#   Introduction
The WHO Housing and health guidelines (HHGL) provide evidence-based recommendations for healthy housing conditions and interventions. This chapter introduces the WHO approach to healthy housing, outlines the key risks to health associated with the housing environment, and assesses the burden of disease associated with housing. Subsequently, the objectives, rational, target audience, scope, and co-benefits of the HHGL are introduced, as well as the relationship between the social determinants of health, housing and health.
- 1 Housing and health

# WHO approach to healthy housing
Healthy housing is shelter that supports a state of complete physical, mental and social well-being. Healthy housing provides a feeling of home, including a sense of belonging, security and privacy. Healthy housing also refers to the physical structure of the dwelling, and the extent to which it enables physical health, including by being structurally sound, by providing shelter from the elements and from excess moisture, and by facilitating comfortable temperatures, adequate sanitation and illumination, sufficient space, safe fuel or connection to electricity, and protection from pollutants, injury hazards, mould and pests. Whether housing is healthy also depends on factors outside its walls. It depends on the local community, which enables social interactions that support health and well-being. Finally, healthy housing relies on the immediate housing environment, and the extent to which this provides access to services, green space, and active and public transport options, as well as protection from waste, pollution and the effects of disaster, whether natural or man-made (1).

# Key health risks related to housing
Exposures and health risks in the home environment are critically important because of the large amount of time people spend there. In high-income countries, around 70% of people's time is spent inside their home (2). In some places, including where unemployment levels are higher, and where more people are employed in home-based industries, this percentage is even higher (3). Children, the elderly, and those with a disability or chronic illness are likely to spend most of their time at home, and are therefore more exposed to health risks associated with housing (2). Children are also at increased risk of the harms from some of the toxins that are present in some housing, such as those in lead paint (4).
Housing will become increasingly important to health due to demographic and climate changes. The number of people aged over 60 years of age, who spend a larger proportion of their time at home, will double by 2050 (5). The changing weather patterns associated with climate change also underline the importance of housing providing protection from cold, heat and extreme weather events (6).
Poor housing can expose people to several health risks. For example, structurally deficient housing, due to poor construction or maintenance, can increase the likelihood that people slip or fall, increasing the risk of injury.
Poor accessibility to homes may expose their disabled and elderly residents to the risk of injury, stress and isolation. Housing that is insecure, sometimes due to affordability issues or weak security of tenure, is stressful. Housing that is difficult or expensive to heat can contribute to poor respiratory and cardiovascular outcomes, while high indoor temperatures can increase cardiovascular mortality. Indoor air pollution harms respiratory health and may trigger allergic and irritant reactions, such as asthma. Crowded housing increases the risk of exposure to infectious disease and stress. Inadequate water supply and sanitation facilities affect food safety and personal hygiene.
Urban design that discourages physical activity contributes to obesity and related conditions, such as diabetes, and poor mental and cardiovascular health. Unsafe building materials or building practices, or building homes in unsafe locations, can expose people to a range of risks, such as injury due to building collapse.
Housing in slums (the preferred term of UN-Habitat) and informal housing pose particular risks to health. Currently, around 1 billion people live in slum conditions today (7), which often develop due to exclusion from planning processes. According to UN-Habitat, a "slum household" is a group of individuals under the same roof, in an urban area, lacking one or more of the following: durable housing (housing which fails to provide shelter from the elements); sufficient living space; security of tenure; sanitation and infrastructure; and access to improved (uncontaminated) water sources.
Slum dwellers are therefore exposed to many of the risks associated with housing, such as structurally defective dwellings, inadequate housing facilities and overcrowding, but also face particular health risks from poor sanitation and unsafe electric connections, toxic building materials, unvented cooking facilities, and unsafe infrastructure, including roads. In addition, such settlements are sometimes in locations that are more likely to expose occupants to hazards such as landslides, floods and industrial pollution. In relation to well-being, the lack of legal title to homes is stressful and can expose slum dwellers to the risk of forced eviction (8).
Slums and informal settlements often house migrants, refugees and internally displaced persons. More people are on the move now than ever before.
There are an estimated 1 billion migrants in the world today: 250 million international migrants, and 763 million internal migrants. This number includes 65 million people, who have been forcibly displaced and require urgent housing solutions (9).

# Prevalence of poor housing conditions
Large numbers of people live in poor housing conditions. For example, 6% of households in Latin America and the Caribbean (compared with 0.4% in the European Union) have more than three people per room (10). Some 9% of the global population has no access to an improved (uncontaminated) drinkingwater source. Nearly half of all people using poor quality or contaminated drinking-water sources live in sub-Saharan Africa, while one fifth live in South Asia (11). In addition, 41% of the world's population cook and heat their housing using open fires and simple stoves that burn solid fuels. These result in polluted indoor air (12) and inadequate ventilation.
Globally, many houses have structural defects. For example, 15% of the European population live in housing with a leaking roof, or damp walls, floors or foundations, or rot in window frames, floors and other structural elements (13). Almost 20% report that their housing did not protect them against excessive heat during summer, while 13% report that their housing was not comfortably warm during winter (13). In the United Kingdom, 72% of adults with mobility problems reported that the entry to their housing was not properly accessible (14). In the United States of America, 5.2% of the housing stock is classified as inadequate, having either severe or moderate physical problems such as deficiencies in heating, plumbing or upkeep (15).

# Burden of disease associated with housing
Health conditions related to housing present an important health burden.
Some of this is attributable to poor access to water and poor indoor environmental quality. Water, sanitation and hygiene (WASH) were responsible for 829 000 deaths from diarrhoeal disease worldwide in 2016. This constitutes 1.9% of the global burden of disease measured as disability-adjusted life years (DALYs) (16). In 2016, 3.8 million deaths globally were attributable to household air pollution from the use of solid fuels for cooking, almost all of which occurred in low-and middle-income countries (17). About 15% of new childhood asthma in Europe can be attributed to indoor dampness, representing over 69 000 potentially avoidable DALYs and 103 potentially avoidable deaths every year (18).
Housing also contributes to the burden of disease through exposing people to dangerous substances or hazards, or to infectious diseases. For example, almost 110 000 people die every year in Europe as a result of injuries at home or during leisure activities, and a further 32 million require hospital admission because of such injuries (19). In Europe, it has been estimated that 7500 deaths and 200 000 DALYs are attributable to lack of window guards and smoke detectors (18). Approximately 10% of hospital admissions per year in New Zealand are attributable to household crowding (20). In 2012, India recorded over 2600 deaths and 850 of various injuries resulting from the collapse of over 2700 buildings (21). In Kyrgyzstan, household crowding causes 18.13 deaths per 100 000 from tuberculosis (TB) per year (18). Exposure to lead is estimated to have caused 853 000 deaths in 2013 (22).
While everyone can be exposed to the risks associated with unhealthy housing, people with low incomes and vulnerable groups are more likely to live in unsuitable or insecure housing, or to be denied housing altogether (23).
Inequalities associated with housing are discussed later in this chapter.

# WHO Housing and health guidelines

# Objectives and rationale for developing the WHO Housing and health guidelines
The impact of housing on health and the prevalence of poor housing conditions around the world, as presented in section 1.1, justify the need for globally acceptable and practical guidelines that will ensure healthy housing and human safety. The underlying principle of such guidelines is for housing to give adequate protection from all potential hazards prevailing in the local environment. This principle should apply to both the existing housing stock and newly constructed dwellings. Although a number of housing and health regulatory frameworks and guidelines exist, they are not comprehensively coordinated to address all aspects of housing, human health and safety. For instance, WHO has guidelines for indoor air quality or water and sanitation but there is a lack of comprehensive, international housing and health guidelines highlighting that these can be a fundamental way of improving population health (1). While improving housing may not be the top policy priority in all countries, reliable global guidance for shaping current and future policy is the first step to protect people living in a range of climatic conditions from unhealthy housing. This is a critical public health priority. The improvements recommended by these guidelines relate to a large array of housing aspects, including vital infrastructure, the physical dwelling, the use of the dwelling, and the location of the dwelling. They must be viewed alongside each other so that policy-makers can make the most of co-benefits and synergies, while avoiding trade-offs (24). Large benefits in cost-effectiveness would arise from addressing the health risks associated with housing simultaneously and this approach is in line with WHO's intersectoral work to create health-promoting environments (25)(26)(27)(28).
These HHGL add to existing WHO guidelines by providing evidence-based recommendations on healthy housing conditions and interventions that are not covered by the other guidelines, and by summarizing those relevant to housing and health. As sectoral guidelines, they represent a proactive step forward, highlighting the need to address the health risks associated with housing through a systems approach. By their nature, land use and building regulations act to address multiple risks, including structures and heating systems, as well as hazard avoidance. These HHGL, by providing access to the science on minimizing multiple health risks associated with housing, will be an important resource for Member States.
Implementing the HHGL will support the achievement of the Sustainable Development Goals (SDGs), including SDG 3 to ensure healthy lives and promote well-being for all age groups and SDG 11 to make cities and human settlements inclusive, safe, resilient and sustainable (29). The HHGL will also be influential for ensuring availability of sanitation for all at household level (SDG 6), meeting targets for renewable energy and energy efficiency (SDG 7)
and taking action to mitigate climate change (SDG 13) (30). The importance of the sectoral approach has been recently emphasized in the New Urban Agenda for sustainable urban development established at Habitat III (31).
The HHGL contribute towards ensuring Member States meet their obligations regarding the human right to adequate housing. This right to adequate housing is recognized in international human rights laws as a component of the right to an adequate standard of living, enshrined in the Universal Declaration of Human Rights (adopted in 1948) and the International Covenant on Economic, Social and Cultural Rights (adopted in 1966). For housing to be adequate, the following seven criteria must be met: security of tenure; availability of services, materials, facilities and infrastructure; affordability; habitability; accessibility; location; and cultural adequacy (32,33). Thus, the HHGL will inform regulations that aim to address and fulfil the above criteria of adequate housing. While the HHGL provide global recommendations, their implementation and prioritization will vary by local context and will require national and local adaptation.

# Target audience
The main target audience for the guidelines is policy-makers who are responsible for housing-related policies and regulations, enforcement measures, and initiating intersectoral collaborations that seek to support healthy housing from a government perspective.
The guidelines are also of direct relevance to the daily work of implementing actors such as government agencies, architects, builders, housing providers, developers, engineers, urban planners, industry regulators, financial institutions, as well as social services, community groups, and public health professionals. These stakeholders are ultimately required to ensure that housing is built, maintained, renovated, used and demolished in ways that support health.
As already noted, "healthy housing" is associated with several factors, inside and outside the home. The HHGL do not address all possible risk factors related to housing but focus on priority areas that have not yet been addressed by existing WHO guidelines and where robust evidence is available. These were identified by the Guideline Development Group (GDG) established for this work (see Chapter 2).
The priority areas addressed by the HHGL are as follows:
- inadequate living space (crowding) (Chapter 3)
- low indoor temperatures (Chapter 4)
- high indoor temperatures (Chapter 5)
- injury hazards in the home (Chapter 6)
- accessibility of housing for people with functional impairments (Chapter 7).
In addition to the above, existing WHO guidelines and recommendations related to housing are identified and summarized in Chapter 8 to cover the following issues:
- water quality (section 8.1)
- air quality (section 8.2)
- tobacco smoke (section 8.3)
- noise (section 8.4)
- asbestos (section 8.5)
- lead (section 8.6)
- radon (section 8.7).
Guidance on other aspects of housing and buildings that relate to healthincluding pests, food safety and ventilation -are listed in section 8.8. Despite the range of issues covered, the list of relevant elements is not exhaustive.
For instance, there are still a number of housing risk factors (such as lighting, height of ceilings and buildings, electric security, housing surroundings and fuel poverty) that have not been covered in the HHGL at this time. WHO is planning to continue investigating and working on other housing-related risk factors to health and to provide future guidance. At the same time, the HHGL do not distinguish between permanent housing and housing that is intended to be temporary, such as emergency shelter arrangements. However, the GDG recognizes that implementing the HHGL is likely to be more challenging in informal and emergency housing and will require different priorities, depending on the context. General implementation considerations and WHO's role in supporting these are discussed in Chapter 9. Important supplementary guidance relevant to emergency shelter arrangements are further provided by the Sphere Project (34). Homelessness, which is the most extreme denial of the right to adequate housing, is not discussed as part of the HHGL (35).
Co-benefits arise from addressing the key health risks associated with housing. In many cases, a dwelling poses multiple risks to healthy housing.
For example, a house may have poor indoor air quality, be cold, and have multiple injury hazards. Housing risks should therefore be viewed holistically and as components of an inter-related system in order to take advantages of the co-benefits presented by different interventions. For example, correcting structural defects reduces the risk of injury, improves thermal comfort, and reduces exposure to outdoor pollutants.
Housing interventions can also have indirect co-benefits for health. Improving thermal insulation, weatherization and ventilation, and installing energyefficient heating (Chapters 4 and 5) can improve indoor temperatures that support health, while also lowering expenditure on energy (24,36) and reducing carbon emissions (37).
Improving housing conditions also supports other positive social outcomes.
As discussed in Chapter 3, reducing crowding supports good health outcomes, but also contributes to improved educational outcomes, as children are able to study more effectively (38). Improving thermal comfort through installing insulation and heating reduces days off school and work (39). Improving housing can also create jobs and stimulate investment (40). Therefore, addressing health risks associated with housing is likely to particularly benefit low-income and vulnerable groups, as these groups are more likely to live in inadequate housing.
In recent years, some countries have instituted new "green" standards for construction practices. These standards are aimed at addressing the design, location, and site of housing; promote water conservation and energy efficiency; encourage the use of building materials beneficial to the environment; and promote healthy living conditions (41). Some green housing elements that are typically included in such standards are associated with health outcomes, including: energy efficient heating; improved ventilation; building materials free from formaldehyde, lead and asbestos; sound insulation; and no carpets in kitchens and bathrooms (42).
The HHGL aim to ensure that occupants of green housing also enjoy health benefits (18,43). Studies of green and energy efficient housing improvements and their influence on health have recently been comprehensively reviewed (44,45).

