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

That study was done in China which is pretty notorious for its air quality. Probably best not to run outside when the AQI is high though. https://airnow.gov/

Anecdotally I noticed a difference when running next to big, busy roads in Northern VA vs. running next to lakes (or even the same busy roads later at night when traffic was negligible). Couldn't go as hard, didn't feel like I was making as many improvements to speed/time/health, etc.