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Air quality index

The purpose of an air quality index is to express complex air quality information in simple terms. Five bands are used in the Irish index:

  • Very good
  • Good
  • Fair
  • Poor
  • Very poor

The index is based on a maximum of four parameters; the one-hour average of SO2 (sulphur dioxide), NO2(nitrogen dioxide) and O3 (ozone), combined with the rolling 24-hour average of PM10 (particulate matter with diameter less than ten microns).  All figures are rounded to the nearest whole number.

The index for each of the four parameters is derived each hour and the overall index for the hour is equivalent to the lowest rating assigned to a parameter.

The key parameter

The key parameter is mentioned, i.e. the one that caused the lowest rating.  For example, if in one hour the rating for SO2, NO2 and O3 is good and that for PM10 is fair, then the index for the hour is fair with the key parameter being PM10.

If one or more parameter is missing, e.g. due to an instrument malfunction, the index is derived using the remaining parameters. At sites where all four parameters are not measured, the index is calculated for whichever parameters are measured.

The index for the day is the second lowest rating (> 95 percentile) for the day. For example, if in one day 20 hours were rated good, 3 were rated fair and one was rated poor, the index for the day is fair.

The two criteria used in developing the index were:

  • Limit values and assessment thresholds in the relevant EU Directives
  • Comparison with existing European and North American indices

Index - values in parts per billion (ppb). PM10 in ug m-3

 SO2 / ppb (1 hour avg.)NO2 / ppb
(1 hour avg.)
O3 / ppb
(1 hour avg.)
PM10 / ug m-3
(24 hour avg.)
Very Good

0 - 19

0 - 19

0 - 19

0 -19

Good

20 - 49

20 - 49

20 - 59

20 - 49

Fair

50 - 79

50 - 74

60 - 89

50 - 74

Poor

80 - 129

75 - 104

90 - 119

75 - 99

Very Poor

≥130

≥105

≥120

≥100

 

Index - values in ug m-3.

 
 SO2 / ug m-3
(1 hour avg.)
NO2 / ug m-3
(1 hour avg.)
O3 / ug m-3
(1 hour avg.)
PM10 / ug m-3
(24 hour avg.)
Very Good

0 - 49

0 - 36

0 - 39

0 -19

Good

50 - 129

37 - 94

40 - 119

20 - 49

Fair

130 - 209

95 - 139

120 - 179

50 - 74

Poor

210 - 349

140 - 199

180 - 239

75 - 99

Very Poor

≥350

≥200

≥240

≥100

Page last reviewed: 27/03/09

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