In the beginning of May each year many schoolchildren in Nordic countries suffers from a stuffy nose with sneezing, itching eyes and often tiredness. Their ability to concentrate on school work may be reduced for several weeks. The same symptoms are seen among the adult population, which may affect their work and leisure time. The reason is that the birch trees, numerous in Nordic countries, are blossoming. The trees send pollen grains into the air, and the pollens can be carried long distances by the winds and affect allergic people even far away.
Some people even get symptoms of asthma, which can result in visits to emergency rooms.
Allergic rhinitis is known from the medical literature since the beginning of the 19th century, but for a century and a half it has been a disease restricted to a very small portion of the population. Nowadays, and this is a new trend during the last three decades or so, an increasing number of people are being affected by allergy to pollens as well as to other allergens. The increase has occurred during a relatively short period of time, which tells us that factors in our environment must be mainly responsible. It has been proposed that the concentration of the population to big cities, with increased exposure to air pollutants, is a risk factor for induction of allergy. There is indeed evidence supporting the view that exposure to air pollutants involves a significant risk of developing allergy, but there are also conflicting findings.