Abstract
Air filters in heating, ventilation and air conditioning (HVAC) systems supplying indoor spaces are proposed as a source of indoor air contaminants. The reasoning is partly based on their main function which is to accumulate airborne micro-organisms - which can proli-ferate given the right conditions. Recent research indicates that the micro-organisms are increasingly implicated in airborne infections and allergies in humans and animals.
Therefore it was the aim of this study to determine the concentration and the survival time of micro-organisms on the material used in air filters of HVAC systems. The main focus was set on the mycological aspect since adverse health effects are increasingly associated with fungal spores, hyphae and mycotoxins in indoor air.
Between January 1996 and June 1997, "Airotester filter devices" were used in the pre-chamber of a HVAC system in a Berlin university-hospital. These filter devices consist of cylinders through which the outdoor air is drawn. Different filter media can be placed a-round the cylinders. Two filter materials (glass fiber and synthetic fiber) were changed at intervalls of 1, 2, 3, 6, 9 and 15 days. The samples of air-filter were washed out ("Schüt-telmethode") and the solution was plated out onto agar media (malt extract agar/blood a-gar) and incubated. The fungi and bacteria colonies cultivated on the agar media were counted and the fungal genera were identified.
Apart from the above mentioned experiments, another method was developed ("Hemmtest" ) to compare the influence of the two different filter materials on the growth rate of fungal mycelium. The "Hemmtest" was conducted between April and December, 1997. An eluate was made from new clean filter material (using the same "Schüttelmethode") and added to agar media. The media was filled in "Race tubes". Cladosporium cladosporoides was ino-culated and its growth rate was measured over 24 days.
The results from the study clearly showed that air filters are a hostile habitat for fungi and bacteria as they appear unable to grow and do not survive very long as viable spores. A different amount of micro-organisms were found. However this seemed more likely due to other factors such as different filter collection efficiency than to any antimicrobial effect of the filter materials. The mycological differentiation showed that most of the identified fun-gal genera, respectively their spores and lysis products, are associated with allergic reacti-ons in humans and animals.
The results underline the importance of keeping the filters of HVAC systems in a well de-signed and maintained state to minimize the growth of potentially allergenic and infectious micro-organisms and contamination of indoor air. The study clearly identifies the further need for investigations on the health implications from inhaled spores or fungal byproducts containing mycotoxins and the consequences of exposure to microbial lysis products such as ß-glucans and endotoxins. |
