Dampness and Mold

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 of related scientific research 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

Indoor dampness or mold in homes, determined visually or via mold odor, is associated with increases in asthma exacerbation, cough, wheeze, upper respiratory symptoms, asthma development, shortness of breath, ever diagnosed asthma, respiratory infections, bronchitis, allergic rhinitis, and eczema. Meta-analyses of published literature often indicate 30% to 70% increases in the prevalence rates of these adverse health effects in homes with dampness and mold. High indoor humidity in homes increases the number of house dust mites present and the allergen from these mites is associated with increases in asthma and respiratory health effects. Dampness and mold in work places and schools are also associated with increased respiratory health effects. The specific agents, e.g., molds, bacteria, or organic chemicals, causing the health effects of dampness and mold are uncertain.

Health Risks of Dampness and Mold in Houses

Based on multiple critical reviews and meta analyses of a large body of research, indoor dampness or mold, determined visually or via mold odor, is associated with increases in asthma exacerbation, cough, wheeze, upper respiratory symptoms, asthma development, shortness of breath, ever diagnosed asthma, respiratory infections, bronchitis, allergic rhinitis, and eczema. The odds ratios resulting from meta-analyses often indicate 30% to 70% increases in the prevalence rates of health effects in homes with dampness and mold. The specific agents, e.g., molds, bacteria, or organic chemicals, causing these health effects are uncertain and insufficient scientific data are available to draw conclusions about the association of dampness and mold with several other health effects. Overall, research indicates that building dampness and mold represents a significant public health problem and that steps should be taken to reduce building dampness and mold.

Health Risks of Dampness and Mold in Workplaces

Based on 16 studies published in 18 papers, visible dampness and mold or mold odor in workplaces is also associated with increases in adverse respiratory health effects, particularly respiratory health effects associated with asthma. Damp microbially contaminated sections of heating, ventilating, and air condition systems may contribute to these health effects.

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 Building 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.

Health-Related Costs of Dampness and Mold

The estimated annual health related costs of dampness and mold in the U.S. are as high as $17 billion for asthma morbidity and mortality, $3.7 billion for allergic rhinitis, and $1.9 billion for acute bronchitis.

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.

Recent Publications

Fisk, William J, Brett C Singer, and Wanyu R Chan."Association of Residential Energy Efficiency Retrofits with Indoor Environmental Quality, Comfort, and Health: A Review of Empirical Data."Building and Environment (2020). DOI

Fisk, William J, Wanyu R Chan, and Alexandra L Johnson."Does Dampness and Mold in Schools affect Health? Results of a Meta-Analysis."Indoor Air 2019 (2019). DOI

Fisk, William J."Review of Some Effects of Climate Change on Indoor Environmental Quality and Health and Associated No-Regrets Mitigation Measures."Building and Environment 86 (2015) 70-80. DOI

Dampness and Mold