AQI Around the World: How to Compare Air-Quality Readings Safely (2026)
Table of contents
Don't compare AQI numbers from different countries as if they were temperatures. First identify the index system, pollutant, concentration, averaging period, data timestamp and health category. The same number can imply a different concentration or message under another system. For today's plans, use the destination authority's current observations and forecast; annual WHO data answers a long-term question and shouldn't be used as a live travel reading.
Key takeaways
- An AQI number is an output of a local formula, not a universal concentration unit.
- Compare the underlying pollutant and concentration before comparing two index values.
- Current, forecast and annual-average products answer different travel questions.
- The overall index is often driven by the worst pollutant; inspect which pollutant controls the reading.
- WHO's ambient database should not be repurposed into a city ranking.
Why AQI is not a universal unit
Micrograms per cubic metre are concentration units. AQI is a communication output created by applying a local formula and breakpoints to pollutant observations.
A system chooses: which pollutants to include; the averaging period for each pollutant; concentration breakpoints; category names and colors; which pollutant determines the overall index; how missing monitors and forecasts are handled; and health advice for general and sensitive populations.
Change any of these and the displayed index can change even when the air sample does not.
Three official products that answer different questions
| Product | What it displays | Best travel use | Don't use it for |
|---|---|---|---|
| U.S. EPA AQI / AirNow | Current or forecast U.S. index on a normalized 0–500 scale | Adjusting today's outdoor plans in the U.S. | Treating 100 as a universal global threshold |
| European AQI | Hourly category from up to five pollutants, with station and modelled layers | Current European conditions and forecast context | Checking legal compliance or assuming a U.S.-style number |
| WHO ambient database | Annual mean city/town measurements | Long-term research and context | Live itinerary decisions, city rankings or simplistic country comparisons |
The WHO explicitly says its database is not designed to rank polluted cities and that differences in monitoring and methodology make direct country comparisons misleading.
The U.S. EPA scale
EPA describes a nationally uniform 0–500 AQI for five common regulated pollutants: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide.
The higher the value, the greater the concern. EPA says a value of 100 generally corresponds to the U.S. national ambient standard for that pollutant. Values above 100 become unhealthy first for susceptible groups and then for everyone as the number rises. That statement is specific to the U.S. system; it is not a conversion rule for another country's index.
| AQI | General label |
|---|---|
| 0–50 | Good |
| 51–100 | Moderate |
| 101–150 | Unhealthy for sensitive groups |
| 151–200 | Unhealthy |
| 201–300 | Very unhealthy |
| 301–500 | Hazardous |
Use the live health message because the controlling pollutant and affected groups matter.
The European index
The European Environment Agency index uses concentration values for up to five pollutants: PM2.5, PM10, nitrogen dioxide, ozone and sulfur dioxide. It is defined hourly. The overall level corresponds to the poorest level among available pollutants.
The official product includes: a station layer using up-to-date reported data, sometimes complemented by modelled data; a modelled layer providing broader geographic coverage; recent observations and forecast values; and transparency indicators when the expected pollutant set is incomplete.
This means two nearby map points can differ because one is a monitor and another a modelled grid. Check the layer and timestamp before interpreting the color.
The six-field AQI decoder
Before acting on a reading, capture:
- System: Who publishes it?
- Pollutant: PM2.5, PM10, ozone, NO2, SO2 or CO?
- Concentration and unit: What was actually measured or modelled?
- Averaging period: Hourly, rolling, daily or annual?
- Timestamp and layer: Monitor, model, forecast or blended value?
- Health message: What does the local authority advise for your group?
This is more defensible than using an online "AQI converter" that accepts only a number.
Can AQI be converted?
A safe conversion is not AQI A → AQI B. It is:
Index A → underlying pollutant concentration and averaging period → official breakpoints for Index B
Even then, the result can be misleading if: the averaging periods differ; one system truncates or rounds concentration differently; the overall index was driven by another pollutant; a NowCast or forecast is being treated as a raw observation; or the target system doesn't include that pollutant in the same way.
For travel decisions, it's usually better to read the destination's official category and health advice than manufacture a cross-index score.
Turn air quality into a trip decision
Weather matters through exposure, not through a color alone. Consider four multipliers:
Personal sensitivity
Children, older adults and people with heart or lung conditions may receive earlier or stronger advice. Follow local guidance and a clinician's plan.
Exertion
Running, steep hiking and cycling increase ventilation. A reading that allows light activity may still argue against a hard workout.
Duration
A short transfer and a four-hour outdoor queue are different exposures.
Micro-location
Roadside pollution, wildfire smoke, elevation and indoor filtration can make local experience differ from a citywide reading.
A traveler action ladder
This is a planning framework, not medical advice:
- Normal local category: proceed, while checking sensitive-person advice.
- Elevated category: move strenuous activity to a better forecast window; shorten exposure.
- Unhealthy category: prioritize indoor or lower-exertion alternatives and follow official health guidance.
- Severe category or symptoms: avoid unnecessary exposure and seek local medical advice when appropriate.
For smoke events, conditions can change quickly. Recheck close to the activity, not only at breakfast.
Current, forecast and annual data
Current observation
Best for deciding whether to leave now. Check recency and monitor distance.
Forecast
Best for sequencing tomorrow's activities. It's a prediction, not a measurement.
Annual mean
Best for long-term destination and public-health context. It can't tell you the air quality on a particular afternoon.
Confusing these time scales is one of the largest errors in travel air-quality content.
Example: two apps both show 100
Don't conclude the air is identical. Ask: are both using the same national index? Is PM2.5 controlling both readings? Is one a forecast and the other a monitor? Do the averaging periods match? Is one app relabeling a provider's data? What health category and advice accompanies each value?
If the systems differ, compare the underlying official pollutant information and follow each local interpretation.
Myths, debunked
"AQI 100 means the same air quality everywhere."
No. EPA says 100 generally corresponds to the U.S. national standard for that pollutant specifically; other systems use different breakpoints, pollutants and averaging periods entirely.
"You can rank the world's most polluted cities using WHO's database."
No. WHO explicitly warns against using its ambient database to rank cities or make direct country comparisons, since monitoring density and methodology differ too much for that use.
Frequently asked questions
Is AQI 100 equally unhealthy everywhere?
No. Index formulas, breakpoints, pollutants and averaging periods differ. In the U.S., 100 generally corresponds to the national standard for the pollutant; another system may not use that scale.
Can I convert one country's AQI to another?
Only if you know the underlying pollutant concentration and averaging period, then apply the target system's official breakpoints. A number-to-number shortcut can be wrong.
Should I use an annual city average for today's plans?
No. Annual data describes long-term conditions; use official current observations and forecasts for today's itinerary.
What should a sensitive traveler do?
Follow the local authority's health advice and personal clinical plan. Reduce exposure or exertion when advised, and seek medical help for symptoms.
Methodology
We reopened EPA, EEA and WHO materials on 15 August 2026. We compared index architecture and intended use. We did not create a universal numeric conversion because the sources do not support one. WHO Version 8.0, released in June 2026, was treated as an annual ambient database, not a live AQI feed.
Sources
- U.S. EPA AQI explanation
- AirNow current conditions
- European Air Quality Index methodology
- WHO Ambient Air Quality Database
Planning around a park visit? See our matrix on which U.S. national parks require timed entry in 2026.
Air quality checked. Now check whether your next hotel has a lower wholesale rate.
Check a hotel →Published August 15, 2026. Based on U.S. EPA, European Environment Agency and World Health Organization materials reviewed on August 15, 2026. Air-quality index systems, breakpoints and forecasts vary and can change; check the destination authority's current reading before you travel.