International efforts related to water in low-income countries often focus on improving water supplies. The WHO/UNICEF Joint Monitoring Program for Water and Sanitation (JMP), which is charged with monitoring progress toward Millennium Development Goal (MDG) 7, counts water supplies as “improved” based on the level of service. Piped water, public taps or standposts, tubewells or boreholes, protected dug wells, protected springs, and rainwater are considered “improved”; unprotected dug wells, unprotected springs, and surface water are deemed “unimproved”. Significantly, the distinction between improved and unimproved supplies is based principally on the quality of water they produce: their perceived, though often untested, potential to deliver safe drinking water sustainably [1
]. Water quantity is not directly part of the current criteria for designating improved water supplies.
This lack of focus on water quantity also appears in much of the epidemiological literature. In recent years, numerous systematic reviews have assessed the impact of water on health, especially diarrheal diseases [2
]. Other reviews have examined the impact of water on nutritional status [9
], soil-transmitted helminth infections [10
] and trachoma [11
]. These reviews, however, either focus on “improved” versus
“unimproved” water supplies, on specific types of water supplies (e.g., piped water), or on improvements in water quality. Only Esrey and colleagues aimed to assess the health impact of improvements in water quantity independently from water quality [13
]. While they reported water quantity interventions to be protective against diarrheal disease, most of the underlying studies did not measure water quantity directly. Instead, they used distance to water supplies as a proxy for quantity [15
]. The reviews, which now date back nearly a quarter century, also rely heavily on studies with before/after comparisons and other methodological shortcomings.
Despite the paucity of evidence on the health impact of water quantity, there are a number of recommendations related to minimum levels of water in the home. The SPHERE project sets out 15 litres (L) of water used per capita
per day as a minimum standard for disaster relief [16
]. A guidance document prepared for the UK Department of International Development (DFID) suggested that a minimum criterion for water supply should be 20 L per capita
per day (lpcd) [17
]. The same figure has been suggested by other researchers [18
]. Gleick suggested that the international community adopt a figure of 50 lpcd as a basic water requirement for domestic water supply [19
]. The WHO has not yet issued guidelines on water quantity, as it has for decades on water quality [20
]. Nevertheless, in a widely quoted background document, the WHO cites a minimum for basic health protection of at least 20 L per person per day, of which 7.5 L is required for consumption, including direct hydration and cooking [21
This review summarizes evidence on the impact of improvements in water quantity at the household level. It examines studies in any population in any region of the world. It should be noted that this review does not include studies where interventions address water supplies and in doing so, might increase the amount of water available in the home. In order to be included in this review, studies must have actually measured a difference in water quantity at the household level, not a proxy such as improved water supplies, improved access, improved storage, or reduced time to collect water.
While water quantity is generally thought to be positively associated with health outcomes, this is mainly based on reviews of studies that rely on access as a proxy for the quantity of water in the home. This review, which was limited to studies that actually measured household water quantity, also found evidence of improved health from increased water quantity. However, the beneficial effect was largely dependent on how that water was used. Differences in study designs, settings, methods, and outcomes made a meta-analysis inappropriate, and the overall strength of the evidence was poor. Nevertheless, this review does provide some useful guidance on the relationship between water quantity and health.
4.1. Low- and Middle-Income Countries
For the communicable diseases examined—trachoma and GI-related illnesses—improved water quantity in the home often appeared to be significantly associated with improved disease outcomes, but in most cases this relationship depended on the manner in which the water was used.
For trachoma, which is a water-washed disease, simply bringing more water to the house or using more water for general household tasks was generally found not to be associated with improved trachoma indicators. Increased water that was used for face washing, by contrast, particularly face washing of children, was found to be significantly associated with lower prevalence of trachoma in the two studies that examined it.
Similarly, higher quantities of water in the home were generally associated with a lower odds of diarrheal disease. However, there was no evidence that increased consumption of water was protective. Rather, the evidence suggests that the health benefits were associated with increased use of water for personal and domestic hygiene.
For the growth indicators, higher levels of water consumption tended to be associated with higher weights and/or increased heights, but again only in certain groups. The finding that increased water quantity was only associated with increased growth in families that owned a latrine reinforces the importance of integrating increases in the quantity of water available with improvements in sanitation and hygiene.
