Inhalant allergy (also known as "atopy") is an allergic reaction caused by breathing in airborne particles such as pollens, dust, and moulds. Humans typically react to inhaled allergies by sneezing and wheezing, however, cats are more likely to develop skin problems.
There are four types of allergies in cats, food, contact, inhalant and insect (particularly fleas, which is known as flea allergy dermatitis). Inhalant allergy is the third most common allergy to affect cats. Allergies are the result of the cat's own immune system reacting to a usually harmless substance (food, chemical, pollen etc), and mounting a response.
What are the symptoms of inhalant allergy in cats?
Symptoms can be seasonal or nonseasonal depending on the allergen, they often begin in young cats, worsening with age.
- Miliary dermatitis (red and crusty rash around the head, neck, and back).
- Eosinophilic granuloma complex.
- Head, ear and neck pruritus (intense itching).
- Facial crusty, scabby lesions.
- Symmetrical alopecia (hair loss).
- In some cases, your cat may bite and chew the itchy skin enough to cause damage to the skin, resulting in a secondary bacterial infection.
How is inhalant allergy diagnosed?
Your veterinarian will obtain a complete medical history from you and perform a physical examination. He will want to rule out other possible conditions such as mange, flea allergy, contact dermatitis, ringworm and food allergy.
- Skin scrapings and fungal cultures to rule out some of the conditions above.
- Intradermal skin test: A small amount of allergen is injected through the skin, which is then observed to determine if elicit an allergic reaction. Certain medications such as antihistamines may interfere with results.
- Blood test: This tests for antibodies to specific antigens in the blood. There are two tests used in cats, radioallergosorbent test (RAST) and enzyme-linked immunosorbent assay (ELISA). 
Treatment of inhalant allergy:
Avoiding the cause of the allergy if possible is the ideal method to treat inhalant allergy but is often not possible because of an inability to identify the allergen or halt exposure.
- Hyposensitisation (allergy shots/allergy desensitization). This involves injecting gradually greater amounts of the allergen in question in the hope that it will re-programme the immune system so it's not hypersensitive to the antigen.
- Corticosteroids are beneficial in relieving symptoms but are not a cure.
- Essential fatty acid supplements have also been effective in some cases of inhalant allergy.
- If an infection has occurred, antibiotics will be prescribed.