# Social determinants, housing and health
Choices of housing types, quality, size and location are shaped by a number of economic, social and demographic factors. These factors affect the features that the house will provide to its occupants (e.g. durability, building materials, accessibility etc.) and whether they can afford the cost of operating and maintaining it. The cost of maintaining and operating a house is of importance to human health and safety and includes: the purchase of safe drinkingwater and of electricity or other fuel for heating the home (27). Transport infrastructure can also be considered as an operational aspect of housing affordability, because it influences how much people need to pay to travel between their homes and work and other places. 3 Globally, across low-, middle-and high-income countries, low-income earners are more likely to live in housing that exposes them to health risks. For example, in Cambodia, toilet facilities are only available to 29% of households in the lowest income quintile, compared with 79% of households in the highest income quintile (47,48). In Guatemala, 89% of the lowest income quintile have dirty floors, compared with 4% of the highest income quintile (49)(50)(51). In the United States of America, repeated hospitalizations for childhood asthma are correlated with residing in the census tract areas with the highest proportion of crowded housing conditions, the largest number of racial minorities and the highest neighbourhood-level poverty (18,52,53).
This inequality in housing conditions goes beyond whether people are rich or poor. In some countries certain groups, including indigenous people, minority populations, single parent families, disabled people and women, are more likely to live in unsuitable housing (54)(55)(56)(57).
Poor health outcomes in turn can contribute to poor economic outcomes.
Poor health can be expensive, because of the costs of treating illnesses. In addition, poor health can affect people's capacity to earn or save money (58).
This creates a cycle between poor health and poor household, local and national economic outcomes. At the same time, housing that is expensive relative to income can affect health, in particular for people on low incomes.
High housing costs can compel people to cut back on other essentials that are connected to health, including food, energy and health care (59)(60)(61). Difficulty with paying rent and mortgage costs exposes people to risks of eviction and foreclosure (62), and increases the likelihood that people have to move often (35,63,64). These factors -eviction, foreclosure and residential mobility -have each been associated with adverse educational and economic effects and poor health outcomes (62,65,66).
Interventions that create healthy homes can help to break this cycle by improving health and broader social and economic outcomes, yielding important benefits for decades into the future. These housing-related interventions need to be complemented by policy interventions relating to education, employment, transport, child care, health systems, taxation, wages, benefit levels and job security. Each of these factors can affect incomes and thus affect people's ability to pay for housing that keeps them healthy (27).
Providing affordable housing can help people to afford housing that fits their needs while improving their health (67,68). Affordable housing, such as public housing, can be promoted through funding a supply of affordable dwellings, or through providing subsidies, such as housing vouchers or tax mechanisms (e.g. low-income housing tax credits) (68,69).

# Guideline development process

#   Guideline development process
- 1 Contributors to the guidelines

# WHO Steering Group
The WHO Steering Group (SG) has been involved in all stages of planning the HHGL, review of evidence and all rounds of consultation on revisions following peer review. Their inputs were provided through face-to-face meetings and email.

# Guideline Development Group
The GDG was made up of people with content expertise in all areas covered by the HHGL, including relevant experience from low-and middle-income countries and expertise in evidence-based guideline development (see Acknowledgements for names and affiliations). GDG selection also took into consideration the need to ensure gender balance and regional diversity. The members of the GDG, under the guidance of the guideline methodologist, worked together to define key questions, priorities and systematic review methods, served as authors of HHGL chapters (including drafting and determining the strength of the recommendations), and responded to external peer review comments. Specified GDG members served as chair, co-chair and rapporteurs.

# External Review Group
External reviewers were drawn from subject experts, implementing agencies and partners working on various aspects of policy to improve health outcomes related to housing to form the External Review Group (ERG) (see

# Systematic review
Systematic reviews of the evidence on the effects of the prioritized interventions and exposures related to crowding, indoor temperature, accessibility and injury hazards were commissioned (see Acknowledgements for names and affiliations).

# Identification of priority questions and critical outcomes
Following discussions with the GDG at the first meeting in Washington (DC) on 9-11 April 2013, a set of priority topics was identified for these guidelines.
This took account of areas of healthy housing that have been covered in other, recent WHO guidelines in order to avoid unnecessary duplication. The priority topics demonstrated the need for a series of distinct systematic reviews of the effects of the relevant interventions or exposures: crowding, low indoor temperatures, insulation, high indoor temperatures, injury hazards, and housing accessibility. It was recognized that the breadth of the topics covered by these systematic reviews meant that different approaches would be needed, and that there would be relatively little, if any, overlap in their included studies and their conduct (e.g. search strategies). Therefore, a mixture of research groups was engaged to form the systematic review teams.
The questions to be addressed by each review were agreed by the WHO Secretariat, the SG and the GDG. These were converted into a PICO or PECO format to show the population, intervention (or exposure), comparator and outcome as the four elements to be considered in the systematic review of the evidence (70). In order to focus the reviews, and their subsequent recommendations, the GDG prioritized the outcomes to be assessed. They ranked the relative importance of a series of health outcomes on a scale from 1 (not important for these guidelines) to 9 (critical) based on their own expert knowledge. The mean scores were calculated and the highest ranked outcomes for each review were included in its PICO or PECO.

# Evidence identification and retrieval
Having agreed on the scope and eligibility criteria for each of the six systematic reviews, the systematic review teams, in consultation with the WHO Secretariat and guideline methodologist, used dedicated search strategies to systematically provide evidence on the effects of the prioritized interventions or exposures. The searches were conducted in 2015 and updated in 2018. GDG members and experts from the specific areas for each review were asked to suggest potentially eligible studies to help inform the design and implementation of the searches. Evidence on the effects of interventions (e.g. insulation and safety devices) stemmed mainly from randomized trials and non-randomized comparative studies (as necessary), while observational designs, such as case-control or cohort studies, were the most relevant designs for studies investigating the effects of exposures (e.g. crowding and heat).
The WHO Secretariat, designated members of the GDG and the methodologist worked together to develop the methods used for each review. The format for the reports of the systematic reviews was agreed with the WHO Secretariat, drawing on experience from previous systematic reviews for WHO guidelines.
This included the inclusion of key elements for each review, such as the PICO or PECO used to define the scope of the review, the search strategy, methods, a table showing the characteristics of each included study and their quality, and an assessment of the overall quality of the evidence for the effects on each prioritized outcome using GRADE (Grading of Recommendations Assessment, Development and Evaluation) evidence profile tables (see the reviews for examples). A timeline was developed for the submission of the draft and final versions of the reviews to allow time for discussion with, and feedback from, the WHO Secretariat and members of the GDG.
Given the substantial differences in topic areas for the reviews, search strategies specific to each review were developed, tested and implemented by the systematic review teams. Details of these searches, including the number of retrieved records and the flow of these through the review process, are provided in the report for each systematic review.

# Quality assessment and grading of the evidence
Each systematic review team used appropriate instruments to assess the quality and risk of bias of each study included in their review. This allowed a tailored approach to the quality assessment of the eligible studies for each review. It also allowed an overall assessment of the quality and relevance of the body of evidence for each review. Details of the methods used and the assessments are provided in the report of each review. The evidence gathered for the effects of an intervention or exposure on the prioritized health outcomes was assessed using the GRADE approach. This led to the categorization of the quality of evidence for each outcome as "high", "moderate", "low" or "very low", based on the risk of bias (and other quality features) in the included studies, along with inconsistency of results, indirectness, imprecision and other factors deemed relevant.
These assessments provided a starting point for the GDG when using the evidence on the health effects of interventions and exposures to develop the specific recommendations for the HHGL.
Where possible, the systematic reviews included information on potential disadvantages as well as advantages of the proposed interventions. This, along with the expert knowledge of the GDG, was used to incorporate information on the feasibility, harms, acceptability and cost-benefit considerations during the development of the recommendations.
The systematic reviews are available online at .

# Formulation of recommendations
The recommendations were formulated in two GDG meetings. The first took place face-to-face in Morges (Switzerland) on 13-15 July 2015, while the second took place by online video conference on 29 May 2018. The interval between the two meetings and the need for the second meeting was due to the time needed to assess the strength of the evidence as a whole, to update the systematic reviews and to draft the guidelines. For both meetings, the systematic reviews and GRADE evidence profile tables were shared with members of the GDG in advance. They were also discussed in preliminary teleconferences and other correspondence between the designated members of the GDG, the WHO Secretariat, systematic review teams and the methodologist. This led to the preparation of draft recommendations, which, together with the systematic reviews (in particular the summaries of evidence for the effects of the interventions or exposures on the prioritized health outcomes and the information on the quality of the evidence), were used by the GDG, WHO Secretariat and methodologist as a basis for discussing and agreeing the recommendations for each topic. The evidence from the systematic reviews was supplemented by the wide range and depth of experience across the GDG and, for instance, this information supported the assignment of the "trivial" categorization for the undesirable effects of some interventions, as detailed in the evidence to decision (EtD) tables.
The EtD framework for public health was used to derive and formulate the recommendations. Each recommendation was defined as either "strong" or healthy housing, the GDG recognizes that not all recommendations will be implementable to the same extent and at the same time in all contexts. The implementation of the HHGL recommendations will require prioritization to best meet a country's most urgent needs, which can subsequently be expanded to longer term commitments to work gradually towards a full implementation.
The GDG also considered resource use, which was not a standard feature of the studies available in the systematic reviews of health outcomes, but was available in some cases (25,71). The GDG discussions covered the perspectives of members of the public who might be the individual payers for some interventions (e.g. smoke alarms, retrofitted house insulation, and home improvements for better accessibility), as well as governments, other parts of the public sector and the private sector who might be responsible for larger scale investments (e.g. to improve the housing stock generally) or actions (e.g. to introduce legislation around housing safety). Details of such considerations are provided in the EtD tables.

# Decision-making during the GDG meetings
The technical consultations were guided by a preset protocol, allowing participants to discuss the draft recommendations where necessary; each of the pre-drafted recommendations was revised through group discussion.
The final recommendations were agreed on unanimously by the GDG; only the recommendation on cold was adopted through a majority vote. Any concerns expressed about the adopted strength of a recommendation are noted in the guidelines. WHO staff, the guideline methodologist and observers at the meetings were not eligible to vote. If an issue to be voted upon involved research (including systematic reviews) conducted by any member of the GDG who had declared a conflict of interest, the members in question would be allowed to participate in the discussion, but excluded from the final vote on that particular issue.

# Declaration of interests by external contributors
In accordance with WHO policy, all members of the GDG and ERG were required to complete and submit a WHO declaration of interest form. The WHO Secretariat, with support of the SG, reviewed and assessed the declarations submitted by each member and agreed on an approach to assess potential conflicts of interest.
A briefing was provided at the beginning of the first GDG meeting in Washington (DC) (April 2013) on the nature of all types of competing interests (i.e. financial, academic/intellectual and non-academic). Each member of the GDG was asked to discuss and declare to the meeting any conflicts they may have.
All appointed members of the GDG and the ERG completed the WHO declaration of interest forms and no conflicts of interest were declared.

# Document preparation and peer review
The first draft recommendations from July 2015 were used as the basis for drafting the chapters of the HHGL. The draft chapters were prepared by the WHO Secretariat, designated members of the GDG and the methodologist, before being circulated to the GDG and the ERG as a whole for revision and approval. The WHO Secretariat used external reviewer comments to make suggested revisions to these initial drafts and further revisions and review took place of the revised draft in June 2018. The reviewers' comments and any suggested changes to the recommendations were then circulated to the GDG and SG for final agreement. No disagreement was expressed by reviewers.

