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A friend of mine lives in a very polluted city and she gets headaches every time she gets stuck in the traffic jams, lasting easily one hour. The cars there are very old and polluting, even if the cars this friend uses is not particularly old.
I wondered if there are devices, besides normal particle/pollen filters, to clean the air getting in the cabin from common gas pollutants such as ozone and NOx in the first place. Ozone would be enough, NOx is less important.
My car features a "Recirc. Air" button which closes the external air vents and instead recycles air around the cabin. This is how I avoid getting pollutants into the car in, for example, a tunnel or heavy traffic.
It may be worth your friend checking to see if their car has this option rather than attempting to filter the pollutants at source.
Most modern cars feature a cabin air filter - it might be worth getting this cleaned or replaced if she's not done so. Othewise, as Steve says, the Recirc button is good, but only for relatively short times...
Ozone can be a respiratory irritant, but there are no effective filters suitable for automotive use, which will substantially filter ozone.
Using recirc reduces incoming pollutants. Cars are not totally airtight, and there will still be some air change. Furthermore, for allergy and asthma sufferers, recirc reduces the incoming irritants and triggers.
The cabin filters are pleated paper filters for dust and pollen, and many have a charcoal filter, but they are not very effective at reducing ozone and for that matter oxides of nitrogen. They do help a little with aromatic hydrocarbons.
Bottom line, recirc can be your friend.
