tldr: "The air becomes cleaner in areas where the economy slumps. The researchers estimate that this cleaner air accounts for more than one-third of the mortality reduction."

From the paper: "Recession-induced mortality declines are driven primarily by external effects of reduced aggregate economic activity on mortality, and recession-induced reductions in air pollution appear to be a quantitatively important mechanism" [1]

[1] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4717244