strange as it sounds it come be, a lady up the road from me has had a lot of tests done on her dog for allergies, as he gets a rash on his tummy in the summer, and it is a pollen/grass allergy.
My old boy had a sinus condition - epistaxis - and due to his age and other conditions he was suffering from, we could not warrant surgery. To keep him comfortable and help his breathing the vet recommended using a menthol spray around the house, and I found one called Bio Air from stock nutrition, which we still use now, great for colds, leaves a lovely fresh smell around the house - but do use sparingly, as it is very strong, one bottle lasts us nearly 6 months, and I spray it in the bedroom every night. It is the last product on the link below.
http://www.doghealth.co.uk/hygiene.htm