This document presents World Health Organization (WHO) guidelines for the
protection of public health from health risks due to dampness, associated micro-
bial growth and contamination of in
door spaces. The guidelines are based on a
comprehensive review and evaluation of the accumulated scientific evidence by
a multidisciplinary group of experts studying health effects of indoor air pollut-
ants as well as those specialized in identification of the factors that contribute to
microbial growth indoors.
Problems of indoor air quality are recognized as important risk factors for hu-
man health in both low-income and middle- and high-income countries. Indoor
air is also important because populations spend a substantial fraction of time
within buildings. In residences, day-care centres, retirement homes and other
special environments, indoor air pollution affects population groups that are par-
ticularly vulnerable due to their health status or age. Microbial pollution involves
hundreds of species of bacteria and fungi that grow indoors when sufficient
moisture is available. Exposure to microbial contaminants is clinically associated
with respiratory symptoms, allergies, asthma and immunological reactions.
The microbial indoor air pollutants of relevance to health are widely heteroge-
neous, ranging from pollen and spores of plants coming mainly from outdoors,
to bacteria, fungi, algae and some protozoa emitted outdoors or indoors. They
also include a wide variety of microbes and allergens that spread from person to
person. There is strong evidence regarding the hazards posed by several biologi-
cal agents that pollute indoor air; however, the WHO working group convened
in October 2006 concluded that the individual species of microbes and other bio-
logical agents that are responsible for health effects cannot be identified. This is
due to the fact that people are often exposed to multiple agents simultaneously,
to complexities in accurately estimating exposure and to the large numbers of
symptoms and health outcomes due to exposure. The exceptions include some
common allergies, which can be attributed to specific agents, such as house-dust
mites and pets.
The presence of many biological agents in the indoor environment is due to
dampness and inadequate ventilation. Excess moisture on almost all indoor ma-
terials leads to growth of microbes, such as mould, fungi and bacteria, which
subsequently emit spores, cells, fragments and volatile organic compounds into
indoor air. Moreover, dampness initiates chemical or biological degradation of
materials, which also pollutes indoor air. Dampness has therefore been suggested
to be a strong, consistent indicator of risk of asthma and respiratory symptoms
(e.g. cough and wheeze). The health risks of biological contaminants of indoor
air could thus be addressed by considering dampness as the risk indicator.
Health hazards result from a complex chain of events that link penetration
of water indoors, excessive moisture to biological growth, physical and chemical
degradation, and emission of hazardous biological and chemical agents. The re-
view of scientific evidence that supports these guidelines follows this sequence of
events. The issues related to building dampness and its effect on indoor exposure
to biological and non-biological pol
lutants are summarized in Chapter 2, which
also addresses approaches to exposure assessment. An important determinant of
dampness and biological growth in indoor spaces is ventilation, and this issue is
discussed in Chapter 3. The evidence for the
health effects of indoor exposure is
presented in Chapter 4, based on a review of epidemiological studies and of clini-
cal and toxicological research on the health effects of dampness and mould. The
results of the epidemiological and to
xicological studies are summarized in the
The background material for the review was prepared by invited experts and
discussed at a WHO working group meeting, convened in Bonn, Germany, 17–
18 October 2007. The conclusions of the working group discussion are presented
in Chapter 5 and are reproduced in this executive summary, as follows.
Sufficient epidemiological evidence is available from studies conducted in dif-
ferent countries and under different climatic conditions to show that the oc-
cupants of damp or mouldy buildings, both houses and public buildings, are at
increased risk of respiratory symptoms, respiratory infections and exacerba-
tion of asthma. Some evidence suggests in
creased risks of allergic rhinitis and
asthma. Although few intervention studies were available, their results show
that remediation of dampness can reduce adverse health outcomes.
There is clinical evidence that exposure to mould and other dampness-related
microbial agents increases the risks of rare conditions, such as hypersensitiv-
ity pneumonitis, allergic alveolitis, ch
ronic rhinosinusitis and allergic fungal
Toxicological evidence obtained in vivo and in vitro supports these findings,
showing the occurrence of diverse inflammatory and toxic responses after
exposure to microorganisms isolated from damp buildings, including their
spores, metabolites and components.
While groups such as atopic and allergic people are particularly susceptible to
biological and chemical agents in damp indoor environments, adverse health
effects have also been found in nonatopic populations.
The increasing prevalences of asthma
and allergies in many countries increase
the number of people susceptible to the effects of dampness and mould in
WHO GUIDELINES FOR INDOOR AIR QUALITY: DAMPNESS AND MOULD
The conditions that contribute to the health risk were summarized as follows.
