Official AQI comes from reference-grade government monitors — accurate, calibrated, but sparse. Low-cost sensor networks fill the gaps with far denser coverage at the cost of precision, especially in high humidity, where uncorrected particle sensors tend to read high.
Two stations a few kilometers apart can legitimately disagree: pollution is local. A monitor beside a freeway sees more NO2 and PM than a park monitor in the same city, and a valley can trap pollution that a nearby hilltop never sees.
AirCheck reads each station’s latest measurement, converts the concentration to AQI using the standard breakpoints, and reports the worst pollutant per location. Where networks overlap we prefer reference-grade data and label the source so you can judge confidence.
Readings are timestamped; a value is only as current as its last update. A station that has gone quiet is shown as stale rather than silently carried forward, so an old reading never masquerades as live air.