A choropleth map of the United States displays county-level PM2.5 air pollution data across years, with a vertical slider and play button to animate through time from 2000 to 2021. Built with React and D3 v5, the visualization uses d3.geoPath with a geoIdentity projection to render county boundaries and a sequential YlGnBu color scale to encode particulate concentrations, allowing users to observe spatial and temporal pollution trends.