Water and FungiBy Chris Karch, M.S., EMLab P&K Analyst Fungi, like all living organisms, require three essential factors for life: air, food and water. Fungal growth in the indoor environment is dictated by the availability of these three factors. Just like humans, what most fungi "breathe" is oxygen, which is generally abundant in indoor environments. Fungi can digest human food, but also many materials that are commonly used in buildings. Water availability, however, is a factor that can be controlled to prevent or reduce fungal growth. Fungi do not ingest their food like animals. Instead, they release enzymes into their surrounding environment to break down complex materials into simpler ones that can be absorbed by the fungus in a process known as absorptive nutrition. For these enzymes to leave the fungus, remain functional and to break down complex substances, water is necessary. Once the complex substances have been broken down and dissolved in water they are absorbed by the fungus. For fungal growth to occur, a certain level of free water (the liquid water found inside plant cells, not in their walls, which is not bound to other molecules) needs to be present. The water that is available to support microbial life in building materials is commonly expressed as water activity (aw). Water activity compares the physical properties of water in the material in question with that of pure water to generate a scale from 0 - 1.0. Pure distilled water has a value of 1.0 and as the value moves closer to 0, more and more solutes are present in the water and less and less water is available for an organism to use, thereby preventing the growth of many fungi. There is no sampling device that directly tests for water activity; instead the equilibrium relative humidity (ERH) is reported. For this measurement, the material is enclosed in a small container, allowed to equilibrate, and the relative humidity in the air above the material is measured. The ERH assumes that the air and materials in an enclosed space have the same concentration of water. The ERH is reported as a percentage, which is the aw decimal multiplied by 100. A definitive aw threshold does not exist for fungal growth. Studies have indicated that fungal growth can occur at and above aw=0.65. The most basic factor to consider when contemplating the potential of growth is the specific type of fungus being examined. Different fungi have different water requirements. These varying requirements allow them to be grouped into three broad categories. Fungi that require aw higher then 0.9 are known as hydrophilic fungi. Some common examples are many yeasts, Stachybotrys chartarum, Fusarium species, and Chaetomium globosum. Fungi that can grow between 0.8 and 0.9 are known as mesophilic fungi, which include species of Alternaria, Cladosporium, some species of Penicillium and Aspergillus, and many other fungi. Finally, fungi that can grow at aw below 0.8 are known as xerophilic fungi. Examples include Wallemia species, Aspergillus restrictus, A. repens and many others, as well as some species of Penicillium. Many physical factors in the environment can affect the water requirements of fungi. Two of the most important factors in the indoor environment are temperature and the type of food that is present. For each species of fungi there is an optimal temperature range for growth to occur. Outside of that optimal temperature range more water is necessary for growth. The same is true if the food source is not something the fungi normally would grow on in nature. Many fungi have evolved to break down plant compounds, thus fungi tend to grow more frequently and quickly on plant-derived materials such as wood and paper. When building materials do not come from a plant origin, or have been altered in some way, fungi require more water in order to grow. Studies have indicated that some fungi can survive periodic drying and rewetting, continuing their growth once free water is available again. Fungi are also susceptible to the rate of drying for a given material or substrate that they are growing on. This process is more detrimental to the fungus when the drying occurs rapidly. The slower the drying process occurs, the more likely the fungus will be able to continue its growth once water is reintroduced to the environment. Besides allergic reactions to the presence of fungi and their spores, another area of concern is the production of mycotoxins. Mycotoxins are secondary metabolites only produced and released under certain conditions that can cause various health effects in humans and animals. Many studies have indicated that Stachybotrys does not start to produce mycotoxins until the aw reaches 0.90, with the level of production increasing once the aw reaches 0.95. Other toxigenic fungi have different water activity requirements for the production of toxins. As with all living organisms on earth, water plays an important role in the growth and life cycle of a fungus. Without water, fungi are not able to grow or reproduce. The key to preventing and controlling fungal growth in buildings is to reduce the availability of free water by removing the water source such as repairing leaks, running a dehumidifier in damp areas and removing wet building materials. References: |