The corresponding specific index of particulate matter < 2.5 µm is determined by fitting the arithmetic average of hourly values recorded in the last 24 hours, in one of the areas the concentrations listed in the following table:
| Field of particulate matter PM 2.5 concentration (µg/m3) | specific index |
| 0-10 |
1
|
| 10-20 |
2
|
| 20-25 |
3
|
| 25-50 |
4
|
| 50-75 |
5
|
| 75-800 |
6
|