Where the air is actually being measured
Every marker is a physical monitoring station. There is no smoothed surface between them, because interpolating a handful of valley monitors across a country would draw a measurement network that does not exist.
How to read these numbers
A station reading is a measurement at that station, not at your street. The distance is always shown for that reason. Where a station has published enough hours to satisfy the US EPA averaging method, an AQI is calculated and labelled with the standard used. Where it has not, the latest concentration is shown as a concentration — a single instantaneous PM2.5 reading is not a daily AQI, and is never presented as one.
A station with no recent measurement shows its last observation and its age. An area with no station says so. Neither is shown as zero: missing data is missing, not clean air.