The data is collected and verified by AirVisual. Where official monitoring data is not available or is not complete, AirVisual collects air quality data from consumer-grade air quality sensors. While readings have been validated against high-grade air quality monitoring stations in multiple locations, reliance on consumer-grade sensors entails a higher degree of measurement error. Greenpeace understands the limitation that the rankings are subject to measurement error and so they are indicative, not absolute.

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