What is the air quality where you are?

You choose a place; our server asks the service of that country and shows what it answers. No value is recalculated, rescaled or smoothed on the way. What the tool does add is the name of the pollutant that drives the index, and the distance to the station or reporting area the reading belongs to.

• In the 38 countries of the European Environment Agency, the index is read on its hourly mosaic, whose cells are 1 km across (the service reports a 1 000 m pixel in EPSG:3035, checked on 19 September 2026). The four sub-indices — fine particles, coarse particles, nitrogen dioxide, ozone — are read at the same time.
• In the United States, Mexico and Puerto Rico, the US AQI comes from the AirNow reporting area nearest to your place, taking the worst pollutant measured today — never a forecast.
• In Canada, the AQHI comes from the nearest Environment and Climate Change Canada station, within 100 km. Beyond that distance nothing is shown, because a reading taken two hundred kilometres away is not your air.

This is the part most air quality pages get wrong. The three indices do not share a range, a list of pollutants, or even a way of combining them. The European and American indices take the worst pollutant; the Canadian one adds the risks of three pollutants together, so it can rise while every single pollutant stays moderate. A conversion table between them would be a fiction, however confident it looked.

So no such function exists anywhere in this tool, and an automated test checks that none appears later: it scans the names the module exports and fails on anything that looks like a conversion, a normalisation or a unification. The scale is printed under the number instead, in every language, on every reading.

• European index (EAQI), 1 to 6, from good to extremely poor: the worst of the four sub-indices. An index of 3 means one pollutant reached level 3, not that all four did.
• US AQI, 0 to 500, with 100 set at the national standard: also the worst pollutant, but on a scale built pollutant by pollutant, so the same air can score very differently from the European one.
• Canadian AQHI, 1 to 10 and above: a health risk, not a concentration. It adds the contributions of nitrogen dioxide, ozone and fine particles, which is why it has no single driving pollutant to name.

These are the readings kept as test fixtures for this tool, so the page and the code are describing the same three answers.

• Paris, 18 September 2026: European index 3, moderate. Fine particles 2, coarse particles 2, ozone 1, nitrogen dioxide 3 — the whole of the 3 comes from traffic, and the ozone that dominates a summer afternoon is at its lowest level.
• Houston: US AQI 115, driven by ozone. Fine particles stood at 55 and coarse particles at 20 the same day; the area's index is the worst of the three, not their average of 63.
• London: the European mosaic answers NoData. The tool reports a country outside the agency's coverage, and says so in those words — it does not print a blank index that would read as clean air.

Air quality indices are published by national agencies, each under its own terms. This tool reads only those whose licence allows a site like this one to reuse them, which is why the map of what it can show is not the map of the world. Outside the countries listed above nothing is requested at all — the tool says the country is not covered rather than fetching data it has no right to show.

The United Kingdom is the clearest example of the difference between the two. Asked for London, the European mosaic answers NoData — not because the air is unknown there, but because the agency that publishes the mosaic has member countries, and the United Kingdom is no longer one of them. The tool reports that as an absence of coverage, never as an absence of pollution.

• Country not covered: no service is read there. The tool says so and points you to your national service, because one certainly exists.
• Covered but no value: the service answered and has nothing for that exact point — too far from a station, or a cell its mosaic leaves empty.
• Outage: the service is down. Nothing is shown rather than a stale number that would look current, and the message says it is an outage and not an absence of data.

An index is a summary, and a summary loses things. It is not a concentration: two cities on the same index number can be breathing different mixtures. It is not your street either — a European cell covers a square kilometre, an American reporting area covers a metropolitan region, and a Canadian station can be the only one for a whole valley. A busy junction can be far worse than the index of the district around it.

• The American readings are preliminary: the agencies that produce them have not validated them, and they may be corrected later. The page says so whenever it shows one.
• The three services publish hourly, so the value can be up to an hour behind the air outside. The time of the reading is shown next to it, in the time zone of the place you picked.
• Health advice belongs to the agency that publishes the index, not to us. If the index is high where you are, the recommendations of your national service are the ones to follow.