Travellers notice it immediately: a "moderate" day in New York might be labelled differently on a European app. US AQI vs EU AQI is not a mistake — countries use different breakpoints, pollutants, and colour bands. Comparing cities fairly means understanding the scale, not just the number.
US EPA AQI (0–500)
The United States Environmental Protection Agency AQI runs 0–500 with six colour categories from green (good) to maroon (hazardous). The index is driven by the worst pollutant at the time — PM2.5, PM10, ozone, NO₂, SO₂, or CO — each with its own concentration-to-index curve.
AQI 100 on the US scale is often described as the upper edge of "moderate" for PM2.5 — not "perfect" air, but acceptable for most people.
European CAQI and national systems
Europe commonly uses the Common Air Quality Index (CAQI) for public communication, with slightly different breakpoints and a focus on urban roadside vs background stations. Some countries still publish their own scales alongside EU reporting. A "medium" CAQI day may not line up exactly with US yellow.
WHO guidelines vs traffic-light apps
The World Health Organization publishes concentration guidelines (µg/m³ for PM2.5) rather than a single public colour scale. Many apps — including Aether — show pollutant concentrations against WHO levels so you can see how far above long-term health targets the air is, regardless of which national index you grew up with.
- Compare PM2.5 µg/m³ directly when travelling between countries.
- Do not assume AQI 80 means the same thing on every website.
- Check which pollutant is dominant — ozone-heavy US summers vs winter PM in Delhi.
- Use hourly trends; scale differences matter less than "getting worse or better".
Aether shows AQI with pollutant detail and WHO context — useful whether you are in Chicago, Paris, or Tokyo.
Check live air quality on Aether