How we rank cities

Correct beats complete: a city is ranked only when enough good measurements back its number.

The pollutant

PM2.5: particles under 2.5 micrometres, in micrograms per cubic metre (µg/m³). We compare against the World Health Organization's 2021 guidelines (15 µg/m³ for a 24-hour mean, 5 µg/m³ for a yearly mean) and colour values by the US EPA's PM2.5 categories (2024):

Good: 0–9.0 µg/m³
Moderate: 9.1–35.4 µg/m³
Unhealthy for sensitive groups: 35.5–55.4 µg/m³
Unhealthy: 55.5–125.4 µg/m³
Very unhealthy: 125.5–225.4 µg/m³
Hazardous: 225.5 and above µg/m³

Which monitors count

Reference monitors first. Government and research stations with regulatory-grade instruments. When a city has any, only they set its value.
Low-cost sensors count only in cities with no reference monitor and at least three sensors, with two or more reporting in an hour. These cities carry a "low-cost sensors" label; such sensors can read high in humid air, and one placed indoors reads low, so low-cost cities are never shown as the cleanest.
Only data we're allowed to use. We use a monitor only when its publisher's licence allows commercial use and redistribution without share-alike terms. Some large national networks don't publish a licence we can rely on, so some countries are missing or thin for now. We would rather leave a city out than show data we may not use.
Mobile sensors and stations silent for 30 days are left out. Readings below zero or above 1,500 µg/m³ are treated as faults, and a monitor that reports the exact same value 24 times in a row is treated as frozen and left out until it changes.

What a city is

Each monitor belongs to the nearest city from GeoNames' list of places with 15,000 or more people. A large city takes in its boroughs and suburbs within its reach (about 4 km for a town, up to 25 km for the largest cities), but never across a state or province line. Monitors far from any such place aren't ranked.

From readings to a ranking

Each hour: the city's value is the median of its monitors. With three or more monitors, a reading over three times the others' median (and 50 µg/m³ above it) is dropped, so one faulty sensor can't make a city the world's worst.
Each day: the mean of the city's hourly values over its own local calendar day, if at least 18 of the 24 hours were measured.
Boards: the last 24 hours needs 18 measured hours; 7 days needs 5 measured days; 30 days needs 21; the year to date needs 70% of days. Ranks are worst first; equal values share a rank.
Past periods: each day, week, month and year gets a board once it ends (a day ends when every time zone has finished it). Weeks, months and years need measurements on 70% of their days.
History before a city's first hourly measurement comes from OpenAQ's daily averages per monitor (days with 75% of hours measured), combined with the same rules.

"Why it ranks here"

City pages compare recent days with the city's own past year: a week far above its typical day, a typical level far above the WHO guideline, or a clear seasonal peak. These are patterns in the numbers. When we name a cause, it's the usual cause for that pattern (smoke, dust, heating season), labelled as likely, not a confirmed source.

Limits

A monitor measures the air where it stands; a city with one monitor next to a busy road may look worse than its average resident's air.
Coverage is uneven: the United States, Europe and Japan have many usable monitors; much of Asia, Africa and Latin America has few we can use yet.
Providers sometimes revise data after publication; our past boards use what was published when they were built, and history refreshes may change them slightly.

Nothing on SmogRank is medical advice. For health guidance during bad air, follow your local health authority.