Try a 3m 4277 combined gas and vapour particulate respirator. With full daily use it may last a month. Its exhausted if you can smell any fumes. If your lucky you might pick them up on ebay as I did for a tenner. £20 plus otherwise. What price a pair of lungs though? If you go the ebay route check the expiry date.
From the medical point of view, P2 (or FFP2) will keep out TB, but if it's a bird flu outbreak you need FFP3. So I guess that means there will be lots of welders living through the flu epidemic if it ever happens.