Summary It is shown by regression analysis that the risk of immediate hypersensitivity to birch pollen, as measured by a skin test, is positively correlated with the quantity of birch male flowers and the number of non‐rainy days in the first birch flowering season (May) met in infancy. The risk is also dependent on the month of birth, with maximal risk associated with birth in Februarys–April. The results suggest that early pollen contacts, particularly during the first 6 months of life, increase the risk of pollen allergy for a period of 20 years. We estimate that elimination of early birch pollen contacts could have reduced the prevalence of immediate hypersensitivity to birch by 28% in our age group 0–19 years.

We studied the prevalence of allergic disorders in an unselected group of 708 adolescents aged 15-17 years. All subjects were physically examined and interviewed by the authors. The prevalence of past or present asthma was 5.7% in boys and 3.1% in girls. The figures for hay fever were 14% and 8%, and for atopic dermatitis (including allergic urticaria) 25% and 30%, respectively. In 24% of all symptomatic subjects, the condition had not been active during the year preceding the study. The sex difference in the prevalence of hay fever was significant. It is associated with higher immediate skin test reactivity in boys. A progressive increase in the frequency of allergic disorders was observed with increasing number of positive skin reactions in both boys and girls. Respiratory allergy was closely related to a positive skin test: 87% of the asthmatics and 83% of all those with allergic rhinitis exhibited at least one positive skin reaction. For atopic dermatitis the association was less pronounced. Nineteen per cent of the population studied had a positive symptom history and a positive skin test to pollens, animal epithelia or dusts indicating a clinically significant relationship. However, 39% of the 346 subjects with a positive skin test, including some with a large number of positive reactions, were completely asymptomatic.

We studied 708 adolescents aged 15-17 years in the 9th grade of school in Imatra. Of the eligible population born in 1962 77% were included. All were skin prick tested with 16 extracts from two manufacturers with 12 common allergens, which included pollens, epithelia, mite, house dust and fish. At least one positive, immediate reaction (weal diameter 3 mm or larger) occurred in 49% and at least two positive reactions in 43% of those studied. The boys were observed to be significantly more reactive than the girls. The allergen preparations to which positive reactions were most prevalent were house dust, cat and horse epithelium, and mite extract. Large differences in the prevalence of positive reactions were observed with different preparations of the same allergen. Pollen allergens tended to cause the largest positive weal reactions, and the weal size distribution with some pollens was distinctly bimodal. A scheme for calculating allergen potency in histamine-equivalent-prick (HEP) units is presented. It is noted that the result is greatly dependent on the population group chosen for testing.

708 unselected adolescents 15-17 years old were studied. They were physically examined, interviewed and skin prick tested with 12 common allergens. Questionnaires were distributed to the adolescents and their parents to investigate past and present symptoms and environmental influences. Emphasis was on the first year of life. Exposition to allergens (pollens and cat epithelium) during the first 6 months of life, eczema during the first year of life, and atopic heredity were found to be major risk factors in the development of positive skin tests and respiratory allergy apparent in the teenage period. Infection and wheezing during the first year of life were also risk factors, but less important.

This report describes a retrospective study of 1421 patients allergic to pollens and 728 patients allergic to animal epithelia. In males, but not in females, the month-of-birth distributiondifferedsignificantly from that in the total population of Finland (P<0 01 in pollen allergy and F<0-05 in epithelium allergy). The phenomenon was most clearly apparent in boys aged 10-14 born in 1960-4 (/'