Indoor Air Pollution by Microorganisms and Their Metabolites

Abstract

Microbial damage in indoor areas is caused most frequently by molds and bacteria. These microorganisms have a very important role in the biogeochemical cycle, as their task consists in disintegrating organic mass to reusable metabolites. In the environment spores of molds and bacteria may become airborne and are therefore ubiquitous. They can enter indoor areas by passive ventilation or by ventilation systems as well. Many genera are also emitted by indoor sources like animals, flowerpots and wastebaskets. This normal flora is in most cases not harmful. But more and more frequently growth conditions like excessive humidity and/or a high water content of building materials are encountered, which are in most cases the limiting factor for microbial growth. This is caused by shortcomings of the buildings like the lack of thermal insulation as well as by the noncorrect behavior of the room users. According to the relative humidity and/or the moisture content of the building material different microorganisms are able to grow on indoor building materials and may cause destruction, adverse health effects and unpleasant smells. Therefore the task of microbial examinations is to differentiate between normal indoor microorganisms, airborne or adherent to walls and floors, and between more or less heavily growing species, attacking building materials and producing microbial products causing adverse health effects. Air sampling of microorganisms is a very favored method, as it allows a direct toxicological evaluation, as results can be related to a concentration expressed in colony forming units per cubic meter. Sometimes even information on the particle size is available, which allows an estimation of how deep those particles may penetrate into the lung. But microorganisms in indoor air are generally not equally distributed, but occur in clouds, so they are often overlooked in air measurements, especially if the microbial damage is hidden by paneling, walls, etc. Another reason for false-negative results obtained by air measurements is that fungal spores are not released at all stages of growth. Therefore other techniques are helpful for example, the sampling of household dust, the sedimentation method or direct sampling from surfaces. The differentiation of bacteria is performed by biochemical methods as a rule, whereas in most cases the differentiation of molds is done by microscopy, especially by the forms of spores. On many occasions the growth behavior and patterns on different nutrient agars also have to be evaluated. Nonsporulating species have to be triggered to produce spores, otherwise “sterile mycelium” will be indicated, which means they cannot be named by genera or even species. Methods of genetic fingerprinting are still in their early stages and only available for some genera or species. But in the meantime enzymatic tests have become available to decide between mold growth and normal quantities on building surfaces. A very difficult task consists in the search for hidden mold growth, for example, if adverse health effects like the fungal syndrome are observed, which is characterized by the occurrence of unspecific symptoms. There are efforts to detect hidden mold growth by the analysis of microbial volatile organic compounds or by the use of specially trained sniffer dogs, but these methods have not been scientifically evaluated. The odor alone perceived by human beings is not reliable to detect mold damage. As far as the rehabilitation of the indoor environment is concerned, it has to be pointed out very clearly that microbial damage has to be removed. The killing of microorganisms is often carried out, but this procedure is not sufficient, as for example, nonviable spores keep their allergenic potential. The acuteness of the rehabilitation procedures is normally considered according to the extent of the microbial damage. Adverse health effects are supposed to be connected with microbial growth in indoor areas, mostly with mold growth. Predominantly allergies have to be mentioned, followed by toxic alveolitis and reactions like (allergic) bronchitis, chronic obstructive pulmonary disease, but also the aggravation of asthma. Infections by molds and bacteria are very rare, but persons with an immunodeficiency are especially susceptible to fungal infections. It has been found that spores of fungi contain fungal toxins (mycotoxins), which are well known from food contaminations. But whether these mycotoxins show toxic effects, if fungal spores are inhaled, has not been confirmed. On the whole, the dose relationship between the concentration of microbial particles mentioned and the adverse health effects described is not very well established. When sanitary effects are observed, very often the susceptibility of the individual is crucial. Therefore guidelines concerning microbial products in indoor areas are sparse and mostly not scientifically sound.