Allergic diseases, eczema, asthma and allergic rhinitis are the most common chronic health disorders in children, 1,2 and several studies have described increasing prevalence rates over the last few decades. [3][4][5] Variations in prevalence, and changes over time, strongly indicate genetic predisposition and environmental influences. 6 While the prevalence is still increasing in lower income countries, 7,8 it seems to have plateaued in many western societies. 3,7 The incidence and prevalence of allergic diseases in Icelandic adults have been lower than in many other European countries. [9][10][11] Allergic diseases in children are more prevalent in Iceland than in neighbouring countries, despite lower concentrations of allergens in Iceland than in most European countries. 9,11 However, high, and sometimes increasing, incidences of allergic diseases have been described in the Nordic countries 12,13 and the need to diagnose asthma in adulthood has been reiterated. 14 Icelandic individuals born in 1987 have been followed by our research group for three decades 10,15-17 and repeatedly evaluated for allergic diseases, namely eczema, asthma and allergic rhinitis. Our previous studies gathered information on family histories of allergic diseases and environmental factors and carried out physical examinations and skin-prick tests and blood tests that included measuring immunoglobulin and identifying specific