Table 3.2, Research recommendations: crowding - WHO Housing and Health Guidelines

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.

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.

From: 3, Household crowding

WHO Housing and Health Guidelines.

Geneva: World Health Organization; 2018.

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Table 3.2Research recommendations: crowding