Air quality guides
How our air pollution ranking is calculated
The PM2.5 ranking compares available cities by one pollutant. It helps identify higher and lower concentrations in that dataset. It is not an overall environmental score or a long-term quality-of-life assessment.
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Which cities are included?
We select directory cities with a numeric, non-negative PM2.5 concentration and a date. We find the latest date in that set and compare only cities carrying that date. Cities with older values remain in the directory but are excluded from this comparison. The latest date need not be today, so we display it beside the ranking.
How is the order determined?
The most-polluted list runs from higher to lower PM2.5; the cleanest list runs from lower to higher. We sort values before rounding them for display. Two numbers that look identical may differ slightly in the source. A tie does not imply a meaningful air-quality difference between adjacent positions.
A shared date is not a simultaneous observation
Card times are local. The comparison can therefore include different hours and time zones even though entries share a local date. It compares the latest available hourly values for that date, not daily averages. Positions can change after an update.
What does “cleanest” mean here?
It means the lowest PM2.5 among the included cities. It does not mean the lowest NO₂, ozone or every pollutant at once. The ranking does not cover every settlement worldwide or measure residents’ personal exposure. Open a city page to inspect its other pollutants, timestamp and recent days.
What about air and sea temperatures?
The homepage compares daytime air temperature and sea surface temperature separately. Each group uses one shared date: today if available, otherwise the most recent earlier date, or the first future date if only forecasts are available. A point represents a country or selected coastal location. These values are not averages over an entire country or body of water.