# Household crowding

#   Household crowding
Household crowding is a condition where the number of occupants exceeds the capacity of the dwelling space available, whether measured as rooms, bedrooms or floor area, resulting in adverse physical and mental health outcomes (72,73). Crowding is a result of a mismatch between the dwelling and the household. The level of crowding relates to the size and design of the dwelling, including the size of the rooms, and to the type, size and needs of the household, including any long-term visitors. Whether a household is "crowded" depends not only on the number of people sharing the dwelling, but on their age, their relationship and their sex. For example, a dwelling might be considered crowded if two adults share a bedroom, but not crowded if those adults are in a relationship (74)(75)(76). Crowding relates to the conditions of the dwelling as well as the space it provides: people may crowd into particular rooms in their home to avoid cold or uninhabitable parts of the dwelling or to save on heating and other costs (54).
The effects of crowding can be broadly defined as the hazards associated with inadequate space within the dwelling for living, sleeping and household activities (77). Crowding is considered to be stressful to health and well-being across different cultures and aspects of life in low-, middle-and high-income countries (78). Several studies have reported a direct association between crowding and adverse health outcomes, such as infectious disease and mental health problems. In addition, researchers have connected crowding to poor educational attainment (79).
Worldwide, crowding is often a marker of poverty and social deprivation (80,81). It has been identified by the United Nations as one of five deprivations that suggest an informal settlement should be characterized as a slum (82).
Crowded households are also often exposed to housing risks discussed in other chapters in the HHGL. For example, the income constraints that compel people to live in dwellings with inadequate space for their needs (78) can also mean that such households struggle to afford housing that is in good repair or to heat homes sufficiently (83,84). In addition, crowding increases exposure to risk factors associated with home injury, social tensions and exposure to second-hand tobacco smoke (SHS) (85,86).
In order to establish clear guidance on minimizing the health risks associated with crowding, a systematic review of the evidence was commissioned.

# Question for the systematic review
In the general population exposed to household crowding, what is the exposure-response relationship between exposure to crowding and the proportion of persons with poorer health compared with the population not exposed to household crowding?
The systematic review focused on the following priority health outcomes:
- close-contact infectious diseases
- gastroenteritis and diarrhoeal diseases
- mental health, including psychological stress - sleep disturbance.

# Guideline recommendation

# Strength of recommendation
Strategies should be developed and implemented to prevent and reduce household crowding.
- Each Member State should choose an appropriate way to measure the amount of crowding in a household, including a threshold that can be used to define a household as "crowded".
- Implementing agencies can draw on a range of existing measures of crowding (also described as "overcrowding") to determine a measure appropriate to their context (see Table 3.1). Specific guidance exists for emergency shelters (87). 4 - While the prevalence of infectious diseases varies between countries, the evidence of an association between crowding and adverse health effects is such that implementing agencies should work to reduce crowding regardless of the local prevalence of specific infectious diseases.
Following United Nations High Commissioner for Refugees (UNHCR) standards, emergency shelters located at public buildings are recommended to have 4.5-5.5 m 2 per evacuee (refugee) in cold climates, as residents remain inside the shelters during daytime (87).
Overcrowding occurs if there are more than three people per habitable room (88).

# American Crowding Index
Crowding occurs if there is more than one person per room; severe crowding occurs if there are more than 1.5 persons per room (excluding bathrooms, balconies, porches, foyers, hall-ways and half-rooms) (89).

# Argentinian National Institute of Statistics and Censuses
Overcrowding represents the quotient between the total number of people in the home and the total number of rooms or pieces of the same (90).
Households with critical overcrowding are considered those with more than three people per room (excluding the kitchen and bathroom) (91).

# Canadian National Occupancy Standard
Overcrowding occurs if extra bedrooms are required to ensure that each of the following have their own bedroom:
- cohabiting adult couple - lone parent - unattached household member aged 18 years or over - same sex pair of children aged under 18 years - each additional boy or girl in the household (unless there are two opposite sex children under 5 years, in which case they can share a bedroom) (75).

# British Bedroom Standard
Overcrowding occurs if extra bedrooms are required to ensure that each of the following have their own bedroom:
- cohabiting adult couple - person aged over 21 years - same sex pair of children aged 10-20 years - two children aged less than 10 years - two children where one is aged 10-20 and one is aged less than 10 years - any other person aged under 21 years that is not paired under one of the preceding categories (76).
Overcrowding occurs if the household does not have at its disposal a minimum number of rooms equal to:
- one room for the household - one room per couple in the household - one room for each single person aged 18 years or more - one room per pair of single people of the same gender between 12-17 years - one room for each single person between 12-17 years and not included in the previous category - one room per pair of children under 12 years (74). - Having considered the certainty of the evidence, the balance of benefits to harms related to reducing crowding, the values and preferences associated with reducing crowding, and the feasibility of reducing crowding, the GDG made a strong recommendation.

# Summary of evidence

# Infectious diseases
When interpreting the following results, it needs to be considered that the relationship between crowding and infectious diseases depends on the background prevalence of the disease in the specific setting.

# Tuberculosis (TB)
Much of the research on the association between crowding and infectious diseases concerns TB.
Twenty-one studies -ten case-control (92)(93)(94)(95)(96)(97)(98)(99)(100)(101), eight cross-sectional (102-109), two ecological (112,110) and one retrospective cohort (111) -were identified that related crowding to TB. These studies were consistent in showing that crowding is associated with increased risks of TB, even though the positive association was not statistically significant in a small number of the studies.
Four studies investigated the effect of different levels of crowding on the incidence of TB (98,99,104,106). In these studies, increasing numbers of persons per room were analysed in relation to the incidence of TB. One of these found a significant increase for two to four persons/room in comparison with one person/room but not at greater than four persons/room (104), while the other three studies did not show a statistically significant relationship between increased crowding and the incidence of TB (greater than one and half, greater than two, one to three, three to five persons per room) (98,99,106). In the two studies that examined an exposure-response relationship for crowding and TB, one found a consistent relationship (112), but the other did not (99). The 15 other studies used a threshold for crowding, comparing crowded with non-crowded households. Although crowding was not found to have a statistically significant association in four studies (92,95,97,105), crowding was significantly associated with TB in each of the other 11 studies (93, 94, 96, 100-103, 107, 108, 110, 112).
The certainty of the evidence that reducing crowding would reduce the risk of TB was assessed as high.

# Respiratory diseases (excluding TB)
Thirty studies reporting on outcomes due to respiratory infectious diseases other than TB were included in the systematic review. These investigated flurelated hospitalizations and illnesses: seven studies (113)(114)(115)(116)(117)(118)(119); pneumonia: six studies (120)(121)(122)(123)(124)(125); acute respiratory illness: 16 studies (126-141); and respiratory syncytial virus: (142). The study designs included 14 cross-sectional, six casecontrol, five cohort (including a randomized trial in which the intervention was not related to housing, which was an incidental variable) and five ecological studies.
Across the majority of studies on non-TB respiratory diseases, the risk of acquiring the diseases was associated with crowding.
The certainty of the evidence that reducing crowding would reduce the risk of non-TB respiratory disease was assessed as moderate to high, depending on the disease.

# Diarrhoea and gastroenteritis
Thirteen studies -two case-control (143,144), seven cross-sectional (145)(146)(147)(148)(149)(150)(151) and four cohort (152)(153)(154)475) -were identified that related crowding to diarrhoea or gastrointestinal diseases or parasites, showing that crowding appears to be associated with gastroenteritis and diarrhoeal diseases. Among the included studies, four looked at the effects of different levels of crowding (145,147,148,155). In two of the studies, the higher levels of crowding (greater than three or four people per room) were associated with significantly more cases of diarrhoea compared with the lower levels (less than two or four people per room) (147,153). In two studies, the level of crowding did not significantly affect the number of cases of diarrhoea, but in one of these studies all levels of crowding were associated with the surrogate outcome of increased intestinal parasite infection (145).
The certainty of the evidence that reducing crowding would reduce the risk of gastroenteritis and diarrhoeal diseases was assessed as high.

# Other infectious diseases
Twenty-five studies investigated an association between crowding and other infectious diseases such as rheumatic fever and heart disease: five studies (156)(157)(158)(159)(160); typhoid fever: one study (161); meningococcal disease: seven studies (162)(163)(164)(165)(166)(167)(168); throat eye and skin infections: three studies (137,169,170); dengue fever: one study (171); Helicobacter pylori: one study (172); methicillin-resistant Staphylococcus aureus: two studies (173,174); parasite Toxoplasma gondii: one study (175); Epstein Barr virus: one study (176); neonatal infections: one study (177); multi-drug non-susceptible enteric infections: one study (178); and risk factors for WASH: one study (179). Study designs included ten cross-sectional, nine case control, one ecological and five cohort studies. In general, the risk of acquiring the infectious diseases was associated with crowding.
The certainty of the evidence that reducing crowding would reduce the risk of other infectious diseases was assessed as moderate to low, depending on the disease.

# Non-infectious health disorders

# Mental health including stress
Of the 13 separate studies in this category (one of which assessed two different mental health outcomes), eight studies reported at least one significant association between household crowding and the mental health outcome. A prospective cohort study (180), a retrospective cohort study (181) and five cross-sectional studies (182)(183)(184)(185)(186) all reported that participants living in a crowded household were more likely to report a mental health problem than those not living in crowded conditions. These mental health concerns included: psychological distress, alcohol abuse, feeling depressed and feeling unhappy about one's health. One cross-sectional study further found that crowding was associated with a lower prevalence of psychiatric disability (187).
Four cross-sectional studies could not detect any relationship between crowding and mental health outcomes such as inattention-hyperactivity and emotional symptoms (130), psychological distress (188), suicidal ideation and self-esteem (189), or drug abuse (186). Further, one retrospective cohort study carried out in Israel reported no association between crowding during infancy and development of schizophrenia in later life (190) and one cohort study conducted in the United States of America found no link between overcrowding and autonomic nervous system reactivity or externalizing behaviour problems (474).
The certainty of the evidence relating crowding to adverse mental health effects, including stress, was assessed as moderate to low.

# Sleep disorders
Two recent cross-sectional (191,192) and one ecological study (193) investigated the associations between crowding and sleep disorders. One cross-sectional study found excessive daytime sleepiness with greater than one per room (192) but the other study concluded that living in a crowded household (greater than or equal to one per room) is not significantly associated with most outcomes relevant to sleep disturbance but did find a significant relationship between crowding and duration of sleep in some analyses (191).
The ecological study found a significant positive relationship between percentage of neighbourhood-level crowding (greater than one per room)
and the apnoea-hypopnoea index (193).
The certainty of the evidence that reducing crowding would reduce the incidence of sleep disorders was assessed as low to very low.
In summary, the systematic review found high certainty evidence that crowding is associated with an increased risk of TB and diarrhoea. There is moderate to high certainty evidence for a positive relationship between crowding and other respiratory infectious diseases. The certainty of the evidence that crowding is associated with an elevated risk of other infectious diseases and poor mental health is moderate to low; and very low for the linkage between crowding and sleep disorders.

# Considerations for implementation of the guideline recommendation
Reducing crowding has implications for national and local governments, which usually need to build and refurbish housing, subsidize social or public housing, regulate private rental housing, implement tax and planning policies that encourage the building of affordable housing, and work with community leaders in informal settlements. Ensuring housing that is not only available, but also appropriate and affordable, is crucial to reducing crowding. If reducing crowding entails people moving to another location, it might have detrimental effects by removing them from social networks, child care support, and work or educational opportunities, affecting health and earning opportunities (194,195). If new housing is situated in low-density or sprawling developments, it can reduce physical activity (196,197). If new housing is not affordable, people may have difficulty paying for other essentials including food, energy and health care (59). Therefore, an integrated policy approach, in which reductions in crowding are supported by appropriate rehousing that takes these considerations about potentially unintended effects into account, is fundamental to equity. Reductions in crowding will be most effective if combined with policies that support employment and improve household incomes to increase the affordability of homes with sufficient space. A supportive social welfare system further ensures that loss of job or other income shock does not entail moving into a dwelling with inadequate space in order to reduce costs.
When developing policies to reduce crowding, policy-makers and technical advisors also need to consider the relevance of crowding measures to different subpopulations (78). Depending on the cultural context, an inhabitant's perception of an overcrowded home might vary and different standards to determine adequate housing space might apply. Table 3.1 provides an overview of different crowding measures that can be applied to assess the prevalence and level of crowding in different settings.

# Research recommendations
The research reviewed shows that crowding is associated with negative health outcomes. However, the study designs, and the close association between social deprivation and crowding, caution against the attribution of causation.
The research base could be further strengthened through focusing on the research priorities shown in Table 3.2. 5

# Current state of the evidence
Although there is good evidence on the association between crowding and poor health outcomes, most studies to date are observational and there is considerable heterogeneity in their design. Meta-analysis is difficult because studies define crowding differently, focus on different outcomes and subgroups of interest, and have used different approaches to adjust (or not) for confounding. Further high-quality studies are required, including randomized trials and comparative studies, perhaps using cluster randomized designs. Such studies might test the impact of new housing policies intended to reduce overcrowding, and subsequent effects on health outcomes. Future research should also examine the exposure-response relationships between crowding and health outcomes, including mental health outcomes and intellectual development of children, and make adjustments for confounding. In order to help others to compare, contrast and combine the results of different studies, researchers should use standard and internationally recognized measures of crowding and common approaches to recording and reporting outcomes.

# Population of interest
Populations living in residential housing. There is a particular need to understand the effects of crowding on different subpopulations (in particular men, women, children, the elderly, indigenous and at-risk populations).

# Interventions of interest
Policies and interventions to reduce crowding, including through extending existing homes, through rehousing and policies that support employment and improve household incomes.

