Much research has been conducted on building dampness. Topics investigated include the causes of excess building dampness, the influence of dampness on indoor biological and organic chemical contaminants, and the effects of dampness and of dampness-related indoor contaminants on people's health. There is also extensive literature on how to prevent building dampness. This document reviews the current scientific knowledge on these topics. The main findings are as follows:

Nature and Causes of Building Dampness: When the materials in a building become sufficiently damp to cause material damage or visible mold growth we often say that the building has a dampness problem or we characterize the building as a damp building. The dampness and mold growth may occur on visible interior surfaces in the building, including within basements or crawl spaces, or be hidden inside walls and air conditioning systems. Building dampness problems arise from a range of sources including, but not limited to, water leakage through roofs and walls, plumbing system leaks, groundwater entry, damp construction materials, high indoor rates of moisture generation, entry of humid outdoor air coupled with insufficient dehumidification, water vapor condensation on cold surfaces of windows and walls, and floods.

Prevalence of Building Dampness: Based on surveys, approximately half of U.S. homes have visible evidence of a dampness problem or mold contamination. The results of other surveys also suggest that dampness and mold are common in schools and office buildings. In a survey of 100 representative U.S. office buildings, 45% had current water leaks. A survey of U.S. schools by the General Accounting Office reported that 30% of schools had plumbing problems and 27% had roof problems; however, the nature of the problems were not described so the prevalence of associated dampness and mold cannot be determined.

Impacts of Building Dampness on Indoor Air Quality: When building materials or furnishings are damp for a sufficient time period, mold and bacteria will often colonize the materials. The molds and bacteria can produce microscopic airborne particles, some containing allergens or chemicals with the potential to induce inflammation in the respiratory system. Molds and bacteria are also sources of odorous volatile organic compounds in the indoor air. High indoor relative humidity in damp buildings also can increase the number of house dust mites present indoors and these mites are an important source of indoor allergens. Many building materials also emit chemicals into indoor air and increased dampness in these building materials may also lead to increases in emission rates of gaseous non-microbial chemicals, for example formaldehyde.

Dampness-Related Health Risks

Health risks of dampness or mold in houses: In 2004, the Institute of Medicine (IOM) of the National Academy of Sciences completed a major review of the available scientific literature pertaining to the health consequences of building dampness and mold. In 2009, the World Health Organization (WHO) completed a similar review and in 2011 Mendell et al. published an update to the review by the WHO. The conclusions of each of these reviews are similar, with the later reviews identifying a few more health effects as associated with dampness and mold. Based on the review of Mendell et al., dampness and mold in homes is associated with increases in several adverse health effects including upper respiratory symptoms, cough, wheeze, difficulty breathing, asthma exacerbation, development of new asthma, bronchitis, allergic rhinitis (allergy-caused inflammation of the nasal passages with runny nose or congestion), and eczema. The specific agents, e.g., molds, bacteria, or organic chemicals, causing these health effects were uncertain and insufficient scientific data were available to draw conclusions about the association of dampness and mold with several other health effects. Nevertheless, the reviews concluded that building dampness and mold represented a public health problem and that steps should be taken to reduce building dampness and mold, including various education efforts, reviews of buildings codes and contracts, and an exploration of financial incentives for reduced dampness.

Since completion of the IOM review, three related analyses were completed for this Scientific Findings Resource Bank:

  • First, a quantitative statistical evaluation of the available scientific literature produced estimates and uncertainty bounds for the average magnitudes of increases in various respiratory health effects in homes with dampness and mold. Building dampness and mold were determined to be associated with 30% to 50% increases in a variety of respiratory and asthma-related health outcomes. The observed increases in these adverse health effects in damp or moldy homes were very unlikely to be the result of chance.
  • The second analysis estimated the U.S.-wide public health impact of dampness and mold in houses, focusing on current asthma, defined as doctor-diagnosed asthma plus recent asthma symptoms, as the health outcome. The proportion of current U.S. asthma cases attributable to dampness and mold exposure was estimated to equal 21% with uncertainty bounds of 12-29%. Approximately 4.6 million cases of current asthma (range: 2.7-6.3 million) were estimated to be attributable to dampness and mold exposure in the homes of the 21.8 million people reported to have asthma in the U.S. The associated annual cost of current asthma attributable to dampness and mold in the U.S was estimated to be $3.5 billion (range: $2.1 - 4.8 billion).
  • The third analysis — another quantitative statistical evaluation of the available scientific literature — found that respiratory infections and bronchitis were increased in residents of homes with dampness and mold. An estimated 8% to 10% of these health effects were attributable to dampness and mold in homes.

