Pro Weather· Updated
Understanding Air Quality Index: AQI vs EAQI
If you have a Davis AirLink sensor, your station measures the same thing: the concentration of particulate matter in the air. But there are two different ways to translate that raw measurement into an air quality index: the US EPA AQI and the European EAQI.
They use the same data but present it very differently, which causes confusion. This guide explains the difference, shows how the same reading maps onto both scales, and helps you choose which one to display on your weather site.
What do AQI and EAQI actually measure?
Both indices are built on the concentration of PM2.5 — particles smaller than 2.5 micrometers — measured in micrograms per cubic meter (µg/m³). The index itself is only a translation layer: it converts that raw concentration into a color-coded category so people can judge health risk without knowing the underlying science.
PM2.5 is the particle size that matters most because it is small enough to enter the lungs and bloodstream, making it the most health-relevant measure. The AirLink sensor uses a laser particle counter to measure PM1, PM2.5, and PM10 simultaneously — the same optical principle behind most consumer air quality monitors. The raw PM2.5 value is identical regardless of which index you choose to display; only the label and color change.
How does the US EPA AQI work?
The Air Quality Index used by the United States Environmental Protection Agency converts pollutant concentrations into a single number from 0 to 500, split into six color-coded categories from "Good" to "Hazardous." An AQI of 100 roughly corresponds to the national air quality standard, and higher numbers mean increasing health risk.
For PM2.5, the categories and their concentration breakpoints are:
| AQI Range | Level | PM2.5 (µg/m³) | Color |
|---|---|---|---|
| 0–50 | Good | 0.0–9.0 | Green |
| 51–100 | Moderate | 9.1–35.4 | Yellow |
| 101–150 | Unhealthy for Sensitive Groups | 35.5–55.4 | Orange |
| 151–200 | Unhealthy | 55.5–125.4 | Red |
| 201–300 | Very Unhealthy | 125.5–225.4 | Purple |
| 301–500 | Hazardous | 225.5+ | Maroon |
Note that these are the 2024-updated breakpoints: in May 2024 the EPA tightened the PM2.5 thresholds as part of its revised particulate standards, so the "Good" band now ends at 9.0 µg/m³ instead of the old 12.0, and the upper categories were lowered too. The same air therefore scores slightly worse today than it did before 2024. The breakpoints are set by regulation and differ per pollutant — AirNow's AQI basics page has the full detail.
How does the European EAQI work?
The European Air Quality Index, published by the European Environment Agency (EEA), reports a named band — Good, Fair, Moderate, Poor, Very Poor, or Extremely Poor — rather than a number. The overall rating is set by whichever measured pollutant currently falls in the worst band, so a single bad reading drags the whole index down.
For PM2.5, the bands (concentrations in µg/m³) are:
| Band | PM2.5 (µg/m³) | Color |
|---|---|---|
| Good | 0–10 | Cyan |
| Fair | 10–20 | Green |
| Moderate | 20–25 | Yellow |
| Poor | 25–50 | Red |
| Very Poor | 50–75 | Dark red |
| Extremely Poor | 75+ | Purple |
Because the EAQI reports a band rather than a number, it is read as a category, not compared on a 0–100 scale. You can see it live for the whole continent on the EEA's European Air Quality Index viewer.
How do AQI and EAQI compare for the same reading?
The European scale is stricter at the clean end, so the same PM2.5 reading usually sounds worse on the EAQI. Air at 30 µg/m³ falls in the EAQI "Poor" band, while on the US scale the same reading works out to about 90 AQI, which is still only classed as "Moderate." The table below translates a few values.
| PM2.5 (µg/m³) | US EPA AQI | US category | EAQI band |
|---|---|---|---|
| 5 | 28 | Good | Good |
| 12 | 56 | Moderate | Fair |
| 22 | 75 | Moderate | Moderate |
| 30 | 90 | Moderate | Poor |
| 40 | 112 | Unhealthy for Sensitive Groups | Poor |
| 60 | 154 | Unhealthy | Very Poor |
The US values use the 2024 EPA breakpoints and treat each concentration as a steady average. In practice, official indices smooth readings over hours, so a brief spike on your sensor will not map onto either scale exactly — but the pattern holds: the two systems describe identical air in very different language, which is why showing the right one for your audience matters.
Which air quality index should you use?
Use the index your audience already knows: the EPA AQI for a US audience, the EAQI for a European one. If your visitors are mixed, display both side by side. Matching the familiar scale means visitors can act on the reading immediately instead of translating between systems.
- Use EPA AQI if your site serves a US audience. It matches what people see on US weather apps, news, and government services.
- Use EAQI if your site serves a European audience. It is the standard used by the European Environment Agency and displayed on European weather services.
- Display both if your audience is mixed or if you want to be comprehensive. Some sites show both indices side by side, which helps visitors understand the difference.
Pro Weather supports both standards. You can choose which one your site displays, and the color-coded panel updates automatically based on your AirLink readings.
Why does the choice of index matter?
Air quality data is most useful when your audience understands what they are looking at. Showing the wrong index for your region leads to confusion: a European visitor used to the EAQI's named bands might read a US AQI number as healthier than their own scale would call it, or vice versa.
By choosing the right index for your audience, you make your air quality data immediately useful rather than requiring visitors to learn an unfamiliar scale. And since the number is only ever a translation of the PM2.5 concentration your sensor measures — the same laser-counting technique we cover in how weather station sensors work — you lose nothing by switching indices later.
Common questions
What is a good PM2.5 level?
Under the 2024 US EPA breakpoints, anything up to 9.0 µg/m³ counts as "Good"; the EAQI's "Good" band runs to 10 µg/m³, so the two scales roughly agree at the clean end. The World Health Organization's guideline is stricter still: an annual mean of 5 µg/m³ and a 24-hour mean of 15 µg/m³.
Why do AQI apps show different numbers for the same place?
Three usual reasons: a different index (US AQI, EAQI, or a vendor's own scale), a different averaging window (official AQI values smooth PM2.5 over hours, while your sensor reports near-instantly), and a different data source (the nearest government monitor versus a crowd-sourced sensor). Placement matters too — just as with temperature, where a sensor is mounted changes what it sees; one next to a road or barbecue will read worse than one in a clear back garden.
Does the Davis AirLink measure ozone?
No. The AirLink is a laser particle counter: it measures particulate matter (PM1, PM2.5, and PM10) plus temperature and humidity. Official AQI figures also fold in gases such as ozone and nitrogen dioxide from government monitors, so on a high-ozone summer day the official index for your area can be worse than your PM-only reading suggests.
Did the US AQI change recently?
Yes. In May 2024 the EPA tightened the PM2.5 breakpoints as part of its updated particulate matter standards: "Good" now ends at 9.0 µg/m³ instead of 12.0, and the thresholds for the upper categories were lowered as well. The same air can therefore report a somewhat higher AQI than it did before 2024, and apps adopted the new scale at different speeds.
See your air quality on your own site
If you have a Davis AirLink sensor, Pro Weather displays your air quality data with the correct color coding, index value, and health recommendations. The panel is updated every 10 minutes alongside your other weather data, and you can pair it with email alerts that fire when PM2.5 crosses a threshold — useful during wildfire smoke events. Start your Pro Weather site and your air quality readings will appear automatically.