This visualization shows the Air Quality Index (AQI) across American counties using a choropleth map, with each county colored on a quantile scale from green to red based on PM2.5 concentrations. The map is rendered as SVG using React and d3.geoAlbersUsa projection, with d3.geoPath drawing county boundaries from TopoJSON data. County-level pollutant data is loaded from a CSV file, and counties with missing data appear in grey. The color scale d3.scaleQuantile maps AQI values to a eight-color gradient, with d3.geoMesh rendering state interiors.