Water Quality: A Foundation for Health and Development

Reliable access to safely managed drinking water (clean, safe, and available at home) is essential for health and well-being. While such access is often taken for granted in high-income countries, 2.1 billion people—about a quarter of the world’s population— still rely on water sources that may be far away, irregular, or contaminated.

Poor water quality has significant human and economic costs. Contaminated drinking water is a major driver of preventable disease. It poses serious health risks, especially for infants and young children, with potential long-term consequences for health and cognitive development.[reference: chemical_risks] These risks are global and affect all regions, including high-income countries.[reference: ecoli_wy] That is why achieving universal access to safely managed drinking water is a Target (6.1.1.) under the United Nations Sustainable Development Goals.

Evaluating the quality of drinking water involves looking at both [emphasis: water sources ](the type of infrastructure providing water) and [emphasis: water service levels ](how well these sources provide water).

A Joint Monitoring Programme for Water Supply, Sanitation and Hygiene (JMP) developed by the World Health Organization (WHO) and the United Nations Children’s Fund (UNICEF) classifies water sources as improved or unimproved. [emphasis: Improved sources] are designed to reduce the risk of contamination, and include pipes, tube wells, and protected wells or springs. [emphasis: Unimproved sources] include unprotected wells, springs, and surface water (from rivers, lakes, and ponds). But while improved water sources are likely to be protected from outside contamination, this does not ensure safe water quality; nor does it consider accessibility or availability.

To provide a holistic perspective of safe drinking water, JMP classifies drinking water services into five levels: [emphasis: safely managed], [emphasis: basic], [emphasis: limited], [emphasis: unimproved], and [emphasis: surface water].[reference: water_levels]

Although global access to safely managed drinking water has improved, significant disparities remain, particularly in rural areas, where access lags behind urban areas.

Next, we examine the overall levels of access to safely managed drinking water at country level.

Because the data for the required components often come from different sources and surveys, they cannot always be measured for the same population. To avoid overstating access levels, the methodology uses a conservative approach, taking the [emphasis: lowest][emphasis: ]value among the three components (free from contamination, accessible on premises, or available when needed) as the estimate of safely managed drinking water access. The component with the lowest value is also referred to as the[emphasis: limiting factor to access.]

Across 94 countries with available data,[footnote: Excludes 29 countries with universal (99%+) access and 25 countries lacking availability data (54 in total). Safely managed drinking water access is estimated when all three components are available, or when water quality data are available with at least one other component.] [emphasis: water quality is the most common limiting factor] —in 48 countries (just over half of all countries)— followed by accessibility, and availability, in 23 countries (about a quarter of the countries) each.

As well as being the most common limiting factor across countries, progress[footnote: For details on the progress speed, refer to “Global Progress” and “Measuring Progress” under the Progress section. ] in improving water quality has also lagged the other two components of safely managed drinking water. While many countries have expanded physical access to water and improved availability, ensuring that water is free from contamination remains a challenge. Tanzania illustrates this pattern clearly.

The presence of the bacteria Escherichia coli (E. coli) is the key measure of fecal contamination of drinking water.

Water contaminated by human feces can lead to acute diseases and pose life-threatening risks, particularly to infants and young children who are most vulnerable as their immune systems are still developing. The scale of the impact is dramatic. In 2019 alone, 270,000 children under five died from diarrheal diseases attributable to unsafe water, sanitation, and hygiene.[reference: burden_disease]

To assess fecal contamination through household surveys, the presence of E. coli is measured in a 100 milliliter (mL) water sample. According to the WHO risk levels, water is classified as low risk if it contains fewer than 1 CFU[footnote: CFU measures the number of viable E. coli bacteria in a water sample, expressed as colony-forming units per 100 mL.]. E. coli per 100 mL, moderate risk if it contains 1–10 CFU, high risk if it contains 11–100 CFU, and very high risk if it exceeds 100 CFUE. coli per 100 mL.[reference: jmp_test]

In household surveys such as the Multiple Indicator Cluster Surveys (MICS), water quality testing is conducted both at the [emphasis: point of collection]—that is, [emphasis: at the household’s primary drinking water source, ]such as a tap, well, or protected spring—and at [emphasis: point of use], that is, [emphasis: within the household, ]typically from a drinking glass or container. The water is tested for E. coli at both points to assess the level of contamination.

In countries with available data, households commonly store water before they consume it, which can increase the risk of contamination.