# Comparisons of interest
Groups living in crowded and non-crowded home environments; groups before and after interventions to reduce crowding. It is also important to compare the effect on health of people living in different levels of crowding (i.e. "crowding" as opposed to "severe crowding") and people living for different lengths of time in crowded housing (exposureresponse relationships).

# Outcomes of interest
Key outcomes of interest are TB and other infectious diseases, gastroenteritis and diarrhoeal diseases, sleep quality, intimate partner violence and mental health.

# Time stamp
Current systematic review included studies published up to April 2018.

#   Low indoor temperatures and insulation
Cold air inflames lungs and inhibits circulation, increasing the risk of respiratory conditions, such as asthma attacks or symptoms, worsening of chronic obstructive pulmonary disease (COPD), and infection. Cold also induces vasoconstriction, which causes stress to the circulatory system (198) that can lead to cardiovascular effects, including ischaemic heart disease (IHD), coronary heart disease, strokes, subarachnoid haemorrhage and death (198)(199)(200)(201)(202)(203)(204)(205)(206). Most of the evidence for the impact of cold on health comes from studies connecting outdoor temperatures to health outcomes. For example, cold spells are associated with increased mortality and respiratory and cardiovascular morbidity (207), and mortality and morbidity rates in countries with cold and temperate climates are higher in winter than in summer (208).
Evidence that cold indoor temperatures have adverse consequences for health is growing (209,210). Cold indoor temperatures are often a consequence of outdoor temperature, structural deficiencies, including a lack of insulation and airtightness, and lack of heating. As outlined in this chapter, cold indoor temperatures have been associated with increased blood pressure, asthma symptoms and poor mental health. Cold homes contribute to excess winter mortality and morbidity. Most of the health burden can be attributed to both respiratory and cardiovascular disease, especially for older people. In children, the excess winter health burden is mostly due to respiratory disease.
Excess winter deaths due to cold housing has been estimated at 38 200 per year (12.8/100 000) in 11 selected European countries (18).
Winter mortality is greater in countries with milder climates than in those with more severe winter conditions (211), in part because countries with mild winters often have homes characterized by poor domestic thermal efficiency that are harder to heat than well insulated houses in more extreme climates. In insulated dwellings, thermal insulation reduces conductive heat loss through the buildings' walls, ceilings and floors. Retrofitted insulation, otherwise known as "weatherization" also reduces convective heat loss by blocking unwanted air leaks through the building envelope. As outlined in this chapter, retrofitted insulation, weatherization, and heating can help mitigate the effect of otherwise cold housing on health.
Socioeconomic factors play an important role in determining whether a dwelling is sufficiently warm. Income constraints force people to live in housing that is older, more likely to be poorly built and lacking insulation.
These deficiencies, in addition to lack of energy affordability, can make it especially difficult for people on low incomes to heat their houses adequately.
For example, a study carried out in South Africa showed that informal dwellings were more vulnerable than other types of dwellings to indoor temperature instability, which affected thermal comfort (212).
In order to assess the evidence on minimizing the health risks associated with cold indoor temperatures and the effects of insulating houses, two systematic reviews were commissioned. that "there is no demonstrable risk to human health of healthy sedentary people living in air temperature of between 18 and 24 C" (213).
The systematic review focused on the following priority health outcomes, as ranked by the GDG:
- respiratory morbidity and mortality
- all cause-mortality in infants
- hospital admissions
- cardiovascular morbidity and mortality

# Question for the second systematic review (intervention)
Do people living in housing with insulation have better health outcomes than those living in housing without insulation?
The systematic review focused on the following priority health outcomes, as ranked by the GDG:
- respiratory morbidity and mortality
- cardiovascular morbidity and mortality
- hospital admissions - all cause-mortality - depression
- high blood pressure.

# Guideline recommendations

# Strength of recommendation
Indoor housing temperatures should be high enough to protect residents from the harmful health effects of cold. For countries with temperate or colder climates, 18 C has been proposed as a safe and well-balanced indoor temperature to protect the health of general populations during cold seasons.
In climate zones with a cold season, efficient and safe thermal insulation should be installed in new housing and retrofitted in old housing.
- There is an association between cold indoor temperatures and adverse health effects, and an association between retrofitting insulation in housing and improved health outcomes. Implementing agencies should work to increase temperatures in cold homes, including through installing insulation with appropriate ventilation, as this is likely to have beneficial effects on health.
- While current evidence is insufficient to establish the precise temperature below which adverse health effects are likely to occur, there is high certainty that taking measures to warm cold houses will have significant health benefits and a minimum of 18 C is widely accepted.
- A higher minimum indoor temperature than 18 C may be necessary for vulnerable groups including older people, children and those with chronic illnesses, particularly cardiorespiratory disease (213).
- The GDG assessed the certainty of the evidence to indicate the extent to which the research supports the recommendation. The certainty of the evidence that warming a cold house reduces the risk of cardiovascular disease is moderate (based on the findings for blood pressure). The certainty of the evidence that installing insulation is associated with improved health outcomes is high but this is qualified by different types of insulation.
- Having considered the certainty of the evidence, the values and preferences associated with indoor thermal condition, the balance of benefits to harm related to increasing indoor temperatures and installing insulation, and the feasibility of taking these measures, the GDG made a strong recommendation regarding cold and a conditional recommendation regarding insulation.

# Summary of evidence
This section summarizes the systematic reviews of the associations between indoor cold and health outcomes, and the benefits to health of thermal insulation in the home environment. The systematic reviews on indoor cold and on insulation against cold and the GRADE tables used to assess the certainty of the evidence are available online at / sustainable-development/publications/housing-health-guidelines/en/index.
html in Web Annex B and Web Annex C.

# Respiratory morbidity and mortality
Of the four studies identified in the systematic review, three found that colder indoor temperatures increased respiratory morbidity. One cross-sectional study in adults with COPD found better health status with more hours of indoor temperature at and above 21 C. A dose-response trend was observed for number of days with bedroom temperatures of 18 C and above for at least 9 hours. The greatest effects were observed in adults who smoked compared with non-smokers (214). Similarly, modelling based on the results of a randomized trial involving children with asthma found that every 1 C increase in room temperature below the threshold of 9 C, was associated with a small but significant increase in lung function. Bedroom exposure was shown to have stronger association with asthmatic children's lung function than living room exposure (215). In addition, one cohort study, including adults with COPD, from China reported reduced respiratory problems with an indoor temperature at 18.2 C regardless of whether indoor humidity was low, moderate or high (216). In contrast, a case-control study in children with and without upper respiratory tract infections showed no consistent associations with indoor temperature (217).
The certainty of the evidence that warming a cold house (perhaps to a minimum indoor temperature of 18 C) would reduce the risk of respiratory mortality and morbidity was assessed as moderate.

# Cardiovascular morbidity and mortality: blood pressure
Of the six included studies that assessed the association between indoor temperature and blood pressure, all showed that lower temperatures were associated with higher blood pressure, including two randomized trials in Japan that found higher blood pressure in people living in colder homes (218,219).
A cohort study in Japan of adults over 60 years of age found that decreases of 1 C in indoor temperatures were significantly associated with increased blood pressure levels at different times of the day, even after controlling for potential confounders (220,221). There was a stronger association of indoor temperature than outdoor temperature with ambulatory blood pressure, which suggested that excess winter cardiovascular mortality could be prevented by improving the housing thermal environment (221). Two cohort studies from Scotland found people in housing heated to less than 18 C had a greater risk of high blood pressure (222,223). This risk increased if temperatures were below 16 C (OR 4.92) (223). Similarly, a cohort study in the United Kingdom found a decrease in systolic and diastolic blood pressure of 0.5 mmHg per 1 C increase in room temperature (224).
The review also identified five studies of temperature and blood pressure that were done under laboratory conditions (225)(226)(227)(228)(229). The studies show a relationship between warming and lower blood pressure but, because this is indirect evidence for the relationship between blood pressure and housing indoor temperature, the studies were not used in formulating the recommendations.
The certainty of the evidence that warming a cold house (to a minimum indoor temperature of 18 C) would reduce the risk of cardiovascular mortality and morbidity was assessed as moderate.

# Insulation and weatherization
Of the 11 studies identified in the systematic review, seven found some association between the benefits of living in an insulated home and improved health. For example, a cluster randomized trial in New Zealand on the effect of insulating existing homes where at least one person in the household had existing chronic respiratory symptoms found that insulation was associated with reduced odds of poor mental health, self-reported wheezing in the previous 3 months, winter colds or flu, and morning phlegm in adults (39).
While mental health was improved in one controlled trial from the United
States of America, the study did not find any differences in general health status between people receiving new insulation and exterior cladding and those in the control group (230). One quasi-experimental study from the United Kingdom found no difference between asthmatic and healthy children with regard to different glazing systems (231). Another quasi-experimental study in New Zealand found that all-cause mortality was significantly lower in people with a history of cardiovascular disease if they lived in an insulated rather than an uninsulated house and non-significantly lower in people with a history of respiratory disease (232). Similarly, a controlled trial from the United Kingdom did not detect any effect of external insulation on general respiratory symptoms, asthma, physical or mental health or subjective well-being (233).
A cross-sectional study from the United Kingdom investigated the effects of different types of insulation on a range of health outcomes (234). The study identified positive effects of loft and external wall insulation on respiratory, mental and general health; but found a negative impact on these outcomes with cavity wall insulation.
Three retrospective cohort studies investigated the effects of living in an insulated home on health. A New Zealand study of 45 000 households, with matched controls, showed no relationship between living in an insulated home and rates of hospitalization. However, mortality rates for adults aged 65 and over who had previously been hospitalized for circulatory illness were lower for people living in insulated dwellings (235). A study from Scotland, looking at the indoor environment and health outcomes as reported by participants, found that rates of coughing were significantly lower in homes with doubleglazed windows but no consistent relationship between wheezing and coughing, and insulation (236). A study from Greenland, of households with children aged 3 to 5 and 8 years who had a previous medical attendance for acute otitis media, found no relationship between episodes of acute otitis media and self-reported poor insulation, defined as "reports of draft along the floors and through doors and windows" (237). A historical cohort study conducted in the United Kingdom reported that double glazing improved the household health status by 4.8% but did not detect effects on quality of life or other measures of well-being (238).
One case-control study from Denmark, which had a high risk of bias, found that eye irritation and throat dryness (connected to respiratory health) decreased slightly when windows were replaced, but the results were not statistically significant (239).
The certainty of the evidence that living in insulated homes is associated with improved health outcomes was assessed as moderate.

# Considerations for implementation of the guideline recommendations
In a cold climate, a healthy indoor thermal environment can be achieved through a combination of thermal insulation and heat supply. Building a properly ventilated and thermally insulated house is more technically advanced and expensive than building a non-insulated house, but is likely to lead to health and other benefits, with some evidence that the cost-benefit ratio can be as high as six (232). On a macro-level, improving energy efficiency of dwellings was found to lead to cost savings and in some countries the clear co-benefits of retrofitted insulation on health and energy efficiency mean that these retrofits are already subsidized by governments. For example, it is estimated that improvements in occupants' health by improving housing in the United Kingdom, including through increasing warmth in bedrooms, would save the United Kingdom health services 1.4 billion in the first year in treatment costs alone (240). An insulation subsidy programme in New Zealand found reduced hospitalization costs due to fewer re-admissions, fewer transfers and shorter stays in hospital, although the rate of hospitalization was unchanged (40). In Cape Town, retrofitting of 2300 houses with solar water heating and roof insulation as part of the Kuyasa low-income housing project had multiple benefits for climate mitigation, respiratory health, poverty reduction, and economic development (241). Besides generating 2.82 tonnes of carbon credit per house annually and lowering heating expenditures, the improved insulation led to a "substantial decline in bronchial and related illness among residents, especially during winter" (241). More broadly, energy efficiency measures contribute to public savings by reducing the burden on energy infrastructure and the climate. Insulation can also help moderate extreme heat situations, as discussed in Chapter 5. Yet, cost-effectiveness will vary significantly for different climate zones and depends on housing quality, the type of insulation, the prior level of insulation and means of heating and ventilation of the housing stock.
At an individual level, there is a clear trade-off between investment costs (installing or retrofitting insulation and heating) and running costs (paying for energy). While people with low incomes are likely to benefit the most from public thermal efficiency programmes because they are more likely to live in cold homes (52), they will also be less likely to be able to afford to install insulation if the costs need to be covered by the inhabitants or home owners.
Therefore, it is essential to ensure that low-income people can afford to live in improved buildings, potentially through providing public support for housing costs; otherwise improvements in insulation might increase inequities (242).  (39).
Implementation also needs to consider the importance of using safe insulation materials, which are free of toxic substances such as asbestos and isocyanate, and are resistant to fire and microbial growth. Improved occupational safety and health protection and training for those involved in installing and maintaining thermal insulation may also be required to ensure that the health of workers is not compromised and that the intervention will be optimally effective (247). Authorization of the building retrofit designers and approval/ inspection of the actual work are necessary to ensure healthy and energy efficient results.

# Research recommendations
The specific mechanisms underlying the association between cold homes, lack of insulation, and poor health may involve both physiological responses or co-exposures to other associated factors causing adverse health problems, such as damp, mould, poor quality housing, poverty and social deprivation.
Further research is needed to investigate these associations and underpin the research priorities summarized in Tables 4.1 and 4.2 for the exposure (cold) and for the intervention (insulation).

