This interactive map shows US county-level PM2.5 air pollution concentration changes from 2000 to 2021. A vertical slider controls the year, with a play button animating through time. The map renders with d3.geoPath and geoIdentity using SVG paths, colored by a sequential YlGnBu scale. Each county path includes a tooltip displaying the state, county name, and pollution value. The visualization updates its 975×500 SVG at each year change, with data loaded from CSV files.