They must have some effect, inasmuch as cyclists and motorbike drivers in highly polluted cities like Kathmandu and Delhi wear at least a cloth over their mouths for their own comfort.
Try cycling in Delhi or Kathmandu without a mask and then tell me a face covering doesn’t do anything but a placebo effect. Yes, without a high-grade mask people are still breathing in dangerous fumes, but putting something, anything over your face will still block certain larger particulates that would otherwise leave you choking and angry at the whole experience.
(A small detour: In Japan you can buy traffic safety charms that are reflective. They might work for pedestrians and people on bicycles - the only charms I've seen so far that look usable.)
