The epidemiological end of the bacterial dust phase is associated with those types of microorganisms that do not lose viability upon drying. The resistance of pathogenic microorganisms to drying is very different. It is known that in the coarse-grained phase of the aerosol even microorganisms, such as influenza, measles, and chicken pox viruses, which are not very resistant to external influences, can remain, since there is a sufficient amount of moisture inside the drop necessary to maintain the viability of the bacteria; bacillus diphtheria, streptococci, meningococci, etc. survive in the small-nuclear phase. Only particularly resistant microorganisms — mycobacterium tuberculosis, spore-forming bacteria, and some types of fungi — can survive in the bacterial dust phase.
Indoor airflows are a significant factor affecting the spread of microorganisms. Horizontal air flows contribute to the spread of microbes within the premises, and in the presence of a common corridor within the floor. Vertical flows caused by convection and mechanical ventilation (for example, in stairwell spaces) transfer microbes to the upper floors.
Sanitary and hygienic studies of microbial pollution of the air. one
Air is a special object of the environment that is not visually detectable; therefore, sampling it has some features. For a hygienic assessment of bacterial air pollution, it is necessary to know how much air was in contact with the nutrient medium, since the standards regulate a certain number of colonies of microorganisms that grow when sown 1 m3 (1000 l) of air.
Depending on the principle of capture of microorganisms, the following methods of sampling air for bacteriological research are distinguished:
• sedimentation;
• filtration;
• based on the principle of impact of an air stream.
The simplest is the sedimentation method (precipitation method), which allows you to catch the spontaneously precipitating fraction of the microbial aerosol. Sowing is carried out on Petri dishes with a dense nutrient medium, which are placed in several places of the room and left open for 5-10 minutes, then incubated for 48 hours at 37 ° C and the number of grown colonies is counted .
This method does not require the use of equipment for sowing, but its disadvantage is low information content, since it is impossible to obtain accurate data on the number of microorganisms due to the fact that their subsidence occurs spontaneously, and its intensity depends on the direction and speed air flows. In addition, the volume of air in contact with the culture medium is unknown. With this method, finely dispersed fractions of bacterial aerosol are poorly captured, therefore, the sedimentation method is recommended to be used only to obtain comparative data on indoor air purity at different times of the day, as well as to assess the effectiveness of sanitary-hygienic measures (ventilation, wet cleaning, irradiation with ultraviolet lamps, etc.).