If you don't want to clog your lungs with exhaust fumes while biking, you have three choices: persuade everyone to stop driving (difficult or downright impossible), avoid riding on or near busy roads (rather problematic if you need to get to work or to shops), or put a filter on your air intake. After a month of terrible bronchitis, I chose the third one.
These masks are produced by a British company Respro. They are designed for sports: unlike industrial protective masks, they are light and built for heavy breathing.
Outer skin of the mask is made of neoprene and fastens behind your head with velcro. Concave shape around your nose is kept by a strip of aluminium (which would break if bent repeatedly, so don't drop heavy things on your mask). There are many colours and patterns available including all-reflective Scotchlite, I chose black. There is also a choice of sizes, just measure several dimensions of your face and the manufacturer selects the best one for you.
There are several types of filters. Sportsta is HEPA for dust, pollen and other solid microparticles, City is activated charcoal for chemicals, Techno is a combination of both. I chose the Techno to be sure. There is some resistance when inhaling, but far from choking. The filter sits directly on your face; contact is maintained by the flexible outer skin and the vacuum created by your inhaling, so there's a caveat: the mask seals (and therefore works properly) only if you breathe heavily during a physical activity. If you breathe calmly (when walking or sitting down), it is useless. New filter is stiff and doesn't seal well, but it softens after first half hour of use and then it's OK. My goatie beard doesn't spoil the seal, but I can't guarantee that for any other type of facial hair.
Exhaled air doesn't sift back through the filter, instead it goes through two one-way valves (a plastic cage with rubber membrane which screws into a hole in the filter and pops through another hole in the neoprene skin). What doesn't escape through the valves, escapes around the edges of the mask. I don't feel any resistance with Techno valves, so I don't think the "high performance" Powa valves would be worth the money.
Now the most important question: what things the mask can block and what is the lifetime of one filter? The activated charcoal absorbs pretty much all odours including petrol or cigarette smoke, but it only stays in this condition for several days, then you start smelling them again, just weaker. The HEPA layer should, according to the manufacturer, capture all solid particles including those smaller than 1 µm. According to my practical experience (uphill starts in clouds of smoke behind buses and lorries), it blocks so much diesel soot that no exhaust makes me cough or choke anymore. The filter is also supposed to block coronaviruses and other horrors, but because they can also infiltrate your body through eyes, the protection is not perfect. And the exhaled air is not filtered at all, so protection of other people from you is zero.
With my intensity of use (I ride in Prague about 50 km per week, wear the mask since autumn till spring, mostly just in the morning for uphill sprints during rush hour, sometimes in the evening too if the smog gets really nasty in winter), I run through about one and half filter per year, average. I guess the replacement date from how black the filter is and how does it smell, I've never got to a point where it would clog or stop filtering.