Trends in outdoor air pollution in the UK. 93/5
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Introduction
Advice on air quality and on the health effects of air pollution
on vulnerable individuals such as those with asthma is now routinely
issued when high levels of air pollution are anticipated.
Figure 1: DOE air quality bands and
WHO air quality guidelines
Increased media coverage reflects both public concern about air
pollution and the growing body of research showing adverse health
effects of exposure to a range of pollutants.
Experimental and population studies have shown that nitrogen
dioxide, ozone, sulphur dioxide and airborne particles may have
acute adverse effects on the respiratory system, at levels at
or near to levels observed during air pollution episodes in the
UK. Exposure to high levels of these pollutants over short periods,
i.e. hours or days, is generally considered more hazardous than
longer-term low level exposure over weeks or months and this is
reflected in the WHO air quality guidelines and the air quality
bands used by the Department of the Environment.
Trends in emissions
Changes in emissions have an affect in air quality, although
other factors are also important.
Nitrogen dioxide
Power stations burning fossil fuels and motor transport are the
two main sources of nitrogen oxides (NOx). Nitric oxide (NO) the
main constituent of these emissions, reacts to form nitrogen dioxide
(NO2). Emissions of NOx (figure 2a) have increased over the last
30 years, mainly as a result of increases in road transport. The
introduction of three-way catalysts to new vehicles will reduce
future emissions from this major source.
Sulphur dioxide and black smoke
Sulphur dioxide (SO2) is created by the burning of fossil fuels
containing sulphur. Nationally, power stations are now the main
source of emissions although coal burning remains a significant
source in some local areas. Emission levels are declining and
this trend is expected to continue. Black smoke consists of small
particles produced by incomplete combustion. Diesel vehicles are
now the main source in urban areas following the decline of coal
burning.
Figure 2: UK trends in emissions of nitrogen
oxides, sulphur dioxide and black smoke
Emissions have dropped dramatically in recent years although
emissions from diesel vehicles are increasing.
Ozone
Ground level ozone is a so-called secondary pollutant. It is
formed by a complex photochemical reaction involving the action
of sunlight and oxygen and NO2, catalysed by volatile organic
compounds. Ozone production is dependent on weather conditions
and may occur considerable distances from the source of precursor
substances. There are no significant direct sources of emission
in the UK.
Trends in air quality
In the UK as a whole, trends in annual average levels of sulphur
dioxide (SO2) and black smoke over the last few decades reflect
the downward trends in emission levels (figure 3).
Figure 3: Average urban concentrations
of smoke and sulphur dioxide in the UK
Underlying trends in levels of nitrogen dioxide (NO2) and ozone
are not easy to determine. There are some indications of a general
rise in NO2 and ozone in central London, for example, shows a
modest downward trend (figure 4).
Figure 4: Trends in annual average
concentrations of ozone and nitrogen dioxide in central London
Variations in air quality
The volume of emissions is only one factor affecting day to day
air quality at any given location. The siting, proximity and height
of emissions sources, weather conditions and other factors affecting
the direction and speed of dispersions are also important. As
a consequence, daily pollution levels often deviate substantially
from average values and levels also vary geographically. In general,
levels of NO2, SO2 and black smoke tend to be higher in urban
areas, while ozone concentrations tend to be higher in rural areas.
Roadside levels of some pollutants may exceed levels recorded
at background urban monitoring sites.
An individual's personal exposure and the dose received by the
lungs will also depend on many factors including where the individual
lives and works, exercise patterns, and exposure to cigarette
smoke and indoor sources of pollution (such as NO2 emitted by
unflued gas appliances).
Air pollution episodes
Annual average figures tend to conceal the occurrence of air
pollution "episodes" (periods when levels of pollution
rise markedly above average) which may be of greater concern from
a health point of view. The pattern of episodes at the one urban
site (central London) is illustrated in figure 5.
Figure 5: Daily maximum hourly
levels of NO2, SO2 and ozone in central London
The occurrence and pattern of episodes varies in different locations.
In 1991, for example, WHO one hour guide values for NO2 were exceeded
at monitoring sites in London, Sheffield, Glasgow, and Manchester;
SO2 guidelines were exceeded at monitoring sites in Belfast (where
WHO guide levels were exceeded on 37 days) and in South and West
Yorkshire, Hertfordshire, Strattforshire, Nottinghamshire and
central London; and ozone guidelines were exceeded at monitoring
sites in the South West and South East, Oxfordshire, Cumbria and
North Ireland. Black smoke guide levels were also exceeded at
various sites in 1991 but excess concentrations of both SO2 and
black smoke (pollutants which are considered more hazardous in
combination) occurred only in Belfast.
We are grateful to Warren Spring Laboratory for advice and provision
of data from WSL monitoring sites and the DOE national network.
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