# Current state of the evidence
While there is good evidence associating low indoor temperatures with adverse health outcomes, further studies, especially in developing countries and particularly in Africa, are required which assess exposure-response associations while controlling for potential confounders (including outdoor temperature). These should take into account peak (occasional low temperatures), chronic (extended periods of low temperatures) and cumulative exposure (215).

# Population of interest
The whole population; in particular, people that are both more likely to spend time at home and experience adverse health effects related to cold (e.g. the elderly, children, and people with long-term illnesses). Studies are required to establish whether 18 C is the most appropriate general target for the minimum indoor temperature and whether the minimum target should vary across different population groups.

# Interventions of interest
Policies and interventions to increase indoor temperatures to healthy levels, including through installing insulation, weatherization, and improved heating, including through alternatives such as solar panels, through subsidies and market-based initiatives (248).

# Comparisons of interest
Groups living in home environments in cold climates that are not heated to an adequate temperature and those that are; groups before and after intervention.

# Outcomes of interest
A range of health outcomes should be included, including indoor temperature and mortality and morbidity, and cardiovascular disease, asthma, COPD, infections and depression.

# Time stamp
Current systematic review on the association between indoor cold and health outcomes included studies published up to April 2018.

# Current state of the evidence
The evidence base on the effects of living in an insulated home on health outcomes should be strengthened through further high-quality studies, including randomized trials. These studies should control for confounding factors, including outdoor temperature, ventilation, and the availability, affordability and efficiency of heating. Research should prioritize professional, independent assessment of insulation levels and health outcomes over self-reports by occupants.

# Population of interest
The whole population; in particular, people that are both more likely to spend time at home and experience adverse health effects related to cold (e.g. the elderly, children, and people with long-term illnesses), who may particularly benefit from insulated housing.

# Interventions of interest
Installing insulation, and the introduction of market-based initiatives intended to encourage the installation of insulation.

# Comparisons of interest
Groups living in insulated and non-insulated home environments; groups before and after intervention.

# Outcomes of interest
Indoor temperature and relative humidity and change in air exchange rates, plus a range of health outcomes. These include indoor temperature and mortality and morbidity; in particular, cardiovascular disease, asthma, COPD, infections and depression.

# Time stamp
Current systematic review on the association between insulation and health outcomes included studies published up to April 2018. the temperature threshold where heat-related deaths begin to increase during a heat wave is lower in cities with cooler climates (262,263). Exposure to heat waves earlier in the season has a greater impact on mortality, because the population has not had a chance to adapt to higher temperatures (264).
The importance of acclimatization (i.e. physiological adaptation to excessive heat exposure) is stressed in the WHO-World Meteorological Organization (WMO) guideline for heat wave warning systems, but complete acclimatization to an unfamiliar thermal environment may take several years. Long-term adaptation results in a lower rise in core body temperature and a lower increase in heart rate at a given heat load (265). In addition, while people may adapt to usual temperatures, they may not be able to adapt to variable temperatures. Unstable temperatures harm the cardiovascular and immune system, and are associated with an increase of mortality (266).
Studies have shown an association between high indoor temperatures and adverse health effects (267). Outside of regions where air conditioning is common, high indoor temperatures are associated with high outdoor temperatures. In a case cross-over study carried out in three Latin America cities, same and previous day apparent temperatures were strongly associated with mortality risk, with susceptibility increasing with age (268). Therefore, studies of morbidity and mortality rates during periods of high outdoor temperatures can also be used to provide indirect evidence of the harmful health effects of high indoor temperatures in such regions. For example, during the 2003 heat wave in France, the number of deaths at home was considerably higher compared with years without extreme heat events (269).
In Japan, a study showed that heatstroke most often occurs at home during summer; elderly people developed heatstroke in their homes with greater frequency compared with patients in other age groups (270,271). In summary,
given that people spend most of their time indoors (2) and that, in the absence of air conditioning, they will be exposed to an increased risk of high indoor temperatures during periods of high outdoor temperature, protection against outdoor heat is a key characteristic of healthy housing. The review identified six studies that included indoor dwelling temperature as an exposure variable. However, none of these studies provided direct evidence on the prioritized health outcomes or the minimal risk temperatures for heat-related health effects. Therefore, no firm answer can be given to the question of whether people living in housing with a temperature above 24 C have worse health outcomes than those living in housing with an indoor temperature below that threshold.

# Guideline recommendation

# Strength of recommendation
In populations exposed to high ambient temperatures, strategies to protect populations from excess indoor heat should be developed and implemented.
- Identification of the minimal risk temperature for heat-related health effects will require research to determine the indoor temperature below which no adverse health effects related to heat are expected. Likewise, research is needed to identify the "maximum acceptable temperature", above which the risk to human health increases drastically. As people are acclimatized to different temperatures in different climate regions, the optimal indoor temperature range is dependent on the specific region.
Examples of minimal risk temperature for heat-related health effects and maximum acceptable temperature are listed in infants, sick and the disabled, below the maximum acceptable temperature.
- The GDG assessed the certainty of the evidence to indicate the extent to which the research on each health outcome supports the recommendation.
As there are so few studies of the direct effect of high indoor temperature on health, the certainty of the evidence that reducing high indoor temperatures would reduce morbidity and mortality was assessed as low to very low.
However, there is higher certainty of evidence that there is a relationship between high indoor and high outdoor temperatures and that high outdoor temperatures are associated with morbidity and mortality.
- Therefore, having considered the certainty of this range of evidence on high temperature and health, the balance of benefits to harms of preventing exposure to high indoor temperature, the values and preferences associated with preventing exposure to high indoor temperature, and the cost and feasibility of preventing such exposure, the GDG made a conditional recommendation.

# Summary of evidence
The systematic review on the association between high indoor temperatures and adverse health outcomes identified eight eligible studies. Further, indirect evidence to support the association between high indoor temperatures and adverse health outcomes was identified in the following manner:
Step 1: Studies on health and outdoor temperature were reviewed to obtain estimates of the relationship between outdoor temperature and health outcomes.
Step 2: Studies that measured both indoor and outdoor temperature were reviewed and used to model the association between indoor and outdoor temperatures. This association was used to derive indoor temperature based on the outdoor temperature.
- Step 3: An assumption was taken that the estimates derived in Step 1 would equally apply to the indoor temperatures calculated in Step 2. These were used to support the recommendations regarding high indoor temperatures.

# Temperature and morbidity

# Indoor temperature and morbidity
Eight studies investigated the effect of indoor heat on health outcomes including, sleep disorders (three studies); general health, blood pressure, respiratory and cardiovascular disease (two studies each); and body temperature, mental health, pregnancy outcomes (one study each).
One quasi-experimental study of 57 people in the United States of America found that reductions in number of days above 27 C corresponded with improved quality of health and life, reduced emotional distress and increased hours of sleep (272).
Three cohort studies explored the association between indoor heat and morbidity. While there were no associations between indoor temperatures and reports of respiratory viral infection or heat illness in a cohort of 40 households in the United States of America, the same study found a significant relationship in sleep problems and prior day's temperature in the summer season but not in winter (273). Similarly, among 113 elderly people in the Netherlands, an increase of 1 C of indoor temperature raised the risk of sleep disturbance by 24% (in the temperature range of 20.8 to 29.3 C) (274).
A third cohort study, in Slovenia, reported worse cardiovascular symptoms with a higher heat burden and low indoor air quality (275).
One case series involving 20 low-income elderly people in the Republic of Korea and one cohort study including 132 women in India, found a nonsignificant positive relationship between indoor temperature and systolic blood pressure but a significant positive association with diastolic blood pressure (276,277).
A case-control study reported that humidity exposure and indoor heat above 26 C non-significantly increased the proportion of emergency calls in New York that were due to cardiovascular cases and respiratory distress calls (278).
Finally, a cross-sectional study among 1136 women in Ghana found a nonsignificant increase in adverse pregnancy outcomes, such as stillbirth or miscarriage, with each additional 1 C increase in atmospheric heat exposure (476).
In view of the mixed findings and the relatively small amount of evidence, the certainty of the direct evidence that reducing high indoor temperatures would reduce morbidity or mortality was assessed as low.

# Outdoor temperature and morbidity
In order to help with the discussions in the GDG, analyses were also done of the effects of outdoor temperature, for which studies on the association between high temperature and morbidity show a non-linear temperature-effect relationship. For example, associations between daily average temperatures and the relative risk estimates for hospital admissions for kidney disease are U-shaped (admissions occur in both the lower and higher temperature ranges) or J-shaped (admissions occur in the higher temperature ranges).
Temperatures at around 25 C present the lowest risk for hospital admission for kidney disease, and high temperatures increase the risk of admission more than low temperatures (254). Although heat waves are significantly associated with elevated risk of cardiovascular hospitalizations (279), recent meta-analysis indicated no apparent association between increased ambient temperature and cardiovascular morbidity (255,280).
The certainty of this evidence linking high outdoor temperature with increased morbidity was assessed as low to moderate and, although indirect, it was used in conjunction with the evidence on the relationship between outdoor and indoor temperature to provide support for the recommendation on indoor temperatures.

# High temperature and mortality

# High indoor temperature and mortality
No eligible studies assessed the effect of high indoor temperature on mortality.

# High outdoor temperature and mortality
Reviews and meta-analyses provide strong evidence of the association between high outdoor temperature and mortality (251,252,264,279,281 (281).
The certainty of the evidence relating high outdoor temperature to mortality was assessed as high, and, although indirect, it was used in conjunction with the evidence on the relationship between outdoor and indoor temperature to provide support for the recommendation regarding indoor temperatures.

# Relationship between indoor and outdoor temperature
Thirty-two studies were identified which reported the relationship between indoor and outdoor temperature. These showed a positive correlation in the higher temperature range (>20 C). The slope of a linear regression between outdoor and indoor temperatures in the warm/hot temperature range (>20 C)
varied depending on several factors including air conditioning, ventilation, insulation, building direction, socioeconomic status and the behaviour of the occupants.
Most of the studies were conducted in temperate climate zones. The studies show that the slope of the correlation curves between indoor and outdoor temperatures in this climate zone was usually not steep, which may be due to the influence of air conditioning in some dwellings (286) and the practice of keeping the windows closed rather than opening them for ventilation (287).
Specific cities have unique correlation curves (288,289). Some studies are better able to predict indoor temperatures from outdoor temperatures by incorporating other environmental factors into a multivariate regression.
These factors include urban heat islands affecting the immediate environment (290), solar radiation (291,292) or dwelling characteristics (293). The relationship between indoor and outdoor temperatures depends on the socioeconomic status of the participants with indoor temperatures of the dwellings of lowincome residents more closely associated with outdoor temperatures because they are not influenced by the use of air conditioners (276,294). Indoor and outdoor temperatures are also more closely correlated where the occupants opened their window for ventilation (295).
Fewer studies have been done in subtropical regions, but these also suggest relationships between indoor and outdoor temperatures. In low-income countries, where air conditioning is seldom available, indoor temperature is directly related to outdoor temperature (296)(297)(298). The relationship becomes weaker further away from the equator (299,300). There is no relationship between indoor and outdoor temperature in countries where air conditioning is widely available everywhere, such as Oman (301). One study found that the indoor temperature could be higher than the outdoor temperature due to cooking activities and inadequate ventilation (302).
The certainty of the evidence that indoor and outdoor temperatures are correlated was assessed as moderate to high, and this indirect evidence was used in conjunction with the evidence on the relationship between high outdoor temperatures and adverse health outcomes to provide support for the recommendation regarding indoor temperatures.

# Considerations for implementation of the guideline recommendation
Thermal insulation, housing location, building materials and house orientation,

# Research recommendations
Some of the evidence summarized in this chapter is indirect, based on the association between indoor and outdoor temperatures, and the association between high outdoor temperatures and health outcomes. However, there is great variation between outdoor and indoor temperatures in terms of the types of heat exposure (direct or indirect sun), the types of activity typically carried out, and the possible interactions with other risk factors associated with housing, including humidity, housing conditions, socioeconomic conditions, the type of structure (including insulation), ventilation/air conditioning, and outside temperature. Further research should therefore focus on the direct effects of high indoor temperatures on health.

# Current state of the evidence
Few high-quality studies have assessed the direct effects of indoor temperature on health.
The current research base is limited by a lack of data on indoor household temperature and the difficulty of designing studies capable of entirely excluding the health impact of outdoor temperatures. High-quality studies that control for confounders are required. These should focus on exposure-response associations and take into account peak (occasional high temperatures), chronic (extended periods of high temperatures) and cumulative exposure.

# Population of interest
The whole population; in particular people who are both more likely to spend time at home and experience adverse health effects related to heat (the elderly, children, women, obese people, and people with long-term illnesses). Studies across different groups will help establish whether the threshold level (the temperature above which temperature poses a risk to health) is different for different population groups.

# Interventions of interest
Installing ventilation and other measures aimed at reducing indoor temperature in housing in hot climates; and moving people to cooler housing in hot climates.

