the United Kingdom, January 1999] confirms that in the UK, particles (PM 10 ) may be regarded as having three predominant source types. Concentrations of PM 10 comprise of primary particles, arising from combustion sources (mainly road traffic); secondary particles, mainly sulphate and nitrate formed by chemical reactions in the atmosphere; and coarse particles, suspended soils and dusts, seasalt, biological particles and particles from construction work.

Analysis of concentrations of PM 10 as measured using TEOM (or equivalent) instruments, shows it is composed of each of the three source types. In general terms, the three source types each make up roughly one third of total long-term average PM 10 concentrations at urban background locations. However, the relative contribution of each source type varies from day to day, depending on meteorological conditions and quantities of emissions from mobile and static sources. The fine particle fraction (PM 2.5) is composed predominantly of primary and secondary particles. Particles in the range from PM 2.5 - PM 10 generally consist of coarse particles.

UK emissions account for about 80% of the primary fraction of annual mean PM 10. The Airborne Particles Expert Group (APEG) reported that emissions in mainland Europe contribute up to about 20% to annual mean levels of primary particles in the UK. This may be much larger during short-term peak episodes. For example, a modelling study of London suggested that in anticyclonic conditions with low wind speeds, typical of winter pollution episodes, European derived primary PM 10 could contribute about 15-30µg/m3 to daily concentrations. London primary PM 10 emissions in the same conditions contributed about 30-40µg/m3 to daily PM 10 concentrations at urban background locations.

Emissions from mainland Europe make a more significant contribution to secondary particles. APEG's findings suggest that in a year with typical meteorology, about 15% of the total annual average of PM 10 concentrations (about 50% of secondary particles) are derived from mainland Europe. In years when easterly winds are more frequent, emissions in mainland Europe account for a much higher proportion particularly in south and east England.

The APEG report shows that across the country, road traffic contributes 25% of national PM 10 emissions. But in city centres traffic contributes typically 30-40% to the annual average concentrations. On high pollution days it can be much more.

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Health Effects

Studies of occupational exposure to PAHs have shown an increased incidence of tumours of the lung, skin and possibly bladder and other sites. Lung cancer is most obviously linked to exposure to PAHs through inhaled air. In its 1999 report EPAQS recommended an air quality standard for PAHs of 0.25ng/m3 as an annual average, based on B[a]P as a marker for the total mixture of PAHs in the UK. This recommendation is intended to reduce any risk to the population from exposure to PAHs to one which the Panel believes would be so small as to be undetectable. The Panel also commented that it does not necessarily follow that all exposure above this standard carries a significant risk, in view of the effective application of an additional 10-fold safety factor in deriving the standard. The EPAQS standard was adopted as an Air Quality Strategy objective in 2002. However, the objective has not been included in regulations for the purposes of Local Air Quality Management due to uncertainties about current releases for key sources and about future concentrations.

Sources

The main sources of PAHs in the UK are domestic coal and wood burning (39%), fires (e.g. accidental fires, bonfires, forest fires etc.) (28%), anode baking and aluminium production (19%) and road transport (8%). Available data indicates a substantial decline in emissions in the UK. Emissions of B[a]P are forecast to have reduced from about 68 tonnes per annum in 1990 to about 9.5 tonnes in 2000. A further fall to about 6.4 tonnes by 2010 is forecast.

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Sulphur Dioxide

Health Effects

Sulphur dioxide causes constriction of the airways by stimulating nerves in the lining of the nose, throat and airways of the lung. The latter effect is particularly likely to occur in those suffering from asthma and chronic lung disease. The effects of sulphur dioxide on sensitive subjects appear almost immediately at the start of exposure. In their 1995 report EPAQS recommended an air quality standard of 266µg/m3 (100ppb) measured over a 15 minute averaging period. The recommendation was intended to reduce the exposure of the population, including individuals who may be particularly susceptible, to levels of sulphur dioxide at which harmful effects are unlikely to occur. EPAQS acknowledged that an averaging time of just a few minutes might be desirable. but concluded that a 15 minute averaging time represented an acceptable compromise between desirability and practicability.

The standard is included as an objective in the Air Quality Strategy, not to be exceeded more than 35 times a year, and to be achieved by 31 December 2005, along with two other objectives derived from limit values in the first Air Quality Daughter Directive - a 24 hour mean of 125µg/m3 (47ppb) not to be exceeded more than three times a year and a one hour mean of 350µg/m3 (132ppb) not be exceeded more than 24 times a year, both to be achieved by 31 December 2004.

Sources

Sulphur dioxide is a gas at normal temperature and pressure. It dissolves in water to give an acidic solution which is readily oxidised to sulphuric acid. In the UK, the predominant source of sulphur dioxide is the combustion of sulphur-containing fossil fuels, principally coal and heavy oils.

For the first half of this century, emissions of sulphur dioxide were dominated by the combustion of coal, not only in the domestic sector but also in commercial and industrial premises, and in power stations which were situated predominantly within towns and cities. Following the smogs in the 1950s and the Clean Air Act of 1956, this pattern changed. Cleaner fuels replaced coal in the domestic, commercial and industrial sectors, and power generation was concentrated in much larger and more efficient stations situated in rural areas. However, in parts of the UK, notably Northern Ireland, use of coal and oil for domestic heating is still a significant factor.

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