322 VENTILATION 



circulation, respiration and metabolism of the body is 

 invigorated." 



354. Humidity Sometimes too Low. While in an ill-venti- 

 lated, crowded room the humidity is likely to be too high for 

 the comfort and well-being of the occupants, in the best venti- 

 lated room or house, where artificial heating is required, the 

 humidity is nearly certain to be too low, in cold weather; 

 unless special effort is made to correct this tendency. It is 

 generally accepted that the best indoor humidity is about 50 

 to 70 per cent. We shall see best just how much and just why 

 this indoor air is too low in humidity, if we study a particular 

 case. 



Suppose we take as an example the case of a typical school- 

 room anywhere in southern Wisconsin, northern Illinois, 

 southern Michigan, or northern Indiana, or Ohio, or anywhere 

 in Pennsylvania, New York, or New England (First Climatic 

 Region, Art. 324). By turning to the map (Fig. 221) showing 

 the average relative humidity of the United States for the 

 months of December, January, and February we see that the 

 average humidity for this region is about 80 per cent, for these 

 winter months. Again, turning to the average temperature 

 map for January (Fig. 219), we find that the average tempera- 

 ture for this region is about 25F. Turning now to the table 

 showing the weight of water vapor in saturated air, i.e., abso- 

 lute humidity at saturation (Table XII, page 220) we see that 

 each cubic foot of saturated air at 25F. contains about 1.6 

 grains of water vapor. Since the outside air is but 80 per cent, 

 saturated, it therefore contains but about 1.3 grains of water 

 vapor per cu. ft. Now, in most cases, when adequate venti- 

 lation is attempted, it is the practice to admit this outside air to 

 the schoolroom, merely heating it to 70F. and without paying 

 any attention to humidity. 



What is the relative humidity of the air when thus admitted 

 to the schoolroom? Again turning to the table of saturated 

 air (Table XII) , we find that to saturate each cubic foot of air 

 at 70F. there must be 7.98 grains of water vapor in each cubic 



