92
(d) International Targets/Recommended Standards: World Health Organization
(WHO) air quality guidelines exist for all the pollutants covered by this indicator
except nitrogen monoxide. Many countries have established their own air quality
standards for many of these pollutants.
(e) Linkages to Other Indicators: This indicator is closely linked to others that
relate to energy use and environmental protection, such as annual energy use per
capita and per unit of gross domestic product (GDP), air pollutant emissions from
energy systems, share of non-carbon fuels and renewables, soil contamination, etc.
METHODOLOGICAL DESCRIPTION
(a) Underlying Definitions and Concepts: This indicator may be designed and
constructed in a number of ways. An important aspect that must be considered is the
definition of the statistic to be used; for example, where monitoring data are available,
the indicator may be expressed in terms of a mean annual concentration, a percentile,
or the
nth highest daily mean, etc., on the basis of either an hourly or daily average.
For health effects, the most appropriate averaging times and statistics are likely to be
different for different pollutants. It is therefore recommended that the basis for the
indicator be the number of days where concentrations exceed an established threshold
(national or international air quality limits) and/or the percentage of the urban
population exposed to concentration levels that exceed the target values (e.g.
according to European Union legislation, 24-hour average PM10 concentrations
above 50
μg/m
3
are not to be exceeded more than 35 times a year). It should be noted
that this type of comparison might need to be made with care because of possible
changes or differences in guideline values. However, a simple count of the number of
exceedances in a country is an inappropriate final measure for the indicator, as the
number of exceedances is likely to increase with increasing numbers of monitoring
stations.
1
Where monitoring data are unavailable, estimates of pollution levels may be made
using air pollution models. Dispersion models, however, depend on the availability of
emissions data; where these are not available, surveys may be conducted using rapid
source inventory techniques. Because of the potential errors in the models or in the
input data, results from dispersion models should ideally be validated against
monitoring data.
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