What happens when a building suffers moisture damage?
Leviticus, the Third Book of Moses in the Old Testament, describes a building that contracts “leprosy” and must be cleaned. If this does not succeed, it must be dismantled, and the stones and wood taken to a dumpsite. This is still sometimes valid advice. There may be no way to clean a mouldy building which has suffered long-term moisture damage. It has been shown in population studies that moisture damage and mould growth in buildings are associated with respiratory symptoms, general symptoms and increased risk of asthma. The symptoms correlate with the time spent in the building, and tend to ease after termination of the exposure. Sometimes also allergic rhinitis, rheumatoid diseases, sarcoidosis and pneumoconiosis are associated with damp buildings.
Even if the association between the damage and the symptoms has been shown, usually one does not know which factors are responsible for the health effects. Moisture causes chemical disintegration of materials and promotes microbial growth on the surfaces. Both chemical disintegration and microbial growth emit chemical impurities to the air, both volatile organic compounds and semivolatile organic compounds. Many mould fungi and bacteria produce toxins when they grow on building materials. These may be very toxic.
Moisture may accumulate in the building because of leaks, through capillary movement of water from the soil, or condensation. Condensation is often due to poor ventilation. It may also be due to wrongly placed non-permeable materials which prevent the escape of humidity out of the wall structures. There are many reasons for moisture problems today: there may be planning mistakes, poor building quality, lack of care and maintenance, and inappropriate use of ventilation systems.
Moisture problems are common. Depending on the definitions of “moisture damage” or “occurrence of mould”, estimates of moisture damage vary from 50 to 80 % and of mould in 5–20 % of buildings. It is difficult to obtain reliable information, because only approximations are possible without opening the wall, floor and roof structures.
The numbers of people exposed to moisture damage and moulds are high, because moisture damage can occur in dwellings, schools, day-care centres, workplaces, hospitals and recreational sites. Since there is no exact information on how extensive and long-lasting the moisture damage has to be to cause health problems, we have to be satisfied with a conservative approximation on the number of exposed people. Probably at least every third person will have contact with some sort of moisture or mould damaged indoor environment. Studies on teachers have suggested that exposure to microbes is higher than one could estimate on the basis of microbe concentrations in the workplaces or at home. This is mainly because people’s own activities cause resuspension of dust, called the “personal cloud”.
Release of chemicals in moisture damage
Moisture damage may cause different disintegration processes in building materials. This produces various chemicals being released into the indoor air. An example is alkaline disintegration of a plastic floor surface, if the surface was installed on a concrete surface which was not yet dry. The process may continue even after moisture is no longer present. The smell of ammonium is a sign of this sort of a problem. In some instances, the moisture comes from the soil below the building.
Release of microbial products to indoor air
Microbial growth releases particulate matter and volatile compounds which often are recognized as the musty smell of a mouldy cellar. Particles can be spores or cells (0.001 to 0.05 mm) or finer particles (0.000,01 to 0.000,1 mm), and they may contain allergens or toxic intermediates. When microbes are growing on surfaces, the release can vary depending on air flows and ambient conditions.
Volatile metabolites released by microbes may be hydrocarbons, acids, alcohols or other organic compounds just like other volatile organic compounds in indoor air. Attempts to identify compounds specific to microbial metabolism have not usually been successful, because there may be other sources such as outdoor air. Even if it is easy to recognise the typical smell of a humid cellar, the chemical characterization of the smell is difficult because of the low concentrations and the huge variety of chemicals present.
Also microbial toxins are products of fungal or bacterial metabolism. These products are usually not volatile, but they are bound to particles such as spores. Moulds producing toxins when growing on construction materials include Stachybothrus chartarum, Aspergillus versicolor and several other species of Aspergillus, and many species of Trichoderma, Fusarium, Chaetomium, and Penicillium families. These species are common in moisture damaged construction materials. There can also be some toxin producing bacterial species found e.g. Bacillus and Nocardiopsis families.
Exposure to microbial toxins via indoor air has not been studied very extensively. This is in contrast to the thorough studies of toxins in food. It is in part due to methodological difficulties, because the concentrations are usually very low, and the number of compounds may be high. Microbial toxins may be irritating, influence the immune and nervous systems, and cause cancer. In animal husbandry, many effects of microbial toxins have been described.
There is little solid information on the effects of low concentrations of microbial toxins in indoor air on humans. It is obvious that the symptoms and diseases attributable to these toxins are too rare to be detected in epidemiological population studies. However, in individual cases, this is one of the most important aspects of indoor exposures.
Indoor moisture problems are as yet poorly characterized. Moisture problems are common; what is not well understood is why sometimes these are associated with health problems and sometimes not.
Notes and references
- Levitictus 14:34-57
- See the chapter "Do we need to monitor the quality of the air in our homes?"
One level up: The air that we breathe
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