Health risks of dampness and mold in workplaces and schools: Relative to research on the health risks of dampness and mold in houses, much less research has been performed on the health risks of dampness and mold in workplaces and schools. However, the available evidence suggests that the health risks of dampness in these buildings may be substantial. Based on review of nine studies, the evidence supporting an association of dampness or mold in offices and institutional buildings with respiratory or other health effects of occupants, or respiratory-health related sick leave, is reasonably robust. Every study identified found one or more statistically significant associations between dampness or mold and adverse respiratory or other health effects. In the several of the studies, the risk for at least one health effect more than doubled. Many of the seventeen studies from schools also showed significant health risks from dampness and mold, but the overall finding of increased health risks in damp schools were not as robust as those from offices because some of the studies were small in size or had scientifically weak study designs.

Health risks of dust mite allergens: Research has clearly demonstrated that dust mite allergen increases the risk of various health effects. Nearly all of this research has been performed in houses. Where concentrations of mite allergen in house dust are greater than approximately 2 micrograms allergen per gram of dust, susceptible people have a much higher risk of becoming sensitized to (i.e., have an allergic response to) dust mite allergen. Those who are sensitized to dust mite allergen have a several-fold increased risk of asthma. Mite allergens exacerbate allergic rhinitis (inflammation of the nasal tissues due to allergen exposures causing sneezing, runny nose, postnasal drip, and congestion) and atopic dermatitis (allergen-caused inflammation of the skin resulting in rash and itching). A National Academy of Science committee concluded that the evidence was sufficient to conclude that dust mite allergen caused development of the disease of asthma and caused exacerbation of asthma in those with asthma. Unfortunately, control of indoor dust mites has proven difficult. At the present time, the health benefits of humidity reduction interventions designed to reduce indoor dust mites are not well documented.

Health risks of mycotoxins from damp buildings: Some molds can produce highly toxic chemicals called mycotoxins under some growth conditions. Some bacteria can also produce toxic chemicals. The potential health effects of mycotoxins have been reviewed by a committee of the National Academy of Sciences, Institute of Medicine. Studies with exposures of living cells and animals to mycotoxins indicate that some mycotoxins are very potent, i.e., only very small amounts of some mycotoxins can produce substantial effects in cells or animals. However, it is not known whether the indoor air concentrations of mycotoxins caused by microbial growth in damp buildings can become high enough to cause health effects.

Health risks of microbial and non-microbial gaseous chemicals associated with indoor dampness: The health risk of the increases in microbial and non-microbial indoor gas-phase chemical chemicals in indoor air from building dampness are not well understood.

Implications for good building practices: Given the extensive evidence that the risks of asthma-related and respiratory health effects area substantially increased in damp or moldy buildings, an Institute of Medicine Committee that reviewed the risks of damp and moldy buildings came to the following conclusions:

"Homes and other buildings should be designed, operated, and maintained to prevent water intrusion and excessive moisture accumulation when possible. When water intrusion or moisture accumulation is discovered, the source should be identified and eliminated as soon as practicable to reduce the possibility of problematic microbial growth and building material degradation. The most effective way to manage microbial contaminants, such as mold, that are the result of damp indoor environments is to eliminate or limit the conditions that foster its establishment and growth."

The committee also concluded:

"When microbial contamination is found, it should be eliminated by means that not only limit the possibility of recurrence but also limit exposure of occupants and persons conducting the remediation."

This document provides references to key sources of information on practices for prevention and remediation of dampness problems in buildings.

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