Fungi and mites on humid indoor walls : a laboratory study

The potential allergen source formed by mites and fungi developing on walls has been studied in a semi-natural model. Gypsum and wooden pieces, representing indoor walls, were artificially soiled with one of two different organic compounds, a yeast/vegetable mixture (Mannite) or a red currant juice (Eden). After acclimatizing to 78% relative humidity (rH), inoculation of fungi and/or mites followed. Water adsorption of the 10x10x0.6 cm pieces was slower than expected, equilibrium was reached after 6 to 8 weeks. Visual mould growth (Aspergillus repens or Penicillium chrysogenum) appeared on soiled pieces after 8 weeks to 5 months, depending on the prevailing humidity. Soiling with Marmite caused a more dense fungal growth than soiling with juice. Storage mites (Glycyphagus domesticus) developed exponentially on all mouldy pieces resulting in a median value of 234 mites on wood and 404 on gypsum 8 weeks after incubation of 50 mites, whereas house dust mites (Derniatophagoides pteronyssinus) survived only barely (less than 10 mites after 8 weeks). Storage mites are presumably living pemanently on mouldy walls, whereas house dust mites are chance visitors. The fungistatic efficacy of four sanitary cleaners for walls, which were sprayed on the mouldy test pieces, was compared. Fungal growth remained high in all 4 cases, which disagrees with earlier publications. A combination of cleaning and spraying with a fungistatic formulation may lead to increased efficacy.