# Comparisons of interest
Groups living in hot climates in home environments at different temperatures; groups before and after interventions to reduce indoor temperatures.

# Outcomes of interest
A range of health outcomes should be included, including mortality and morbidity generally, but, in particular, cardiovascular disease, blood pressure, respiratory symptoms, sleep disturbance, heatstroke, hyperthermia, dehydration.

# Time stamp
Current systematic review on the direct association between high indoor temperatures and health outcomes included studies published up to April 2018. The literature review to provide evidence on the indirect relationship between high indoor temperatures and health was done by members of the GDG in April 2016.

#   Injury hazards
Injuries in the home present an important health burden worldwide (307).
Globally, around a third of injuries occur in the home (308), and, in 2016, half of all unintentional injury-related deaths occurred in the home (309). Although injuries in the home affect people of all ages, home injury rates are highest in the youngest and oldest age groups (310). They are also more common in people with functional impairment and interventions to improve their housing conditions are discussed in Chapter 7.
Injuries in the home include falls, burns, poisonings, ingestion of foreign objects, smoke inhalation, drowning, cuts and collisions with objects, and crushing and fractured bones as a result of structural collapse. This chapter focuses on injuries caused by hazards that can be eliminated or controlled with proper attention to housing design and maintenance (311).
Falls account for the largest proportion of the injuries in the home that require medical attention (56). Worldwide, about 424 000 individuals die each year from falls, of which the vast majority are in low-and middle-income countries, and more than 37 million falls require medical attention (312). In Thailand, for example, almost 70% of severe injuries related to accidental falls occur in the home or residential areas (313). Similarly, falls are the leading cause for hospital admissions due to injuries in Fiji (314). In India, the rate of deaths from falls in urban areas, including the home, was 15.6 per 100 000 in 2005 (315). In high-income countries, about 26% of falls can be attributed to the environment, both inside and outside the home. In low-and middle-income countries, about 31% of falls are attributable to the environment (316). Hazards that encourage slips and falls that could result in injuries include: uneven floor surfaces; inadequate or inappropriate lighting; steep stairs, stairs of varied height, stairs without handrails or in disrepair; lack of guarding of stairs, landings and balconies; lack of grab-rails or handles to baths and showers;
and windows and doors without child safety locks (317)(318)(319). In Europe, in 2010, around 10 deaths (0.007 per 100 000) and 3310 DALYs (2.0 per 100 000) were attributable to a lack of window guards (that is, bars and other products that prevent people falling from windows) (18).
The home environment can also put people at risk of injury and death from burns. Injury from exposure to heat, fire and hot substances results from hazards such as an absence of smoke detectors, unsafe electric installation, open fires, unprotected hot surfaces and hot water. Around 268 000 deaths occur each year worldwide due to burns from exposure to fire, heat or hot substances, including in the home environment (49). In the developing countries of Africa, Asia and Latin America, kerosene use for cooking and lighting remains widespread, with fires and explosions being well-documented kerosene hazards (320). More specifically, in one hospital in Sri Lanka, 41% of patients admitted for burns had been injured by falling kerosene lamps (321).
In Bangladesh, the overall burn mortality rate was 2.2 per 100 000 in 2003, with 90% of burn incidences occurring in the home mostly caused by cooking fire, heating fire and fire from kerosene lamps (322). In Europe, 7523 deaths (0.9 per 100 000) and nearly 200 000 DALYs (22.4 per 100 000) are attributable to a lack of smoke detectors (18). Nearly a quarter of scald burns in children in the United States of America were caused by hot tap water (319). Unvented gas or solid fuel burning stoves also expose people to dangerous levels of carbon monoxide (discussed in Chapter 8).
Injury rates at home are sometimes higher for low-income people (323). This is partly because homes that contain hazards are more likely to be within the price range of people with low incomes. For example, a United States of America survey reported that fire extinguishers, fire escape plans and carbon monoxide alarms were much less common in low-income homes (310). In Thailand, severe injuries are more common for unskilled labourers than other population groups (313); and in Kenya, a low level of education was identified as a key risk factor for burn injury among patients admitted to a public hospital (324). Reducing hazards in homes can therefore contribute to reducing health inequalities (325,326). And, a study carried out in Nigeria, South Africa, Uganda and the United Republic of Tanzania found that living in an urban environment was associated with increased odds for injury (327).
In order to establish clear guidance on minimizing the health risk associated with hazards in the home, a systematic review of the evidence was commissioned.
Question for the systematic review Do residents in housing with fewer hazards have fewer injuries than residents living in housing with more hazards?
The systematic review focused on the following priority health outcomes, as ranked by the GDG:
- electrocution - broken or fractured bones
- mortality due to injuries
- burns or scalds
- hospitalization (outpatient or inpatient) due to injuries.
Although it was recognized that injuries due to building collapse are an important health issue; the focus for these guidelines was on hazards present within the house.

# Guideline recommendation

# Strength of recommendation
Housing should be equipped with safety devices (such as smoke and carbon monoxide alarms, stair gates and window guards) and measures should be taken to reduce hazards that lead to unintentional injuries.
- There is strong evidence for an association between hazards in the home and injuries. Implementing agencies should work to reduce the number of hazards in housing, because reducing these hazards is likely to have beneficial effects on health.
- In addition to increasing the risk of burn injuries, fireplaces and fuelburning appliances can contribute to poor health outcomes and poor indoor air quality. WHO guidelines around household fuel combustion are summarized in section 8.2.
- The GDG assessed the certainty of evidence to indicate the extent to which the research on each intervention supports the overall recommendation, and, in general, the certainty of the evidence for reducing hazards in the home is moderate to high. In relation to specific safety devices and safety measures, the certainty of the evidence that reducing fire hazards (through installing smoke detectors) reduces the risk of burns is moderate. The certainty of evidence that the use of guards to protect against fires and hot surfaces reduces the risk of burns is low to moderate. The certainty of the evidence that installing stair gates and window guards reduces the risk of falls is low to moderate. The certainty of the evidence that home safety modification programmes reduce the risk of injury is moderate.
- Having considered the certainty of the evidence, the balance of benefits to harm of modifying the home to prevent home injury, and the values, preferences and feasibility of doing so, the GDG made a strong recommendation.

# Summary of evidence
The recommendations were informed by a systematic review of the evidence on the exposure-response relationship between housing factors and injury.
The systematic review and the GRADE tables used to assess the certainty of the evidence are available online at  in Web Annex E.

# Smoke detectors
Properly installed and functioning smoke alarms were found to reduce the incidence of burn injuries. A randomized trial in the United States of America found that burns and fires in the homes were prevented by smoke alarms and carbon monoxide detectors at baseline and at 12 and 24 months' followup (328).
A Canadian case-control study found an increased risk of burns and scalds in children if their house did not have a smoke alarm (329). Similarly, a casecontrol study in Iraq found that, among children admitted to hospital, there were fewer smoke alarms in the homes of those who had sustained a burn injury (330). Another study, in the United Kingdom, reported that among children seeking primary care, admitted to hospital, or presenting to the emergency department, those with burn injuries were less likely to have working smoke alarms in the home (331). However, another case-control study, in the United States of America, reported that burn cases had similar rates of smoke alarm usage and use of carbon monoxide detectors (332).
The evidence that smoke alarms reduce the risk of hospitalization is supported by two additional cohort studies. One found that the introduction of legislation for compulsory smoke alarm ownership in an Australian state decreased hospitalization rates by 36.2% annually (333). The other found that fire-related death and injury were lower in the population with an installed smoke alarm than in the population without a smoke alarm (334).
The certainty of the evidence that the presence of smoke detectors reduces the risk of injury was assessed as moderate.

# Stair and safety gates
Three studies reported on the effects of stair or safety gates on injury in children. A cohort study in the United Kingdom found that among children under 5 years of age, those who lived in homes that had been fitted with stair safety gates were less likely to be admitted to hospital, to attend primary care or to access the accident and emergency department (331). A case-control study in Bangladesh found that children living in homes where the kitchen did not have a door were more likely to sustain burns (335). This finding is supported by a case-control study from the United Kingdom, in which not using safety gates was associated with a significant increase in scalds (336).
The certainty of the evidence that installing stair and safety gates reduces the risk of injury was assessed as low to moderate.

# Window guards
One cross-sectional study from the United States of America assessed the effect of window guard legislation. Window guards were found to be twice as effective in preventing falls than windows without guards (337).
The certainty of the evidence that window guards reduce the risk of injury was assessed as low to moderate.

# Fireplace guards, stoves and unprotected hot surfaces
Three studies investigated the effects of fireplace and stove guards on injury. A case-control study in the United States of America found that people admitted to hospital to treat burns were likely to use fireplace guards less often than the control group (332). In contrast, a cohort study from the United Kingdom found no association between fire guard use and burn injuries (338). A Canadian case-control study reported an increased risk for burns or scalds when there was no stove guard to prevent children from grabbing pots (329).
Other studies have looked at how the presence of specific lamps or heaters affects the risk of injury. A case-control study in the Islamic Republic of Iran found that the odds of burn injury were twice as high in homes with unvented air heaters rather than conventional vented kerosene-or gas-burning heaters (339). A pre-post intervention study in India found that the number of unintentional burns was reduced to zero from 23 among 1042 households in the 6 months after kerosene lamps were replaced with solar or LED lamps (340). In the Bangladesh case-control study, burns were more likely in children in the presence of a traditional kerosene lamp (335), and a case-control study in Iraq found that people who had been burned or scalded were more likely to use kerosene heaters for space heating (330).
The certainty of the evidence that guarding against fireplaces, stoves and dangerous unprotected hot surfaces reduces the risk of injury was assessed as low to moderate.

# Home safety modification programmes
Five randomized trials studied the effect of home safety assessment and modification programmes on injuries (26,328,(341)(342)(343). These had mixed results depending on the comparator for the home safety assessment and modification programmes, some of which are effective interventions for, for example, reducing falls. However, in general, people living in homes in which hazards had been reduced were less likely to sustain injuries than those who received no injury prevention interventions. For example, a randomized trial in New Zealand of adults over 75 years who had severe visual impairments found that there were fewer falls in the group of participants in the home safety programme, compared with those who did not receive this programme (341). Similarly, a randomized trial in the United States of America showed that the rate of medically attended injuries was reduced in children who had the programme compared with controls who did not (328). This is supported by Keall et al. (2015), who found that medically treated falls were rarer for the group of dwellings that had been assessed and modified for safety (26). And a randomized trial from Japan found that falls occurring in the home 1 year after introducing a home hazard modification programme were reduced more in the intervention group than in the control group (343). In contrast, a randomized trial of older adults in the United Kingdom found that the home modification programme did not reduce the incidence of falls (342).
The certainty of the evidence that home safety modification programmes reduce the risk of injury was assessed as moderate.

# Association between the number of hazards in the home and the incidence of injuries
Four case-control studies found a dose-response relationship between the number of home hazards and the need for medical consultations or visits to health care services. A New Zealand study reported an estimated increase of 22% in the odds of injury occurrence associated with each additional home injury hazard (326). A Canadian study of adults aged 65 years and over found that an increase in the number of home hazards was associated with an increased risk of a second fall-related medical visit (344). While at the other end of the age spectrum, a study of children (aged 9 months to 3 years) in the United Kingdom found that those who lived in homes without any of the four hazards measured (no fire guard, safety gate, smoke alarms and electric socket covers) were approximately 20% less likely to have been injured than those with all four hazards (338). Finally, a study that compared residents in a high fall rate building and a low fall rate building in Florida, United States of America, found fewer environmental hazards in the low fall rate building (345).
In addition, several other studies investigated the association between single hazards such as slippery floors, uncovered furniture corners or a lack of grab bars in the shower (345)(346)(347)(348)(349)(350)(351)(352), with the majority of studies finding a positive relationship between the presence of hazards in the home and unintended injuries.
The certainty of the evidence that a higher number of hazards in the home is associated with an increased risk of injury was assessed as moderate to high.
While many injury prevention interventions have been tested in high-income settings, the GDG deems the suggested prevention measures generalizable to middle-and low-income countries. This is because injuries have certain antecedents. For example, burns are caused by excessive heat. Similarly, injury prevention has certain universal interventions applicable to all contexts, e.g. burns can be prevented by smoke alarms.

# Considerations for implementation of the guideline recommendation
Several tools are available to help assess and improve home safety. These include housing-based health risk assessment methodologies such as the English Housing Health and Safety Rating System, the United States National Healthy Housing Standard, or the New Zealand Rental Warrant of Fitness (developed from the Healthy Housing Index) (242,(353)(354)(355). Using such checklists relevant to local housing type, including adapting these to low-income settings and a variety of housing environments, can help prioritize interventions and ensure that serious hazards are not ignored or overlooked (356,357).
Taking action to reduce hazards in the home can be efficiently carried out alongside housing improvements that address other risk factors covered by these guidelines. One key instrument in reducing hazards in housing is mandated requirements, accompanied by effective enforcement measures.
Measures should be put in place to ensure that codes rapidly integrate injury and building science. In addition, subsidies and tax incentives can encourage safety modifications. Such measures can be supported by education campaigns to teach the public what they can do to reduce the risk of injury in the home (358).
Reducing injury hazards in the home is likely to provide considerable health benefits, particularly for the poorest people, children, the elderly, the disabled and other at-risk populations living in the lowest quality housing. This action is likely to be cost-effective because the benefits from fewer injury-related hospitalizations have been shown to be larger than the costs of improving home safety, resulting in net benefits (26,71,(359)(360)(361)(362)(363)(364)(365)(366). Housing that is in such a state of disrepair that it cannot be modified may have to be demolished.
Therefore, implementing this HHGL recommendation will probably need to be accompanied by the provision of new low-income housing to avoid potentially inequitable consequences.

