This interactive choropleth displays PM2.5 air pollution concentrations across US counties, with a color gradient from light to dark blue indicating higher values, alongside a slider to animate through years 2018–2021. Built with React and D3 v5, it uses `d3.geoPath` with a `geoIdentity` projection to render county and state boundaries from TopoJSON, while `d3.scaleSequential` with `interpolateBlues` maps data values to fill colors. A play button triggers a JavaScript interval that advances the year, updating the rendered SVG paths and the static color legend.