For more than a century, society has recognized that improved housing generally leads to improved health.1 The typhoid and tuberculosis experiences in the 19th and early 20th centuries showed that basic sanitation, ventilation, reduced household crowding, and other improvements in housing powerfully contributed to conquering these epidemics. Other housing-related interventions, such as improved sanitation through indoor plumbing, the creation of smooth and cleanable interior surfaces, and better food preservation and storage facilities in homes were clearly linked to further advances in public health. Many housing and building codes trace their ancestry to the public health responses to epidemics that occurred with the rapid industrialization and urbanization in Western countries more than 100 years ago.
As attention has shifted from communicable disease to more chronic afflictions, such as asthma, cancer, lead poisoning, injuries, and mental health disorders such as anxiety and depression, the link between housing and health has received new appreciation and further investigation. Several reviews completed in the past decade have amply demonstrated the connection between housing and health in the modern era.2 – 6 Both the Centers for Disease Control and Prevention (CDC) and the US Department of Housing and Urban Development have expanded their efforts on the interaction of housing and health. Increased recognition of this link has led to important advances in healthy and safe housing both nationally7 , 8 and internationally.9 , 10
Links Between Housing and Health
However, knowing the link between housing conditions and health effects is only part of the solution in promoting health in homes. For example, recognizing the link between exposure to house dust mites and increased risk of asthma exacerbations does not necessarily provide information about specific methods that can effectively reduce exposure to house dust mites and result in improved asthma status.
We need to better understand the housing interventions that demonstrably improve health. For example, many state and local programs are beginning to adopt healthy homes approaches when they conduct a home visit. The healthy homes approach holistically views the health of families and may recommend several interventions based on family needs. To have the greatest health impact, these programs need to recommend interventions that have demonstrated positive health effects. Understanding which interventions have positive health outcomes can also lead to policies that will substantially improve our quality of life. For example, residential lead paint hazard control policies have helped to reduce children's exposure to lead. In addition, this better understanding can guide decisions about which interventions to implement for economies of scale. For example, one intervention can address multiple hazards, such as replacing a rotted handrail covered with deteriorated lead paint to prevent both lead poisoning and injury from falls. Furthermore, identifying knowledge gaps can help to inform and update a research agenda in this field.
Few reviews of the scientific literature have been published on the effectiveness of housing interventions, either at the household or community level, to improve health status.11 – 13 This article describes the methods used by 4 panelists of subject matter experts who systematically reviewed housing interventions to control biologic and chemical agents and to address structural deficiencies as well as community-level interventions to improve health. The panelists also reported the sufficiency of the evidence. Detailed results of this review are contained in companion articles.14 – 17
The goal of this systematic review was to assess the effectiveness of housing interventions on improving health. A team from the CDC and the National Center for Healthy Housing identified several intervention topic areas on the basis of a preliminary search described later. The team identified and invited nongovernment researchers who were experts in the topic area fields and who represented diverse disciplines, backgrounds, and work settings. These experts formed the topic area review panelist. The team created a priority list of interventions for review by polling the panel reviewers and other specialists in the field regarding their perception of the strength of the evidence.
The CDC and National Center for Healthy Housing identified 5 broad areas of housing intervention research that could be associated with improved health. One panel, external exposures (drinking water and sewage treatment), is not included in this series. This report and the companion articles describe the findings from the following 4 panelists:
- interior biological agents (toxins) interventions,
- interior chemical agents (toxics) interventions,
- structural deficiency (injury) interventions, and
- community-level housing interventions.
The association between housing and health is complex, and causal relationships can be hidden or otherwise influenced by a host of confounding variables and effect modifiers. Several different frameworks have been proposed to characterize this relationship.2 , 18 , 19 These frameworks show that both proximate and distal factors are important to understanding the relationship between housing and health. Each of the 4 panel reviews includes a logic model showing these relationships for the topic area.
The CDC conducted a preliminary literature review by using relevant key words and search terms on MEDLINE, a public health database. This search covered articles added between 1990 and December 2007 and included the following general terms and key words: public housing; housing; home; intervention studies; health effects; mitigation; program evaluation; prevention; primary prevention; clinical trials, randomized controlled trials; and domestic.
In addition, each intervention area included the following terms and key words for its topic:
- Biological agent interventions: allergens; dust; mites; asthma; cockroaches; animals, domestic; mice; and rats;
- Chemical agent interventions: air; air pollution; integrated pest management; pesticides; safe chemical storage; storage; pest control; particulate matter; filtration; ventilation; volatile organic compounds; formaldehyde; organic chemicals; radon; and lead;
- Structural (injury) interventions: burns; burn prevention; accident prevention; accidents, home; protective devices; accidental falls; fall prevention; falls; and accidents, self-help devices; and
- Community-level housing interventions: environmental justice; universal design; ordinances; law; law enforcement; and public policy.
