Change them at regular intervals, or, in extra heavy traffic, put the recirculator on.
It'll make the air stuffy, but will keep it cleaner than it would have been otherwise. Don't keep it on for too long though, as the CO2 levels will start to rise and you might begin to lose concentration.
There also needs to be better emission standards on the other end of the vehicle as well. I've been riding a motorbike, and I hate getting stuck at red lights. I lost my fask mask with an N99, but should we be pumping that much pollution out in the first place?
What percentage of the air in the cabin actually passes through the cabin filter? Cabin filters are too small to filter the amount of air required for more than one human, let alone a packed car with 5 or more. Air gets in through seams and gaps and cars aren't very airtight.
Change them at regular intervals, or, in extra heavy traffic, put the recirculator on.
It'll make the air stuffy, but will keep it cleaner than it would have been otherwise. Don't keep it on for too long though, as the CO2 levels will start to rise and you might begin to lose concentration.