# Research recommendations
The research reviewed suggests that interventions aimed at reducing hazards in the home are associated with positive health outcomes, but there are continuing uncertainties about the size of the benefits and the impact of specific interventions. The research base would be strengthened by focusing on a number of priorities.

# Current state of the evidence
Some studies were not included in the review because they assessed multicomponent interventions, making the effect of home structure modification on health outcomes difficult to distinguish from other interventions to decrease injuries (such as behaviour change). Future studies might consider using factorial designs to enable analyses of the effects of the specific interventions that make up a multicomponent intervention in different locations. These need to be large enough to have adequate power to detect the effect on different outcomes. In addition, more research is needed to assess the exposureresponse relationship between the number and type of hazards, the amount of time spent in the home, and the frequency and magnitude of injury.

# Population of interest
The whole population, in particular, people from low-and middle-income countries and those both more likely to spend time at home and experience adverse health effects due to injury (e.g. the elderly, children, women and people with long-term illnesses).

# Interventions of interest
Reducing injury hazards in housing, including through installing safety devices and removing hazards.

# Comparisons of interest
Groups living in home environments where home modifications aimed at reducing the risk of injury have been carried out and where home modifications have not been carried out; and groups before and after such interventions.

# Outcomes of interest
Injuries, including electrocution, broken or fractured bones, mortality due to injuries, burns or scalds, and hospitalization (outpatient or inpatient) due to injuries.

# Time stamp
Current systematic review on the association between hazards in the home and injury included studies published up to April 2018.

#   Housing accessibility
Disability is an umbrella term describing physical or psychological impairments, activity limitations or participation restrictions (367). At least 1 billion people, or 15% of the world's population, have some form of disability (368). The disabled population is increasing as the world's population ages (369). Disability disproportionately affects low-income households, and has a higher prevalence in low-and middle-income countries (368). Disability can cause and contribute to poverty (368). People with functional impairment are more likely to be discriminated against when looking for housing, and more likely to pay high costs for housing relative to their income (370).

# According to the United Nations Convention on the Rights of Persons with
Disabilities, Member States have an obligation to identify and eliminate all barriers to accessibility, including in housing (371).
As functioning and disability result from an "interaction between the person's health condition and both personal and environmental factors" (372), contextual factors substantially determine whether an impairment is perceived as disabling. Accessible dwelling can improve a person's domain-specific functioning within their home. Life span housing (also known as life cycle housing, lifetime homes or adaptable housing) can accommodate changes in human functioning over a person's life span, enabling the occupants to remain in their homes as long as possible. Universal design (or universal housing) is an approach to the design, construction and adaptation of housing to meet the needs of all occupants regardless of their age, functioning or social situation (373).
Most homes are not currently built with accessibility in mind. For example, only about one fifth of public buildings in Enugu city, Nigeria, were found to be accessible for wheelchair users (374). However, there is a high chance that they will be occupied by people with disablities at some time, especially considering the trend of ageing populations. In the United States of America, it has been estimated that there is a 60% probability that any new house will be occupied by a person with a functional impairment over its life span (375). Nonaccessible home environments expose people with functional impairments to risk of falls and injuries, restricts social participation, negatively affects quality of life, and increases the burden on caregivers and external social services (376,377). They limit a person's ability to manoeuvre in different spaces.
In order to establish clear guidance on maximizing the health gains associated with accessible housing, a systematic review of the evidence on the effects of home improvements for people with functional impairment was commissioned.

# Question for the systematic review
Do residents with functional or cognitive impairments living in accessible/ usable home environments have better health/social outcomes than residents with functional or cognitive impairments living in conventional or unmodified home environments?
The systematic review focused on the following priority health outcomes, as ranked by the GDG:
- injury rates (especially falls)
- well-being/quality of life
- mental health/depression
- dependency on external social or care services
- social participation.

# Guideline recommendation

# Strength of recommendation
Based on current and projected national prevalence of populations with functional impairments and taking into account trends of ageing, an adequate proportion of the housing stock should be accessible to people with functional impairments.
- People with functional impairments living in accessible home environments have better health and are better able to accomplish everyday tasks and manage living independently than those living in conventional or inaccessible home environments. Implementing agencies should work to ensure that people with functional impairments live in accessible housing, because this is likely to have beneficial effects on health. The recommendation is limited to the effect of housing accessibility on people with functional impairments because of the small number of studies focusing on people with cognitive impairments.
- Each implementing agency should determine what percentage of the housing stock is required to be "adequate" to meet the needs of the population with functional impairments. They should do this by considering current and projected national prevalence of people with functional impairments and trends in ageing. A good discussion on estimation methods is available (378). The level required for "adequate" provision will need to be reviewed as the population changes; in particular, as it ages.
Given that accessible housing is not exclusively occupied by people with functional impairments, it is likely that the proportion of accessible housing stock would be higher than the proportion of households that include people with functional impairments.
- Implementing agencies can draw on a range of existing programmes that target an increase in the supply of accessible dwellings.
- The GDG assessed the certainty of the evidence to indicate the extent to which the research supports the recommendation and, in general, the certainty of the evidence for interventions to improve the accessibility and usability of the houses of people with functional impairment is moderate, supporting the decision to provide a strong recommendation. This overall certainty arises from a consideration of the accumulated evidence as a whole, which suggests important benefits for this disadvantaged population even though the evidence is of low certainty for some interventions and outcomes. In relation to specific aspects of the findings from the systematic review and other information, the certainty of the evidence that people with functional impairments are better able to accomplish activities of daily living when living in accessible environments was assessed as low to moderate; the certainty of the evidence that people with functional impairments are less likely to fall and be injured when living in accessible environments was assessed as moderate; the certainty of the evidence that living in accessible environments reduces the mortality rate of people with functional impairments was assessed as low; and the certainty of the evidence that people with functional impairments experience positive psychological effects and improved quality of life when living in accessible environments was assessed as low to moderate.
- Having considered the certainty of the evidence, the balance of benefits to harm related to increasing the supply of accessible dwellings, and the feasibility of increasing the supply of accessible dwellings, the GDG made a strong recommendation.

# Summary of evidence
This recommendation was informed by a systematic review of the evidence on the impact of accessible housing on residents with functional or cognitive impairments. The systematic review and the GRADE tables used to assess the certainty of the evidence are available online at / sustainable-development/publications/housing-health-guidelines/en/index. html in Web Annex F.
Twenty studies, including six randomized trials, were eligible for the systematic review. Almost all the studies focused on people with functional impairments, with only one study of people with cognitive impairments (379). Interventions implemented to enhance home accessibility features were carried out either as a sole intervention or as part of a multicomponent programme. Home modifications were mainly focused on architectural changes or fitted devices (such as grab bars) targeting mobility issues. Some focused on lighting improvements or adjustments targeting vision.

# Activities of daily living
Functional impairments are often operationalized in terms of whether a person can accomplish activities of daily living (ADL) or instrumental activities of daily living (IADL). IADL refers to basic tasks of everyday life such as bathing, dressing, transferring, toileting, continence and feeding.
ADL refers to a range of activities that are required for independent living in the community, such as preparing meals, housekeeping, taking medication, shopping, managing one's own finances, travelling and using the telephone.
Some studies reported the effects of interventions on ADL/IADL related outcomes. Three studies reported considerable decreases in perceived difficulties performing ADL/IADL after home modifications (380)(381)(382); but difficulty with mobility/transfer did not change (382). Self-efficacy, which was defined as confidence in managing difficulty, was improved in one intervention group (382). Increased safety with ADL/IADL was also identified 2 months after home modifications among adults with impairments. The greatest safety benefits were in regard to bathroom use and entry access (380) and to difficulty in bathing and toileting (382). In addition, one population-based survey identified a strong association between self-recognized difficulty managing ADL and perceived unmet needs for home accessibility features among people with physical impairments, after adjusting for the severity of their physical limitations (383). This suggests that people who already have difficulties functioning in everyday life can benefit from home accessibility features, possibly delaying further deterioration of their functioning.
One randomized trial involving people with visual impairments did not identify a significant improvement overall in self-rated certainty in performing specific activities 6 months after lighting adjustments (384). In addition, two studies found no significant change in dependence with ADL/IADL at 2 months and up to 9 months after home modifications (380,385), although it was noted that dependence in bathing was significantly decreased between 2 to 3 months and 8 to 9 months after home modifications (385). These non-significant findings may be because the participants were elderly meaning that the participants' functions declined so rapidly that specific home modifications had an effect only for a short period of time (386).
Three further studies investigated the impact of interventions on general ADL.
One randomized trial involving adults above the age of 60 who had had surgery for hip fracture in Finland did not detect any significant effects on general ADL or IADL (387). However, a quasi-experimental study conducted in Thailand found that home modifications improved abilities in all function areas except for participants with severe degrees of difficulties (388), and a cross-sectional study involving participants from Sweden and Germany showed improvements in various aspects of ADL (389).
The certainty of the evidence that people with functional impairments living in accessible environments are better able to accomplish activities of daily living was assessed as low to moderate.
Studies showed that home modifications aimed at reducing the likelihood of falls and injuries for people with functional impairments were successful in doing so. One randomized trial reported 41% fewer falls after 1 year in the home safety programme with a group of older adults with severe visual impairments, compared with those who did not receive this programme (341).
A longitudinal prospective cohort study identified a significant reduction in falls at home and post-fall hospitalizations among frail older adults after the use of a light path coupled with tele-assistance (390). Two other studies reported a significantly lower mortality rate in the intervention group over the control group after the implementation of the multicomponent home modification programme. The programme included training to promote healthy behaviours (382,391).
The certainty of the evidence that people with functional impairments living in accessible environments have reduced fall and injury rates was assessed as moderate.
One randomized trial, reported in two papers (382,391) reported a significantly lower mortality rate in the intervention group over the control group up to 2 years after the implementation of the multicomponent programme, which included home modifications as well as training strategies to promote healthy behaviours. However, there was no statistically significant effect on survival 3 years after the intervention.
The certainty of the evidence that living in accessible environments reduces the mortality rate of people with functional impairments was assessed as low.

# Quality of life
Two randomized trials (384,392) identified the positive effect of interventions on quality of life. Ahmad et al. (2013) found that 2 months after the homes of paraplegic wheelchair users were modified, quality of life was significantly enhanced in the intervention group, compared with the control group (392). Brunnstrom et al. (2004) found that adjustments to living room lighting increased quality of life and well-being among adults with low vision (384).
Conversely, a cross-sectional study with adults with dementia found no association between quality of life and home safety and accessibility factors such as hazards, grab bars and visual cues (379).
The certainty of the evidence that living in accessible environments improves the quality of life of people with functional impairments was assessed as low.

# Psychological effects
Three studies identified positive psychological effects of home accessibility interventions for people with functional impairments (382,393,394). A randomized trial found that older adults with functional difficulties reported significantly less fear of falling following a multicomponent home intervention (382). A quasi-experimental study from Sweden found a significant decrease in fear of falling at 3 months but not at 6 months after the intervention (394). Heywood (2004) reported that 62% of people whose homes had modified for accessibility (through for example installing handrails and grab-rails), reported "feeling safer from accidents" and 77% perceived a positive effect on their health (393).
The certainty of the evidence that people with functional impairments living in accessible environments experience positive psychological effects was assessed as moderate.
One cross-sectional study conducted in Sweden concluded that accessibility problems were significantly associated with less participation and autonomy and more participation problems (395).

# Considerations for implementation of the guideline recommendation
A report from New Zealand concluded that it would be approximately 22 times more cost-efficient to build housing that includes key accessibility features than to retrofit when an unplanned need arises (396). However, in 2006, only 56% of countries had accessibility criteria in their building standards (397). Introducing such national regulations in a large number of countries could therefore lead to more inclusive societies and avoid extensive expenditures on retrofitting. It needs, however, to be noted that globally only a very small proportion of the housing stock will be newly built, while most of the housing stock requires retrofitting. Accessibility needs therefore to be ensured in regulation both for existing and new dwellings. The international standard "ISO 21542:2011 Building construction -Accessibility and usability of the built environment", published by the International Organization for Standardization, specifies a range of requirements and recommendations including on construction aspects of housing accessibility (398). Information on low-cost home modifications to increase housing accessibility in low-income settings is available from community-based rehabilitation programmes in India, using guidelines for care and community integration after spinal cord injury produced by the Government of India and the WHO Community-based rehabilitation guidelines (399).
Besides the public sector, the private sector is a major partner in promoting accessible housing. One such a multistakeholder collaboration has been initiated by the National Cooperative Housing Union in Kenya by linking government, disability groups and the private sector to identify available land and providing technical assistance and loan capital to facilitate accessible housing construction (400). In Australia, the housing industry developed a target of having all new homes meet the Australian Liveable housing design guidelines by 2020. Those guidelines describe a number of elements that make a home more responsive to the changing needs of home occupants (401). However, voluntary schemes require extensive education and training programmes to highlight the benefits of accessible housing.
Progress towards increasing the stock of accessible housing should be carefully monitored. If sufficient progress is not being made using a voluntary programme, it may be necessary to introduce a mandatory programme. Such schemes are usually introduced gradually. For example, Portugal's compulsory scheme was phased in over 8 years (402). In Sweden, every local authority is legally obliged to provide home modifications for people with impairments (380). However, planning for home modification requires consultation with service users as well as health and architectural professionals to ensure the users' needs are met appropriately. Home modifications that are poorly implemented due to bad planning or administrative errors may have a negative impact on physical and mental health of persons with impairments (393).
Accessible housing should consider other factors related to healthy housing in addition to usability for occupants. If providing a household with an accessible dwelling entails people moving to another location, it could potentially remove them from social networks, child care support, and work or educational opportunities, affecting health and earning opportunities (194).