Most included studies examining infectious diseases and growth outcomes in low and middle-income countries are cross-sectional studies, and many have serious methodological flaws that put these findings at a high risk of producing biased outputs. The relative consistency of the relationship of the findings and their accordance with the understood pathways of diseases add some credibility to the above summaries. Unfortunately, the described weaknesses in the included studies preclude any definitive statement of causality between water quantity and health outcomes; at best, these studies suggest an association between the two. In most of the included studies, it was also not made clear whether the groups using less water did so due to differences in preference (e.g., not wanting to wash the child’s face) or due to differences in availability of water (e.g., not having enough water to both wash the child’s face and provide adequate drinking water).
4.2. High-Income Countries
All of the studies that examined mortality and/or NCDs took place in high-income countries, namely the United States, Australia, and Canada. These studies were all either case-control or cohort studies, both of which generally produce more valid estimates than the cross-sectional studies more common in the included studies conducted in low- and middle-income countries. As noted in the risk of bias table, however, several of these studies had methodological flaws that increase the chance that bias affected their results. In particular, several studies had extremely low response rates and relied on recall periods of one to two years for self-reported water consumption.
For two of the three NCDs examined—renal cell carcinoma and bladder cancer—higher levels of water consumption were associated with increased odds of disease, while Type II diabetes incidence was not found to be associated with levels of water consumption. In both studies that examined mortality, neither all-cause mortality rates nor cardiac-related mortality rates were found to be associated with different levels of water consumption. Together, these findings suggest that increased levels of water consumption among residents of high-income countries might be associated with increased risk of a few specific NCDs, but these increased risks are not enough to significantly affect all-cause mortality rates.
4.3. Limitations and Further Research
This review identified several limitations in existing research on household water quantity that should be addressed by future studies. First, there is a paucity of studies that actually assess water quantity at the household level. While previous reviews have identified dozens of studies on the health impact of water quality and water access, few studies actually quantify the amount of water available. Even the proposed post-2015 targets simply rely on time to collect water as a proxy for water quantity [44
]. The current available guidelines vary widely in their recommendations for the required amount of water per person per day, with a range of 15 to 50 L per capita per day [16
]. Additional studies that directly measure the effects of changes in water quantity on health will allow for more valid, consistent, and evidence-based water quantity guidelines. Studies that examine water usage and the associated health outcomes in different settings, such as examining NCDs in low-income countries, would also be useful.
Second, the studies that do include a direct measurement of water quantity mainly follow a cross-sectional design with significant risks of confounding and bias. We identified no randomized controlled trials that assessed the impact of water quantity and health. In future, when researchers deliver interventions that are hypothesized to improve the quantity of water used by a household—such as building a standpipe in a household’s yard—they should measure the amount of water used by the household before and after the intervention and compare it to a valid counterfactual group. They could also measure the health impact of interventions that improve the efficiency of water use and so free up the quantity of water to be used for other tasks. In either case, if the intervention proves successful, the researchers could provide the counterfactual group with the intervention at the study’s close. These studies would allow for the conduct of randomized trials on water quantity without unethically restricting people’s water use, and they would also allow research on how people’s habits and preferences influence the quantity of water they use when restrictions on water quantity are eased. A supplementary source of evidence could be well-conducted, long-running surveys such as the Nurses’ Health Study cited in the section on NCDs, which could allow for much larger sample sizes than the randomized studies described above.
Third, the manner in which water quantity was actually measured varied significantly among studies and in some cases relied on self-reports whose accuracy has not been confirmed. There is a need for standard and validated approaches for measuring water quantity in the home, ideally using sensors or other technologies that are accurate and do not cause reactivity [45
Finally, this review suggests that research to assess the impact of water quantity on health must also measure how the water is actually used—a gap in much of the existing research. People do not merely collect water; they use it for hygiene, drinking, cleaning, and other purposes, all of which provide different health benefits. Depending on the disease outcome of interest, studies should include measures of the amount of water used for consumption, for personal hygiene, and for cleaning, as these were identified in this review as being associated with at least one health outcome.