Hay fever is caused by an allergic reaction to pollen from trees, grasses and plants. When these pollens come into contact with the mucous membranes of the eyes, nose or throat, the immune system overreacts and releases histamine. This substance causes inflammation and irritation, which leads to the typical hay fever symptoms. Because pollens are microscopic and easily spread by the wind, it is almost impossible to completely avoid exposure. The hay fever season often starts in early spring with pollens from trees such as birch, alder and hazel. Grass pollens follow later in the year, which usually cause the most severe reactions. Some people also experience symptoms in late summer and early autumn as a result of herb pollens, such as mugwort and ragweed.

Heredity plays an important role in hay fever. If one of the parents has hay fever, there is a good chance that a child will also be sensitive to pollen. If both parents have hay fever, the chance is even greater. In addition, external factors, such as air pollution, can increase the risk of allergies. The weather conditions also affect the severity of the symptoms. Dry, windy days cause a higher concentration of pollen in the air, while rain washes the pollen out of the air and can provide temporary relief.