Frequently asked questions
What is the daily air quality index?
In deriving a daily air pollution indicator an approach must be developed to combine concentrations of different measured pollutants. A number of alternative approaches have been proposed to present air pollution information from a range of pollutants in a single index. The most widely known of these is the United States Pollution Standards Index which is used in a number of countries. In the UK, DEFRA have established an Air Pollution Public Information System. DEFRA's System is based upon three air pollution thresholds (Standard, Information and Alert) and four bands (Low, Moderate, High and Very High Pollution). Thus:
|Low||1, 2, or 3||Effects are unlikely to be noticed even by individuals who know they are sensitive to air pollutants|
|Moderate||4, 5, or 6||Mild effects, unlikely to require action, may be noticed amongst sensitive individuals.|
|High||7, 8, or 9||Significant effects may be noticed by sensitive individuals and action to avoid or reduce these effects may be needed (e.g. reducing exposure by spending less time in polluted areas outdoors). Asthmatics will find that their 'reliever' inhaler is likely to reverse the effects on the lung.|
|Very High||10The effects on sensitive individuals described for 'High' levels of pollution may worsen.|
Employing the DEFRA Air Pollution Public Information System limits the Indicator to those pollutants for which there are potential acute health effects. An indicator, based upon this approach does not therefore include pollutants in the National Strategy with only chronic health impacts: benzene, 1,3-butadiene and lead.
Combining chronic and acute health effects from air pollution within a single indicator would require impacts to be evaluated in terms of a life-time or annual risk. The resulting indicator would be extremely complex to calculate and highly dependant upon the assumptions employed. In particular, long-term exposure to each pollutant would need to be assessed. A recent study (Development of Aggregated UK Indicators of Air Quality, 1998, DETR, HMSO) concluded that there is no current methodology for estimating population exposure [to air pollution] which is sufficiently robust to derive a meaningful indicator.
An Indicator based upon the frequency of pollution episodes (and potential acute health effects), as opposed to one of chronic air pollution toxicity is considered to be more:
- scientifically robust,
- readily understood by the public, and
- readily reflects at least some public concerns (air pollution as a precursor to acute respiratory disease).
The Index is therefore based upon the five pollutants included in the DEFRA Air Pollution Public Information System. If, in the future, the National Air Quality Strategy is extended to include other pollutants with acute health effects, it would be appropriate to re-examine the pollutants to be included both within the Indicator and Public Information System. If the national standards and/or Public Information System were amended, for example to reflect improved scientific knowledge of air pollution health impacts, it would be necessary to re-examine the Indicator.
The overall daily air quality index for Swansea or an individual site is calculated from the highest concentration of the five pollutants included within the banding system. All data presented within these pages is shown as micro grams per cubic meter of air (µgm-3).
Boundaries Between Index Points for Each Pollutant
Running 8 hourly mean
15 minute mean
24 hour mean
24 hour mean