+------------------------------------+-------------------------------------------------------------------+
| Pollutant | Index level |
| | (based on pollutant concentrations in µg/m3) |
| +-------+---------+----------+---------+-----------+----------------+
| | Good | Fair | Moderate | Poor | Very poor | Extremely poor |
+====================================+=======+=========+==========+=========+===========+================+
| Particles less than 2.5 µm (PM2.5) | 0-10 | 10-20 | 20-25 | 25-50 | 50-75 | 75-800 |
+------------------------------------+-------+---------+----------+---------+-----------+----------------+
| Particles less than 10 µm (PM10) | 0-20 | 20-40 | 40-50 | 50-100 | 100-150 | 150-1200 |
+------------------------------------+-------+---------+----------+---------+-----------+----------------+
| Nitrogen dioxide (NO2) | 0-40 | 40-90 | 90-120 | 120-230 | 230-340 | 340-1000 |
+------------------------------------+-------+---------+----------+---------+-----------+----------------+
| Ozone (O3) | 0-50 | 50-100 | 100-130 | 130-240 | 240-380 | 380-800 |
+------------------------------------+-------+---------+----------+---------+-----------+----------------+
| Sulphur dioxide (SO2) | 0-100 | 100-200 | 200-350 | 350-500 | 500-750 | 750-1250 |
+------------------------------------+-------+---------+----------+---------+-----------+----------------+