Assessment and Summary of the Evidence
All panelists reviewed only those studies that were published in English. The majority of studies reviewed by the panelists were conducted in the United States. However, the panelists did include some relevant studies from other countries, particularly in areas where little research has been conducted in the United States. Future reviews should examine research published in other languages and countries.
We included 2 broad categories of evidence: clinical evidence and environmental measurements. Each of these sources of evidence has strengths and weaknesses. Clinical evidence (or other health data such as self-reported health) is likely to be the most direct measure of health status. However, many health conditions do not have adequate biomarkers or have long time horizons before an adverse health event occurs, making clinical evidence problematic. For example, lung cancer from radon gas exposure may not be clinically observable for many years, yet evidence shows that radon environmental measurements can be linked reliably to risk of lung cancer. Similarly, asthma is a complex set of symptoms for which a single, reliable biomarker has yet to be identified. Thus, an intervention may be judged successful if it reduces environmental exposures for which the evidence shows a dose-response relationship. Ideally, both clinical and environmental data can make the most compelling case for a given intervention. We reviewed studies that contained clinical, health, or environmental measurements or a combination of these.
Reviewers grouped the intervention studies according to recommendations in the Guide to Community Preventive Services (Community Guide), which identifies similarities in (1) the type of intervention (eg, activities undertaken, content, and scope); (2) the delivery of the intervention (eg, who delivers it, time period, frequency, and duration); (3) target population (eg, high-risk population or general population); and (4) the setting of the intervention. (See http://www.thecommunityguide.org/pubhealthpro.html.)
At least 1 panelist reviewed each publication; in some cases, 2 panelists reviewed a publication. Panelists evaluated each publication by using a structured review instrument and review procedure adapted from the Community Guide.
The instrument (Figure) allowed reviewers to assign each intervention study a score by using the following factors:
- study size and population,
- overall value, and
- direction of effect and degree of impact.
The panelists assigned each of the interventions into 1 of 4 broad categories on the basis of the evidence in the literature:
- sufficient evidence,
- needs more field evaluation,
- needs formative research, or
- no evidence of effectiveness.
The Community Guide recommends that an effective intervention results in “improvements in health or leads to changes in behaviors or other factors that have been shown to result in better health.” In addition, interventions must demonstrate independent impacts on health or key factors that will result in better health. Reviewers based a determination of sufficient evidence on either a small number of well-designed, well-executed, and consistent studies or a larger group of studies that may be weaker in design, execution, and effect but, when taken together, provide convincing evidence for an intervention. We did not consider other factors such as implementation barriers or economic analyses.
Panel members were provided copies of manuscripts for review before meeting in person on December 11-12, 2007, in Atlanta, Georgia. The experts reviewed more than 170 scientific studies of housing interventions that improve asthma and respiratory problems, cancer, injuries, and other health concerns. Each reader presented a review, and each of the 4 panels worked to provide summary advice on the category for the intervention. Each panel then presented its assessment of the body of literature to the entire group of experts from the other panels. Some panel members and CDC subject matter experts identified additional literature that was included in the review after approval by the panel chair. One exception to the process described earlier involves the effectiveness of residential lead hazard control, which has been extensively reviewed elsewhere and is summarized in the companion article by Sandel et al15 on chemical agents.
The deliberations of the 4 panelists, an overview of each panel's topical area, and the results of their reviews are detailed in the companion articles. Table 1 provides a summary of the deliberations at the meeting and the interventions by category. More details on each of the interventions reviewed are provided in the companion articles. The full report (which also does not necessarily represent the official position of, nor has it been officially endorsed by, the CDC) may be viewed at http://www.nchh.org/LinkClick.aspx?fileticket=2lvaEDNBIdU%3d&tabid=229.
Use of the Findings
The evidentiary categories can be used in the following ways:
- develop policy on those interventions that currently have sufficient evidence of effectiveness to recommend immediate implementation;
- conduct research on those interventions (1) in which the evidence shows promising outcomes that need more testing and evaluation in the field before recommending widespread implementation or (2) that have demonstrated success in nonresidential settings, such as schools or offices, but need to be tested in homes;
- implement formative research to determine the biologic plausibility of a link between a housing and health condition and an intervention's effectiveness; and
- identify those interventions in which the evidence is clear that the interventions should not be pursued.
The ability to use the Community Guide process improved the rigor of the reviews and enabled the panelists to comprehensively and systematically examine a large and varied literature. Using 1 or 2 readers for each article, followed by in-depth discussions at the group level, strengthened each panel's confidence in its assessment, although additional readers for each article would have been desirable. A limitation was the fact that panelists essentially volunteered their limited time for this exercise. In short, the process was successful. Future reviews should attempt to make more extensive use of both studies from other countries and government high-quality data that have not yet been published in the peer-reviewed literature.
Although many housing conditions are associated with adverse health outcomes, sufficient evidence now shows that specific housing interventions can also improve certain health outcomes. Further research is needed to strengthen the evidence base for many other housing interventions that may improve health but for which evidence is still weak. This research is important because investing in housing quality can yield important savings in medical care and improvements in quality of life.
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