The US Air Quality Index is a compression algorithm. It takes five pollutants — particulate matter, ground-level ozone, carbon monoxide, sulphur dioxide and nitrogen dioxide — measured in different units on different scales, and returns a single integer. Compression loses information. That is the point of it, and it is also the problem.
ZEPHIRA prints that integer next to the fare because the two questions belong on the same screen. What will it cost, and what will it feel like to breathe. The industry has answered the first for eighty years and left the second to chance.
The scale we publish runs in six bands. Up to 50 is Good. From 51 to 100 is Moderate, which for most people is an ordinary day. From 101 the index turns to Unhealthy for sensitive groups — and if you are asthmatic, pregnant, elderly, or travelling with a small child, that band is written about you. Above 150 the qualifier drops away and it is simply Unhealthy, for everyone. Then Very unhealthy. Then Hazardous.
Three things about that scale are worth knowing before you use it to decide anything.
It is not linear the way a price is. The gap between 40 and 60 crosses a boundary and changes the advice; the gap between 260 and 280 changes nothing. Read the word, not only the number.
It is a moment, not a month. Air moves. A city can spend a fortnight in the low tens and a Tuesday in the high hundreds because the wind turned, and a monthly average would have hidden both. Everything here carries the time it was read, and we would rather show you a stale reading labelled as stale than a fresh-looking one we made up.
It describes where the sensor is, not where you will be. A reading at an airport on the edge of a city is not a reading in the middle of one. We say which point we scored, and on a connecting itinerary we score the worst point on it rather than the most flattering — a clean arrival does not undo four hours somewhere else.
None of this makes the number less useful. It makes it honest, which is the only thing that makes it useful. Colour alone will never carry enough, which is why every figure on this site ships with an integer, a set of notches and a word — and why, where we have no measurement, we print Not scored instead of a guess.