86 WORLD-POWER AND EVOLUTION 



due to the season at which the weather happens to be dry. If we 

 compare the dry Januaries with the wet Januaries of the same 

 temperature at specific places such as New York, Chicago, 

 San Francisco, or elsewhere, we find that the dry are systemati- 

 cally less healthful. The same is true if we compare the wet and 

 dry Februaries or Marches. Neither can the wind be the cause 

 of the many deaths, for when we compare the most windy Jan- 

 uaries or Februaries and so forth with the least windy, we find 

 by no means such a difference as when we compare wet and dry. 

 The only satisfactory explanation of the harmful effect of dry 

 air in winter seems to be that our houses are heated. Southern 

 Italy and southern Japan, the two places where dry winter air 

 does no harm, are also the only two where the houses are not 

 heated in winter. At temperatures from 65" to 70° such as pre- 

 vail in our houses in winter, dry air is harmful, as is clearly 

 evident in most of the diagrams. In Figure 14, which represents 

 the dry interior of the United States, a relative humidity of less 

 than 30 per cent increases the deathrate by at least 10 per cent 

 above what prevails when the relative humidity is 70 per cent or 

 higher. In winter, when we heat the outside air from near the 

 freezing point to about 70°, we give our houses a climate as dry 

 as that of the driest parts of the continental interior. Such 

 dryness there in summer causes an increase of at least 10 per cent 

 in the deathrate, as appears in Figure 14. It causes similar harm 

 to health elsewhere in winter, as appears in the lower parts of 

 most of our climographs. Apparently during the long processes 

 of man's evolution he has become adapted not only to certain 

 strictly limited conditions of temperature, but also to a relatively 

 high degree of humidity. This does not complete the tale of 

 climatic adjustments, but it shows how extraordinarily delicate 

 is man's adjustment to the air in which he lives. It proves that 

 "air is the first necessity of life," and also that changes in the 

 air must be the most potent causes of changes in human health. 



