**3. Exposure evaluation of biological contamination of indoor air**

Indoor air levels of bacterial concentrations of <1, 000 CFU/m3 can be regarded as "low, " and concentrations of >5, 000 CFU/m3 can be regarded as "high" [1]. These numbers reflect mainly the crowdedness of the room and the efficiency of the ventilation and thus are a measure of the hygienic quality of the indoor air.

Characterizing the bacterial flora in the sample may give additional information about the sources of the bacteria present. If Gram-positive cocci and corynebacteria, the common bacteria of the human skin, are the dominating groups, this indicates human sources [1].

The presence of Gram-negative bacteria shows an active bacterial growth due to the presence of water indoors.

Gram-positive bacteria with exo- and endospores like *Streptomyces* and *Bacillus* can grow on moist building materials. Their spores are very resistant and can survive even if the air humidity is low.

The presence of fecal bacteria like the group of *Enterobacteriaceae* is considered heavy contam‐ ination of indoor air, coming probably from a sewage leakage. The presence of actinomycetes shows the growth of microbes in the building material.

The presence of two groups of bacteria, that is, *Legionella* and actinomycetes, has a special importance in indoor air.

*Legionella* spp. are Gram-negative environmental bacteria that have found a remarkable ecological niche in man-made water systems, especially in those serving air conditioning systems for heat transfer [1].
