This interactive choropleth map shows annual PM2.5 air pollution levels across U.S. counties from 2000 to 2021, with a slider and play button animating through years. County polygons are colored using d3.scaleSequential with a reversed YlGnBu interpolator, while uncolored areas appear grey. The visualization uses React hooks for state management, d3.csv for data loading, topojson.feature for county and state boundaries, and d3.geoPath with geoIdentity projection for rendering SVG paths. State borders are drawn as a separate mesh layer, and tooltips display county names and values.