n Clean Air Journal = Tydskrif vir Skoon Lug - Methodology for developing an air pollution index (API) for South Africa




The Dynamic Air Pollution Prediction System (DAPPS) involves the development and integration of the following elements: Downscaling the current numerical urban-scale weather prediction to a finer spatial and temporal resolution, and establishing a comprehensive air pollutant emission inventory that will include industrial, motor vehicle and domestic emissions, and temporal variations in these emissions. The enhanced meteorological data and the emission inventory data will be used as inputs into a photochemical dispersion model, the Comprehensive Air Quality Model with Extensions (CAMx), to produce air pollution fields for the forecast meteorology.

Local air quality affects how we live and what we breathe. Like the weather, it can change from day to day - sometimes from hour to hour. The known and recognized health effects of air pollution include the increased risk of the exacerbation of respiratory symptoms such as increased asthma attacks and reduced lung function, increased hospital admissions for respiratory and cardio-vascular diseases, and increased mortality. An Air Quality or Air Pollution Index (API) is a quantitative tool through which air pollution data can be reported, providing information on how clean or polluted the air is, and the associated health concerns the public should be aware of. These indices usually focus on short-term health effects - those that can happen within a few hours or days of exposure to polluted air. A key feature of the DAPPS is that the final model output is a set of Air Pollution Indices.
Several countries employ some type of air pollution index to communicate the quality of their air. Some of these systems rely on relating measured (monitored) or predicted concentrations of air pollutants to a numerical scale, for example ranging from 0 and 100. This scale may be enhanced by verbal descriptors such as high or moderate. The advantage of such a system is that the public does not have to interpret a number of different concentrations - one for each pollutant. They also do not need to recall that, the health effects of, for example 1 ppm of ozone is very different from those of 1 ppm of carbon monoxide.
The simplistic use of a single index to reflect air pollution levels creates several difficulties. Different pollutants may have different health endpoints, information that may be lost through the use of a single index. Members of the public may also find it difficult to obtain details of how to translate a unified pollution index back into the disaggregated 'real' pollutant levels. In addition, it can be difficult to use an index to compare pollutant levels with national or international standards or guidelines, or with indices used in other countries. The use of a single standardized or unified scale doesn't solve the problems of how to report raised concentrations of a number of pollutants.
The DAPPS proposes the development of a health-based Air Pollution Index as opposed to an Air Quality Index. The basic concept of this index is that of using a combination of modelled pollutant concentrations and exposure-response functions. The initial modelling output would be pollutant-specific numerical values indicating the degree of pollution in an area of the modelled domain. Normalising these values with exposure-response functions will result in normalised bands corresponding to a scale (for example, a scale of 1 to 10) and colour coding system that reflects the possible health impacts. Advice and information on possible health effects associated with each value on the scale would reflect information applicable to both the 'normal healthy' population and 'sensitive' groups within the exposed population (such as asthmatics, the aged or the very young). The index will be modified where possible to account for known additive and/or synergistic effects.


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