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Exposure to pets (e.g. cats, dogs, guinea pigs, horses, rabbits, mice, rats etc.) at home or at work can trigger asthma in some people. Cats and dogs are a major source of allergens in the home environment. The allergens come from the sweat glands in cats and salivary glands in dogs. All cats and dogs have sweat and salivary glands, so there are no breeds that do not contain allergens (although the amount of allergen released can vary between breeds).

Allergens stick to the hair and skin of pets and become airborne when the pet sheds their hair. This is called animal dander. Cat allergen is especially difficult to remove from houses. It can remain in the house for months after the cat is removed. Cat allergen can be found in places where cats have never lived. For example, it can be carried around on clothing to schools and offices.

The most effective method of allergen avoidance for people with asthma who are allergic to cats or dogs is removal of the pets from the home.