In one sense, all allergies are environmental because an allergic reaction kicks in only when you are exposed to the relevant allergen from which you suffer. If you can avoid it, then you may never suffer from an allergic reaction – or even know you have an allergy at all. That said, the development of allergies is known to have a hereditary correlation, so purely environmental explanations of their presence are not possible. In other words, you cannot explain allergies entirely by the presence of car fumes nearby, for example.