During the past four years a study has been made by us of the seasonal variation in the antirachitic effect of sunshine. From the results reported1 it was concluded that the sun's rays during the latter part of October, all of November, December and January and the first part of February in the latitude of Toronto produced a slight but definite antirachitic effect, which sharply increased about February 15 and continued throughout the summer at approximately eight times the winter level. A marked decrease in the antirachitic effect occurred about October 15. It was further found2 that this change in antirachitic effect is apparently largely dependent on the altitude of the sun above the horizon. When the sun is low in the sky the rays necessarily pass through a greater amount of the earth's atmosphere, and the short, biologically active ultraviolet rays are markedly diminished. A critical altitude
TISDALL FF, BROWN A. SEASONAL VARIATION IN THE ANTIRACHITIC EFFECT OF SUNSHINE. Am J Dis Child. 1931;42(5):1144–1147. doi:10.1001/archpedi.1931.01940180094015
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