People most often associate allergies with respiratory symptoms, not with headaches. When allergy symptoms flare and include headache pain, patients might suspect unrelated causes, such as viruses or vision problems. In fact, the facial and headache pain that follows allergic reactions develops from the body's natural response, an influx of histamine chemicals in the bloodstream. The action of histamine causes a number of inflammatory symptoms, which are shared by people with perennial and seasonal allergies alike. When sinus and eyelid inflammation persist, headaches may develop.
Animal
Unlike seasonal allergies, allergic reactions to animals in the indoor environment take place on a continual basis, whenever individual threshold exposure levels are reached. Pet and insect life cycles and household cleaning cause airborne allergen levels to rise and fall, so allergy symptoms won't always be present or noticeable.
Elements in dog, cat and other warm-blooded animal dander and fluids dry, enter the air and are inhaled by humans, as the Asthma and Allergy Foundation of America (AAFA) reports. People ingest the waste of cockroaches and dust mites in a similar manner. Because high allergen levels from these ever-present creatures can persist, respiratory inflammation may become strong enough to cause headaches.
Fungus
Outdoor mold fungi can provoke lengthy periods of seasonal allergies, as mold colonies reproduce in succession between spring thaws and winter freezes. Allergy patients may experience extreme symptoms after disturbing mold spores by raking or walking through decaying leaves. According to MayoClinic.com, indoor molds reproduce best in humid environments. Poor ventilation or recirculation of mold allergens can produce repeated allergic reactions that affect the sinuses. Long-term inflammation may add facial pressure and headaches to respiratory allergy symptoms.
Plant
Plants that give off large amounts of pollen at one time generate the most extreme allergic reactions, but at least these are contained within seasonal boundaries. Allergy patients who know the source of their health problems can take steps to minimize the impact of their seasonal allergies. The AAFA notes that autumn ragweed reproduction produces the majority of U.S. seasonal cases of plant allergies, and it grows in significant numbers only in the Eastern half of the country. This prolific weed can loose 1 billion pollen grains from a single plant, overwhelming patients' tolerance levels. The body's inflammatory response, intended to keep additional allergens out, may produce sinus headaches. Some trees, such as cedar and alder, also emit pollen "bombs" during late winter or spring. Because it may take weeks for viable cones to distribute their contents, allergy symptoms may linger, worsen and cause headaches. The National Institutes of Health report that allergy patients in grassy areas may get similar symptoms during the summer.
