A choropleth map of the United States displays county-level PM2.5 concentration for each year from 2000 to 2018, using a yellow-to-blue color scale to indicate air pollution levels. An interactive vertical slider and a play button animate through the years. Built with React and D3, it uses `d3.geoPath` for the map, `d3.interpolateRdYlBu` for the color gradient, and TopoJSON for state and county boundaries. Data comes from county PM2.5 measurements, with missing areas in grey.