﻿122 
  

  

  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  194 
  3 
  

  

  The 
  seasonal 
  and 
  geographic 
  differences 
  in 
  antirachitic 
  effect 
  are 
  

   more 
  impressive 
  when 
  one 
  considers 
  the 
  actual 
  exposure 
  to 
  sunlight 
  

   necessary 
  to 
  protect 
  a 
  child 
  against 
  rickets. 
  This 
  figure 
  is 
  not 
  easy 
  

   to 
  arrive 
  at, 
  since 
  it 
  is 
  difficult 
  to 
  control 
  the 
  factors 
  in 
  any 
  single 
  

   direct 
  experiment 
  with 
  sufficient 
  accuracy. 
  However, 
  there 
  is 
  indirect 
  

   evidence 
  from 
  which 
  we 
  may 
  arrive 
  at 
  15 
  minutes' 
  exposure 
  per 
  day 
  

   of 
  a 
  nude 
  child 
  to 
  zenith 
  clear-day 
  sunlight 
  under 
  tropical 
  (2.0 
  milli- 
  

   meters 
  ozone) 
  conditions 
  as 
  entirely 
  adequate 
  to 
  protect 
  against 
  

   rickets, 
  even 
  in 
  the 
  absence 
  of 
  other 
  vitamin-D 
  intake. 
  A 
  Negro 
  

  

  2900 
  

  

  2950 
  

  

  3100 
  

  

  3IS0 
  

  

  Figuke 
  9. 
  — 
  Product 
  curves 
  of 
  solar 
  spectral 
  energy 
  by 
  antirachitic 
  efficiency. 
  (2.0 
  

   mm. 
  ozone 
  in 
  zenith 
  atmosphere.) 
  

  

  child 
  may 
  require 
  somewhat 
  greater 
  exposure 
  because 
  of 
  loss 
  of 
  radia- 
  

   tion 
  in 
  the 
  skin 
  pigment, 
  although 
  this 
  is 
  by 
  no 
  means 
  demonstrated. 
  

   If 
  we 
  use 
  the 
  above 
  figure 
  of 
  15 
  minutes 
  per 
  day, 
  the 
  scale 
  is 
  immedi- 
  

   ately 
  set 
  for 
  figures 
  11 
  through 
  19. 
  Thus 
  for 
  sun 
  in 
  the 
  zenith 
  and 
  2.8 
  

   millimeters 
  ozone 
  in 
  the 
  atmosphere, 
  27 
  minutes 
  per 
  day 
  (clear 
  sky) 
  

   would 
  be 
  required. 
  For 
  midwinter 
  conditions 
  at 
  the 
  higher 
  latitudes 
  

   the 
  exposures 
  become 
  so 
  long 
  as 
  to 
  be 
  entirely 
  out 
  of 
  the 
  question. 
  

   For 
  example, 
  in 
  midwinter 
  at 
  43° 
  latitude 
  with 
  2.8 
  millimeters 
  ozone 
  

   in 
  the 
  zenith 
  atmosphere 
  the 
  exposure 
  required 
  even 
  at 
  noon 
  on 
  a 
  

   clear 
  day 
  would 
  be 
  66 
  times 
  15 
  minutes, 
  or 
  more 
  than 
  16 
  hours, 
  and 
  

   even 
  with 
  only 
  2.0 
  millimeters 
  of 
  ozone 
  in 
  the 
  atmosphere 
  the 
  expo- 
  

  

  