THE MICROBIOLOGY OF THE ATMOSPHERE 



Ways of expressing rates of removal or death of bacteria are discussed 

 by Bourdillon et al. (1948), and are based on the constant, 'K', in the 

 equation : N = NoC"^'^^', where No is the number present at time T = 

 (e = base of Naperian logarithms) ; K, the 'die-away', is the rate of 

 removal of bacteria by all processes during the period, and may be sub- 

 divided. Thus Ki) is the death-rate, K^ is rate of removal by ventilation 

 only and is identical with the ventilation rate in air changes per hour, 



rooo 



100 - 



MINUTES 



Fig. 22. — Exponential form of the die-away of bacteria-carrying particles from the 

 air of a room. Line A: In an observation military canteen after the occupants had left 

 suddenly. Line B : Observations on the die-away following a group of sneezes in a small 

 room. (From Lidwell (1948), reproduced from M.R.C. Special Report No. 262, Studies 

 in Air Hygiene, by permission of the Controller of H.M. Stationery Office.) 



and Kj.. is the rate of removal by sedimentation. In an example of die- 

 away rates of bacteria from all causes in a bedroom with open windows 

 during fine weather at midsummer K, was equivalent to 6- 1 air changes 

 per hour after the occupants settled down to sleep at 23-00 hours; K = 4-9 

 after they went down to breakfast at 07-55 hours; and K = 6-8 after the 

 making of the beds at 08-50 hours (Lidwell, 1948, p. 253). Another 

 example is shown in Fig. 22. The case of die-away with stirred settle- 

 ment has been discussed by C. N. Davics (1947). 



Spore Movement in Convection Currents 



Convection currents alone, in an enclosed space without access of 

 outside air, are often sufficiently active to diffuse fungus spores evenly 



156 



