Microbes come into the atmosphere across different way. Human skin scales are probably the principal source of pollution; in fact it is estimated that one human being loses into environment 3g of scales a day and that every scale transports 3-4 bacteria.Other important sources of pollution are air conditioners and humidifier apparatuses, the boaking wood, humid wall, moquette coated floors with, dish-washe, domestic animals, w.c. pans, domestic dust, the spit and the sneeze.The amount of pollution agents sent into the atmosphere can lay down rapidly or travel ben about ten kilometres. Their storage is helped by lapse pressure, which is formed next to buildings, by wind velocity, by density and by forms of polluters themselves. Moulds, bacteria, viruses, dust, pollens and chemical compounds are the most frequent contaminators.

The main kinds of bacteria may be: Pneumococcus, Pseudomonas, Micobacter, Staphylococcus, Streptococcus, Bacillus, Enterococcus, Streptomyces, Rickettsie, Yersinie.
Concentration per m³ of air are very variable, from 200 to 12000 UFC, according to the sources concentration and to the conditions in which are kept.

The most abundant fungi are Aspergillus, Cladosporium, Chaetomium, Clamydia, Alternaria, Botrytis, and Ophiobulus.
The concentration per m³ of air changes between 10-20000 units, with the maximum level in very humid zones of 360000 units.

The kinds of viruses are represented prevalently by Rhinovirus, Adenovirus.

The risk of microbic pollution is more frequent in schools, ships, barracks, restaurants, halls for meetings and other crowded places.