Factors Affecting Air Microflora

Factors Affecting Air Microflora

	Factors Affecting Air Microflora - A number of intrinsic and environmental factors influences the kinds and distribution of the microflora in air. Intrinsic factors include the nature and physiological state of microorganisms and also the state of suspension. Spores are relatively more abundant than the vegetative bacterial cells. This is mainly due to the dormant nature of spores which enables them to tolerate unfavourable conditions like desiccation, lack of enough nutrients and ultraviolet radiation. Similarly fungal spores are abundant in the air since they are meant for the dispersal of fungi.
The size of the microorganisms is another factor that determines the period of time for which they remain suspended in air. Generally smaller microorganisms are easily liberated into the air and remain there for longer period. Fungal mycelia have a larger size and hence mainly fragments of mycelia will be present in air. The state of suspension plays an important role in the settling of microorganisms in air. Organisms in the free state are slightly heavier than air and settle out slowly in a quiet atmosphere. However, microorganisms suspended in air are only rarely found in the free state.
Usually they are attached to dust particles and saliva. Microorganisms embedded in dust particle settle out rapidly and in a quiet atmosphere they remain airborne only for a short period of time. Droplets which are discharged into the air by coughing or sneezing are also remain suspended in air for a short period of time. When their size decreases by evaporation they remain for a longer period in air. Environmental factors that affect air microflora include atmospheric temperature, humidity, air current, the height at which the microorganisms are found etc. Temperature and relative humidity are the two important factors that determine the viability of microorganisms in aerosol. Studies with Serratia marcesens and E. coli show that the airborne survival is closely related to the temperature.
There is a progressive increase in the death rate with an increase in temperature from -18°C to 49°C. Viruses in aerosols show a similar behaviour. Particles of influenza, poliomyelitis and vaccinia viruses survive better at low temperature from 7 to 24°C.The optimum rate of relative humidity (RH) for the survival of most microorganisms is between 40 and 80 percent. Low and high relative humidity cause the death of most microorganisms. Almost all viruses survive better at a RH of 17 to 25 percent.
	A notable exception is that of poliomyelitis which survives better at 80 to 81 percent. survival has been found to be a function of both RH and temperature. At all temperatures, survival is best at the extremes of RH. Irrespective of RH, an increase in temperature leads to decrease in survival time. Air current influences the time for which either the microorganisms or the particles laden with microorganisms remain suspended in air. In still air the particles tend to settle down. But a gentle air current can keep them in suspension for relatively long periods. Air current is also important in the dispersal of microorganisms as it carries them over a long distance.

Air currents also produce turbulence which causes a vertical distribution of air flora. Global weather patterns also influence the vertical distribution. High altitudes have a limiting effect on the air microflora. High altitudes are characterized by severe conditions like desiccation, ultraviolet radiation and low temperature. Only resistant forms like spores can survive these conditions. Thus high attitudes are characterized by the presence of spores and other resistant forms.

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