The Major Cities Ranking (or just 'Ranking', as seen on our app) is made up of 110 major cities and includes some of the largest and most well-known metropolises around world ordered according to AQI (Air Quality Index). The data used to generate the ranking is aggregated from thousands of regulatory air quality monitoring stations and low-cost air quality sensors operated by governmental bodies, research institutions, non-profit organizations, companies, and citizen scientists. The data shown for each major city is an average (median) value from all the stations in that city.

The goal of the major cities ranking is to provide a simple snapshot of the real-time air quality conditions of the most popular urban hubs. Comparing the air quality of only major cities rather than an exhaustive list of all cities provides a concise, meaningful and eye-opening comparison of air quality in urban centers across the globe.

The immediacy of the ranking is able to highlight the impact of events like wildfires, biomass burning, industrial activity, sandstorms and inversion weather events on the air quality in a city.

The ranking can be accessed either on the website or air quality app (Android or iOS) and updates at least once an hour as new air quality monitoring station data becomes available.