338 BACTERIA IN AIR 



The number of bacteria in the air varies with the season, increas- 

 ing from winter to summer and decreasing from summer to winter. 

 There is also a marked decrease in the number of bacteria in the 

 air after a rainstorm. The rain carries them to the ground and 

 also moistens the surface so that particles of dust are not carried 

 into the air by every breeze. But the added moisture of the soil 

 greatly increases the speed of multiplication so that later as the 

 surface soil dries out more dust and with it a greater number of 

 bacteria are carried into the air. It is also true that the number 

 of microorganisms in the air decreases in the winter months not 

 because cold is inimical to the life of the microorganisms for just 

 the reverse is true but the conditions are not as good for them 

 to find their way into the atmosphere. This is due to the fact that 

 the soil is covered with snow or the greater moisture prevents 

 the dust from being carried into the atmosphere. 



It is quite evident that there would be a relationship between 

 the number of bacteria in the atmosphere and the climate of that 

 region. Bacteria would multiply rapidly in the soil of a warm, 

 humid district and these in turn may be carried into the atmosphere, 

 but the rains would quickly wash them out. Hence, there would 

 be a great variation in a short time, whereas in an arid region the 

 number in the air may be smaller but will not vary as greatly as 

 in the humid region. 



The stay of the bacteria within the atmosphere will vary, depend- 

 ing upon a number of factors: 



1. The hardy spore-forming saprophytes may remain suspended 

 in the air for days or even weeks, whereas the frail non-spore- 

 forming pathogens soon perish due to either drying or the steriliz- 

 ing action of the sun's rays. 



2. Small particles settle out more slowly than do large ones, 

 for as the size of an object is decreased the surface area decreases 

 less rapidly proportionately than does the volume. Hence, those 

 bacteria which are floating free in the atmosphere would subside 

 more slowly than those attached to dust particles. 



3. The time of suspension is also determined by the velocity 

 of the air current. Organisms settle out of a still atmosphere 

 more readily than from one in motion, whereas it may require an 

 air current of considerable velocity to dislodge microorganisms 

 and bring them in suspension a slight current will sustain them. 



4. Moisture in the atmosphere tends to cause particles to adhere 

 together and as they grow in size the tendency for them to settle 

 out is increased proportionately. 



5. Although the air of London and many large cities contains 

 numerous particles of dust, the number of living organisms is com- 

 paratively small as the various gases thrown into the atmosphere 

 have a slight germicidal effect upon the bacteria. 



