Air quality guides
How to read AQI and air quality categories
AQI is an air quality index; µg/m³ is a concentration unit. They describe different quantities. Before comparing results, check the scale, pollutant and data time.
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What do our labels describe?
Our cards assign a category to an individual pollutant using the revised European Environment Agency (EEA) thresholds. We apply them to hourly concentrations, including cities outside Europe. This does not mean EEA measures that city. A PM2.5 label describes that particle fraction only; other pollutants can have different categories.
Six categories and five pollutants
The categories are good, fair, moderate, poor, very poor and extremely poor. Thresholds differ for PM2.5, PM10, NO₂, O₃ and SO₂. The overall EEA index is driven by the poorest category among the included pollutants. Our individual concentration labels are not numeric AQI scores. We display CO as a concentration, without assigning a category from this five-pollutant scale.
| Pollutant | Good | Fair | Moderate | Poor | Very poor | Extremely poor |
|---|---|---|---|---|---|---|
| PM2.5 | 0 - 5 | >5 - 15 | >15 - 50 | >50 - 90 | >90 - 140 | >140 |
| PM10 | 0 - 15 | >15 - 45 | >45 - 120 | >120 - 195 | >195 - 270 | >270 |
| NO₂ | 0 - 10 | >10 - 25 | >25 - 60 | >60 - 100 | >100 - 150 | >150 |
| O₃ | 0 - 60 | >60 - 100 | >100 - 120 | >120 - 160 | >160 - 180 | >180 |
| SO₂ | 0 - 20 | >20 - 40 | >40 - 125 | >125 - 190 | >190 - 275 | >275 |
Example: PM2.5 at 18 µg/m³
An hourly concentration of 18 µg/m³ falls in the moderate PM2.5 category. It does not mean “AQI 18”, and it does not automatically describe overall air quality. Check ozone too, for example: its thresholds differ. We determine categories before rounding the displayed concentration.
Why do daily averages have no hourly label?
Daily and monthly averages smooth out short peaks. We do not apply an hourly category to them. A low daily average can include hours with much higher concentrations. AQI is also not a tool for establishing legal compliance or assessing an annual average.
Check the date and source first
A label describes the stated hour even when that data is old. A missing reading appears as a dash, never as zero or good air quality. Another service may use EEA, US AQI or a national scale. Equal index numbers across different scales need not mean equal conditions.