I've worn a mask every hike I've taken this summer when passing (or getting passed by) others. No CO2 poisoning, and no viral infection either!
But were you blinded to the fact that you were wearing a mask? And only the summer? Could be a placebo.
Without this necessary data carried out in rigorous studies for years, we will never truly know if masks stop droplets of spit that come out of your mouth. I choose only to believe my definition of science.