This interactive choropleth maps fine particulate matter (PM2.5) concentrations across U.S. counties from 2000 to 2021, with a slider and play button animating through years. Each county’s fill color transitions smoothly via `scaleSequential` and `interpolateYlGnBu`, while hovering reveals state and value details. Built with React and D3 v5, it uses `geoIdentity` with `geoPath`, TopoJSON from the us-atlas library, and CSV data fetched via `d3.csv`, rendering SVG paths that update with `transition` styling.