Mite sensitization in the Scandinavian countries and factors influencing exposure levels

Mites are the common sources of indoor allergen and play a major role in sensitization and elicitation of allergic disease. In the Scandinavian countries, mite infestations in the home were not common in the past decades. Recent studies show that sensitization to mites among children, particularly in Sweden, is increasing. Similar trends have also been reported in Norway. Poor indoor climate, e.g., high humidity and poor ventilation as a consequence of energy-saving measures, are cited as a possible explanation of this increase. Modern furnishings, e.g., carpets and various kinds of upholstery, may also serve as reservoirs of indoor allergens. At least 2 microg of mite allergen per g of dust is considered to be a risk level for sensitization and symptoms of asthma. As compared to experience from other parts of the world, mite-allergen levels are generally low in the northern Scandinavian regions. Recent studies from Sweden and other European countries show that mite sensitization may occur below 2 microg/g dust. Various environmental adjuvant factors may affect the threshold levels of allergen. In this review, the importance of the indoor environment for the accumulation of mite allergens, sensitization to mites in the Scandinavian countries, and various environmental factors that could influence exposure levels will be discussed.