Cool, the study was done at the University of North Sumatra.

From the Abstract/ Conclusion: "All three type of masks such as surgical mask, N95 mask, and carbon mask was effective to reduce CO levels from air pollution with the most significant was N95 and carbon mask. Fabric mask has the poorest protection from CO levels."

I'm surprised at the N95 being effective. And expect the best results would come from a fresh, well fitted, carbon filter. As for myself, I do my best to ride where the motor vehicles are not present. Our town, like many, has a decent bike/ walking trail system.