I don't know why this never occurs to people but it can save you much grief. You want one that can do a massive air exchange if you can find one. Even if you only get 8-12 hours a day of clean air it will give your body time to recover so your immune system can deal with the time away from home. My son had SEVERE allergies after moving to Texas and developed allergies to everything he ate, drank, breathed because his poor little 3-year old immune system just could not take it. He had immediate relief from much of the nasal/head stuff and within a few months of filtering the air he lost all his food allergies and had much less of a reaction to the outdoor stuff even though he spent just as much time outdoors as before. This was 12 years ago so I do not have a good current air purifier to recommend but it made a HUGE difference for him.
New homes rarely have 'clean air', they build them so air tight now it is usually the most polluted air you will encounter. Even if you can only really filter your bedroom WELL it will give you time during sleep of pure fresh air to recover.
Also, try oil pulling, great detox that costs very little (check out the forum). Be sure to saltwater rinse and use some up the nose afterwards, really helps stave off the mucous.
Sorry for your suffering!
D.