Coal-fired power plants release sulfur dioxide and nitrogen oxides as gases. Over hours to days those gases react in the atmosphere and become fine particles — the PM2.5 people actually breathe — drifting hundreds of kilometres on the way. Henneman et al. 2023 estimates that ~75% of U.S. deaths attributable to coal PM2.5 occur outside the state the plant sits in. A map of where the pollution starts (the source map) tells you where the problem begins. It cannot tell you who ends up affected. That takes a source–receptor model: a pre-computed table that links every emitting location to the concentration it adds at every location downwind.
A box drawn round the Ohio River Valley — its edges reach as far as Maryland and New York.