I remember a paper that found that its worse to run near roads than to run at all, with huge amounts of absorption of contaminants into the body. Contaminants being "recorded" in the runner's hair.

Out of the blue I found this paper finding a relationship between worse performance and pollution in marathons.[1]

[1] https://mpra.ub.uni-muenchen.de/79473/1/MPRA_paper_79473.pdf