# Research recommendations
The research reviewed suggests that living in accessible housing supports the health of people with impairments. However, high-quality research in this area is difficult because allocating people to a comparison group is not always possible: the home modifications might be mandated under law, or it may be unethical to deny or delay accessibility improvements. Table 7.1 lists the research priorities in this area.

# Current state of the evidence
There are relatively few high-quality studies, with most studies to date being observational or small. The participants and types of intervention reviewed vary greatly. Some studies rely on subjective self-reporting rather than objective performancebased measures, and use different psychometric instruments to identify quality of life outcomes. There are few studies conducted outside of high-income settings, and most of the research focuses on the experience of adults. For some studies, it is not clear which component of the intervention was most effective. Future studies might consider using factorial designs to enable analyses of the effects of the specific interventions that make up a multicomponent intervention. Longitudinal studies, using standardized outcome measurements, are required to provide a stronger evidence base for the health and social benefits of home accessibility interventions.

# Population of interest
Populations with a range of physical and cognitive impairments. Much of the current evidence is based on research with adults with physical impairments in high-income settings; future research should consider people with cognitive impairments, people in low-income settings, and children and young adults.

# Interventions of interest
Improving accessibility of housing. This could include modification of specific furniture and fixtures in the house; structural changes to the inside and immediate outside of the house; and assistive devices that are part of the house. Studies should consider how different accessibility features affect health and social outcomes for people with specific impairments. Research should determine which accessibility features affect these outcomes for people with different impairments.

# Comparisons of interest
Groups living in accessible and conventional/unmodified home environments; a group before and after intervention.

# Outcomes of interest
Injury rates (especially falls), well-being and quality of life, mental health and depression, dependency on external social or care services, and social participation.

# Time stamp
Current systematic review on the association between accessible home environments and the health of people with impairments included studies published up to April 2018. guidelines, the introductory paragraphs, which outline the current health burden of each risk factor rely on other sources and have been drafted for the purpose of these guidelines.
Water is a physiological requirement to sustain adequate hydration, to prepare food, and to maintain hygiene. Contaminated water transmits infectious disease and sometimes non-infectious disease such as lead poisoning from leaded pipes and plumbing. Lack of access to sufficient water discourages hygiene practices. Water can be contaminated by microbes and chemicals at its source, in its storage, and in its transportation, whether by handheld vessels, tankers or distribution pipes. It is important to ensure that drinking-water supply is reliable, that it is protected from contamination by waste water, and that pipes and storage systems are correctly installed and maintained (403).
Everyone has the right to sufficient, safe, acceptable and physically accessible and affordable water for personal and domestic use, such as for drinking and for hygiene (404). However, in 2015, 844 million people lacked even a basic drinking-water service, including 159 million dependent on surface water. Only eight out of ten people could use improved sources with water available when (405). At least 1.8 billion people use a drinking-water source contaminated with faeces (406). Climate change, increasing water scarcity, population growth, demographic changes and urbanization pose challenges for water supply systems. By 2050, 40% of the world's population will be living in river basins experiencing severe water-stress (407).
Unsafe water and poor sanitation are linked to transmission of diseases such as cholera, diarrhoea illnesses, dysentery, hepatitis A, typhoid and polio, and lead poisoning from lead drinking-water service lines, or leaded solder, or brass or leaded fixtures. Diarrhoeal disease alone amounts to an estimated 3.6% of the total DALY global burden of disease and is responsible for the deaths of 1.5 million people every year. It is estimated that 58% of that burden, or 842 000 deaths per year, is attributable to inadequate water supply, sanitation and hygiene, mostly in low-and middle-income countries (132). With children particularly at risk from water-related diseases, access to improved sources of water can result in better health and therefore better school attendance (408).

# WHO Guidelines for drinking-water quality
Threshold values for water contaminants in drinking-water are available in the WHO Guidelines for drinking-water quality (fourth edition published in 2011 and complemented by the first addendum in 2017) (409, 410). 6 WHO documents related to water management in buildings and other aspects of WASH provide more technical guidance on implementation and are listed at the end of this section.
Guideline values related to drinking-water contamination 1) Chemical contaminants: The selected chemical contaminants displayed in Table 8.1 are associated with potential contamination through pipes and plumbing components and therefore are considered relevant from a building perspective.

# Compound Guideline value
Antimony 0.02 mg/l (20 g/l) As the most common source of antimony in drinking-water appears to be dissolution from metal plumbing and fittings, control of antimony from such sources would be by product control.
Benzopyrene 0.0007 mg/l (0.7 g/l) The presence of significant concentrations of benzopyrene in drinking-water in the absence of very high concentrations of fluoranthene indicates the presence of coal tar particles, which may arise from seriously deteriorating coal tar pipe linings.
It is recommended that the use of coal tar based and similar materials for pipe linings and coatings on storage tanks be discontinued.
Copper 2 mg/l (2000 g/l) In most instances where copper tubing is used as a plumbing material, concentrations of copper will be below the guideline value. However, there are some conditions, such as highly acidic or aggressive waters, that will give rise to much higher copper concentrations, and the use of copper tubing may not be appropriate in such circumstances.
Lead 0.01 mg/l (10 g/l) This is a provisional guideline value as the key effects of lead seem not to have a threshold. Lead is exceptional compared with other chemical hazards, in that most lead in drinking-water arises from plumbing in buildings, and the remedy consists principally of removing plumbing and fittings containing lead. All other practical measures to reduce total exposure to lead, including corrosion control, should be implemented.
Nickel 0.07 mg/l (70 g/l) Where nickel leaches from alloys in contact with drinking-water or from chromium-or nickel-plated taps, control is by appropriate control of materials in contact with the drinking-water and flushing taps before using the water.
Vinyl chloride 0.0003 mg/l (0.3 g/l) As vinyl chloride is a known human carcinogen, exposure to this compound should be avoided as far as practicable, and levels should be kept as low as technically feasible. Vinyl chloride is primarily of concern as a potential contaminant from some grades of polyvinyl chloride pipe and is best controlled by specification of material quality. The selected chemical contaminants displayed in Table 8.2 are of greatest health concern in some natural waters, and may pose significant health risks in non-piped water supply conditions.
Guideline value Arsenic 0.01 mg/l (10 g/l) The guideline value is designated as provisional on the basis of treatment performance and analytical achievability.
Arsenic is usually present in natural waters at concentrations of less than 1-2 g/l. However, in waters, particularly groundwaters, where there are sulfide mineral deposits and sedimentary deposits deriving from volcanic rocks, the concentrations can be significantly elevated. Signs of chronic arsenicism, including dermal lesions, such as hyperpigmentation and hypopigmentation, peripheral neuropathy, skin cancer, bladder and lung cancers and peripheral vascular disease, have been observed in populations ingesting arsenic-contaminated drinking-water. For local non-piped water supplies, the first option for control is often substitution by, or dilution with, microbially safe low-arsenic sources. It may also be appropriate to use alternative sources for drinking and cooking but to use the contaminated sources for purposes such as washing and laundry.
Fluoride 1.5 mg/l (1500 g/l) Traces of fluorides are present in many waters, with higher concentrations often associated with groundwaters. Skeletal fluorosis (with adverse changes in bone structure) may be observed when drinking-water contains 3-6 mg/l fluoride, particularly with high water consumption. Crippling skeletal fluorosis usually develops only where drinking-water contains over 10 mg/l. The risk of dental fluorosis will depend on the total intake of fluoride from all sources and not just the concentration in drinking-water. A management guidance document on fluoride is available. In some countries, fluoride may also be added to drinkingwater in order to provide protection against dental caries, such that final concentrations are usually between 0.5 and 1 mg/l. Nitrate 50 mg/l (50 000 g/l) Methaemoglobinaemia has most frequently been associated with private wells. The most appropriate means of controlling nitrate concentrations, particularly in groundwater, is the prevention of contamination. This may take the form of appropriate management of agricultural practices, the careful siting of pit latrines and septic tanks, sewer leakage control, as well as management of fertilizer and manure application and storage of animal manures. It may also take the form of denitrification of wastewater effluents.

# Escherichia coli
Escherichia coli (E. coli) or thermotolerant coliform bacteria must not be detectable in any 100 ml sample. The presence of E. coli indicates faecal contamination of drinking-water due to cross-contamination. Such cross-contamination may occur in buildings due to cross-connection with non-drinking-water systems or during water transport or storage in non-piped water supply conditions where households need to fetch water at a source outside their home.

# Table 8.3 WHO guideline values for drinking-water quality: bacteria
Guidelines related to quantity and continuity of drinking-water supply 7 Access to drinking-water should be optimal. Interventions to increase levels of water service and supply should be prioritized as follows: Interruptions to drinking-water supply, either because of intermittent sources or resulting from engineering inefficiencies, are a major determinant of the access to and quality of drinking-water.
These values included in the WHO Guidelines for drinking-water quality refer to the WHO publication on Domestic water quantity, service level and health. This latter is being updated and new reference values will be ready during 2018.

# Guidelines related to water collection and transport
Maintaining the quality of water during collection and manual transport is the responsibility of the household. Good hygiene practices are required and should be supported through hygiene education. Hygiene education programmes should provide households and communities with skills to monitor and manage their water hygiene.
In many low-and middle-income countries, consumers purchase water from kiosks and then carry the water home in a variety of containers of varying size. Measures should be taken to protect vended water from contamination during transport, as well as during storage in the home. These measures include transporting and storing water in containers that are clean, free from both faecal and chemical contamination, and either enclosed, or with narrow openings, and ideally fitted with a dispensing device such as a spigot that prevents hand access and other sources of extraneous contamination. Good hygiene is required and should be supported by educational programmes.
In other cases, particularly in low-income countries, vendors transport and deliver the water to users in tanker trucks. If large volumes are being transported, the addition of chlorine to provide a free residual concentration of at least 0.5 mg/l at the point of delivery to users is desirable. Tankers should also be used solely for water or, if this is not possible, should be thoroughly cleaned prior to use.

# Guidelines related to plumbing
Significant adverse health effects have been associated with inadequate plumbing systems within public and private buildings arising from poor design, incorrect installation, alterations and inadequate maintenance.
Numerous factors influence the quality of water within a building's piped distribution system and may result in microbial or chemical contamination of drinking-water. Outbreaks of gastrointestinal disease can occur through faecal contamination of drinking-water within buildings arising from deficiencies in roof storage tanks and cross-connections with wastewater pipes, for example. Poorly designed plumbing systems can cause stagnation of water and provide a suitable environment for the proliferation of Legionella.
Plumbing materials, pipes, fittings and coatings can result in elevated heavy metal (e.g. lead) concentrations in drinking-water, and inappropriate materials can be conducive to bacterial growth. Potential adverse health effects may not be confined to the individual building. Exposure of other consumers to contaminants is possible through contamination of the local public distribution system, beyond the particular building, through crosscontamination of drinking-water and backflow.
The delivery of water that complies with relevant standards within buildings generally relies on a plumbing system that is not directly managed by the water supplier. Reliance is therefore placed on proper installation of plumbing and, for larger buildings, on building-specific water safety plans (WSPs).
To ensure the safety of drinking-water supplies within the building system, plumbing practices must prevent the introduction of hazards to health. This can be achieved by ensuring that:
- pipes carrying either water or wastes are watertight, durable, of smooth and unobstructed interior and protected against anticipated stresses;
- cross-connections between the drinking-water supply and the wastewater removal systems do not occur;
- roof storage systems are intact and not subject to intrusion of microbial or chemical contaminants;
- hot and cold water systems are designed to minimize the proliferation of Legionella; - appropriate protection is in place to prevent backflow;
- the system design of multistorey buildings minimizes pressure fluctuations;
- waste is discharged without contaminating drinking-water;
- plumbing systems function efficiently.
It is important that plumbers are appropriately qualified, have the competence to undertake necessary servicing of plumbing systems to ensure compliance with local regulations and use only materials approved as safe for use with drinking-water.
Design of the plumbing systems of new buildings should normally be approved prior to construction and be inspected by an appropriate regulatory body during construction and prior to commissioning of the buildings.