The prevalence of indoor dampness varies widely within and among coun-
tries, continents and climate zones. It is estimated to affect 10–50% of indoor
environments in Europe, North America, Australia, India and Japan. In cer-
tain settings, such as river valleys and coastal areas, the conditions of damp-
ness are substantially more severe than the national averages for such condi-
The amount of water on or in materials is the most important trigger of the
growth of microorganisms, including fungi, actinomycetes and other bacteria.
Microorganisms are ubiquitous. Microbes propagate rapidly wherever water
is available. The dust and dirt normally present in most indoor spaces provide
sufficient nutrients to support extensive microbial growth. While mould can
grow on all materials, selection of appropriate materials can prevent dirt ac-
cumulation, moisture penetration and mould growth.
Microbial growth may result in greater numbers of spores, cell fragments, al-
lergens, mycotoxins, endotoxins, ?-glucans and volatile organic compounds
in indoor air. The causative agents of adverse health effects have not been
identified conclusively, but an excess level of any of these agents in the indoor
environment is a potential health hazard.
Microbial interactions and moisture-related physical and chemical emissions
from building materials may also play a role in dampness-related health ef-
Building standards and regulations with regard to comfort and health do not
sufficiently emphasize requirements for preventing and controlling excess
moisture and dampness.
Apart from its entry during occasional events (such as water leaks, heavy rain
and flooding), most moisture enters a building in incoming air, including that
infiltrating through the building envelope or that resulting from the occu-
Allowing surfaces to become cooler than the surrounding air may result in
unwanted condensation. Thermal bridges (such as metal window frames), in-
adequate insulation and unplanned air pathways, or cold water plumbing and
cool parts of air-conditioning units can result in surface temperatures below
the dew point of the air and in dampness.
On the basis of this review, the following guidelines were formulated.
Persistent dampness and microbial growth on interior surfaces and in build-
ing structures should be avoided or
minimized, as they may lead to adverse
Indicators of dampness and microbial growth include the presence of conden-
sation on surfaces or in structures, visible mould, perceived mouldy odour
and a history of water damage, leakage or penetration. Thorough inspection
and, if necessary, appropriate measurements can be used to confirm indoor
moisture and microbial growth.
As the relations between dampness, microbial exposure and health effects can-
not be quantified precisely, no quantitative health-based guideline values or
thresholds can be recommended for acceptable levels of contamination with
microorganisms. Instead, it is recommended that dampness and mould-re-
lated problems be prevented. When they occur, they should be remediated be-
cause they increase the risk of hazardous exposure to microbes and chemicals.
Well-designed, well-constructed, well-m
aintained building envelopes are crit-
ical to the prevention and control of excess moisture and microbial growth, as
they prevent thermal bridges and the entry of liquid or vapour-phase water.
Management of moisture requires proper control of temperatures and ventila-
tion to avoid excess humidity, condensation on surfaces and excess moisture
in materials. Ventilation should be distributed effectively throughout spaces,
and stagnant air zones should be avoided.
Building owners are responsible for providing a healthy workplace or living
environment free of excess moisture and mould, by ensuring proper building
construction and maintenance. The occupants are responsible for managing
the use of water, heating, ventilation and appliances in a manner that does not
lead to dampness and mould growth. Local recommendations for different
climatic regions should be updated to control dampness-mediated microbial
growth in buildings and to ensure desirable indoor air quality.
Dampness and mould may be particularly prevalent in poorly maintained
housing for low-income people. Remediation of the conditions that lead to ad-
verse exposure should be given priority to prevent an additional contribution
to poor health in populations who are already living with an increased burden
The guidelines are intended for worldwide use, to protect public health un-
der various environmental, social and economic conditions, and to support the
achievement of optimal indoor air quality. They focus on building characteristics
that prevent the occurrence of adverse health effects associated with dampness
or mould. The guidelines pertain to various levels of economic development and
different climates, cover all relevant population groups and propose feasible ap-
proaches for reducing health risks due to dampness and microbial contamina-
tion. Both private and public buildings (e.g. offices and nursing homes) are cov-
ered, as dampness and mould are risks everywhere. Settings in which there are
particular production processes and hospitals with high-risk patients or sources
of exposure to pathogens are not, however, considered.
While the guidelines provide objectives for indoor air quality management,
they do not give instructions for achieving those objectives. The necessary action
WHO GUIDELINES FOR INDOOR AIR QUALITY: DAMPNESS AND MOULD
and indicators depend on local technical conditions, the level of development,
human capacities and resources. The guidelines recommended by WHO ac-
knowledge this heterogeneity. In formulating policy targets, governments should
consider their local circumstances and select actions that will ensure achieve-
ment of their health objectives most effectively.