HYGIENIC ASPECTS OF NUTRITION 269 



dust. " Mouth breathers " are likely to accumulate dust in 

 the passages leading to the lungs, and in large cities with 

 dusty air such persons may accumulate enough dust to prove 

 a menace to the work of the lungs. Collections of dust may 

 become sources of irritation and may be the starting points 

 for the growth of disease germs, such as those of tubercu- 

 losis. In wintertime, air that is breathed inward through the 

 nose is warmed before entering the lungs, but if taken in 

 through the mouth, its temperature is changed but little before 

 reaching the lungs and may chill the lung tissue. " Mouth 

 breathers " should consult a physician to determine whether 

 there is any structural reason for their practice. If not, care- 

 ful attention will erjable them to change the habit to normal 

 breathing. 



