I have allergic rhinitis and nasal polyps. would it be bad for me to take a job that required spending 4mo of each year in a polluted region of china?

Maybe (not) . . .: Granted, it's best to avoid polluted areas whenever possible, but perhaps that area of china won't have allergens to which you are allergic such that your allergies might improve w/your move. Unfortunately, it's all theoretical conjecture. You'll need to decide for yourself whether the experience & remuneration is worth the physical & emotional toll of living in a distant country w/foreign culture.

No sure way.: Living in a polluted area is never a good idea regardless of your health status. Since I do not know the cause for your nasal polyps (chronic sinusitis, aspirin-exacerbated rhinosinusitis etc), i can only venture that pollution is unlikely to be good for you since it may slow down the movement of the hair-like structure at your nose and sinuses ever further and make your condition worse.