This choropleth map visualizes PM2.5 pollution levels across U.S. counties from 2000 to 2018, with each county colored on a sequential YlGnBu scale. Users can scrub through years via a vertical slider to animate changes over time. The map uses React to manage state and render SVG paths generated with `d3.geoPath` and a `geoIdentity` projection, with county boundaries drawn from the us-atlas TopoJSON. Data is loaded from a CSV of county-level pollutant measurements, and the animation uses a timed `setInterval` loop to advance the year automatically.