If your dog has skin allergies it is usually something in your environment.

Toxic allergens are everywhere - are they making your dog sick and causing dog allergies?

Just as fragrance filled detergents, soaps and floor cleaners can make a human sneeze, a dog, with his much more sensitive nose can also cause dog allergies. 

if your dog licks the floor he might be getting toxins on his feet then when he licks his feet it is ingested. If food is dropped on the floor of the kitchen and your dog is faster than you are getting to it, he might also be ingesting whatever cleaner is on the floor.

What about outdoors where your dog plays? Do you spray the grass with weed killers? Do you apply other chemicals to the areas around the house? These are all toxic to your dog.

Be careful during wet weather. Old pesticides can be reactivated when it rains. Read more about pesticides

The most common type of dog allergies is the inhalant type. Dogs may be allergic to all of the same inhaled allergens that affect humans. These include tree pollens, grass pollens, ragweed pollens, molds, mildew, and the house dust mite. Many of these allergies occur seasonally, such as ragweed, tree, and grass pollens. Others are with us all the time, such as molds, mildew, and house dust mites. The dog's reaction usually produces severe, generalized itching. In fact, the most common cause of itching in the dog is something your dog has inhaled. Watch out for your dog and keep household allergens in mind if your dog looks a little under the weather. By all means, get your dog to the vet once a year!