Click on the air pollutants listed below for information
on their health effects and sources.
More detailed information can be found in the
Air Quality Strategy for England, Scotland, Wales and
Northern Ireland and an
Addendum
to the Strategy.
Further
information on polycyclic aromatic hydrocarbons in
Northern Ireland was published in April 2004.
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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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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