This paper examines the emergence of Black-White disparities in exposure to air pollution from fossil-fuel power plants in the United States over the past century. We combine newly digitized data on annual power plant fuel consumption, emissions factors, and an atmospheric transport model to estimate county-level PM2.5 exposure over 1940-2020. We show that the exposure gap emerged in the 1950s, widened until 1980, and declined thereafter. To understand these patterns, we combine data on power plant openings (1900-2020) with U.S. Census demographics (1870-2020). We find little evidence that plants were disproportionately sited in counties with larger Black populations. Instead, event study estimates reveal that Black population shares increased by an average of 4 percentage points in the 50-70 years following a county's first plant opening. These shifts are concentrated in counties that first hosted a plant between 1900 and 1949 and reflect the contributions of the Great Migration, differential sorting in response to pollution, and other factors. Our findings highlight that the equity implications of siting long-lived infrastructure can differ dramatically depending on the time span considered.