Household water treatment—such as boiling, chlorination, filtration, or solar disinfection—is therefore an important public health intervention, reducing bacterial and other microbial contamination. But treatment practices are often uncommon, and when reported, are not always effective due to incorrect or inconsistent application.[reference: ecoli_research]

For example, [emphasis: in Fiji, 56 percent of rural residents use water sources classified as high or very high risk,] compared to 4 percent of urban residents. Many rural communities are geographically isolated and depend on untreated sources, such as surface water, rainwater, wells, and boreholes. Climate change and increasingly severe extreme weather events intensify the risk of microbial contamination.[reference: fiji1] The picture[emphasis: ]is similar[emphasis: ]in [emphasis: Togo ]and the[emphasis: Congo, Rep.,] where 72 and 56 percent of rural residents use high-risk water sources, compared to 26 and 10 percent of urban residents, respectively.

Beyond risks from fecal contamination, [emphasis: chemical contaminants such as arsenic, fluoride, nitrates, and heavy metals also pose serious long-term health threats]. Among these, arsenic and fluoride are considered priority concerns for drinking water.[reference: sdg_priority] At low concentrations, fluoride strengthens teeth; but excessive exposure can have adverse effects on skeletal tissues, including dental and skeletal fluorosis,[reference: who_guideline] characterized by changes in bone and tooth structure. Arsenic contamination is also harmful, and prolonged exposure is associated with skin lesions, cancers, cardiovascular disease, and cognitive impairment.[reference: arsenic_exposure]

Arsenic is a naturally occurring element found in the Earth’s crust. About 140 million people across at least 70 countries consume water containing arsenic at levels above WHO’s guideline value of 10 parts per billion (ppb).[reference: who_guideline] [emphasis: The largest numbers of people exposed to arsenic in drinking water are in][emphasis: Bangladesh and India].[reference: arsenic_exposure][reference: arsenic_bgd]

Bangladesh faces the world’s most severe arsenic crisis: since the 1990s, millions of people have been exposed, leading to cancers, skin disease, and adverse pregnancy outcomes. Around 17 percent of the population drinks water with arsenic concentrations above the WHO guideline.

[emphasis: Arsenic exposure is particularly high among users of wells, including tubewells, which are the most common source of drinking water in Bangladesh. ]Tubewells are a relatively low-cost way to access shallow groundwater, which is generally less prone to bacterial contamination than surface water. But in Bangladesh, due to geological conditions, many of these water sources are contaminated with arsenic.[reference: arsenic_bgd]

Among people who rely on wells, roughly one in five (about 20 percent) are exposed to arsenic concentrations above the WHO guideline of 10 ppb. This risk is substantially lower for other water sources: only about 5 percent of those with piped water and 2 percent of those using alternative sources exceed this level. At higher concentrations, above the national standard of 50 ppb, the same pattern holds. About 12 percent of well users are exposed at these levels, compared with 3 percent of piped-water users and just 1 percent of those relying on other sources.

Since the 1990s, the Government of Bangladesh has introduced a range of programs to address arsenic contamination in drinking water. This includes nationwide tubewell screening, installation of arsenic-safe water sources, such as piped systems drawing on safe surface water or deep groundwater, the use of household and community filtration technologies, public awareness campaigns, and health sector initiatives to identify and manage arsenic-related illnesses.

Yet arsenic contamination remains a pressing challenge. Millions of people continue to rely on unsafe water because arsenic-free options—such as deep wells, piped systems, and rainwater harvesting—are often limited, costly, or difficult to access.[reference: arsenic_bgd][reference: arsenic_bgd_jpal] Research indicates that limited resources, gaps in local information, and the sheer number of people and wells affected further complicate mitigation efforts.[reference: arsenic_research] Countries like Bangladesh face the dual burden of microbial risks and arsenic contamination, a problem that is particularly difficult for low- and middle-income countries with constrained resources.

Major challenges remain in ensuring water quality for all. While global access has progressed over the past two decades, deep inequalities persist across regions, countries, and income groups, and between rural and urban areas. A major challenge is that in many low- and middle-income countries drinking water is still contaminated by E. coli or harmful chemicals, such as arsenic.

At the same time, strengthening sanitation and hygiene systems is crucial, but sanitation programs, in particular, remain chronically underfunded in many countries.[reference: SWS] Without adequate sanitation and hand hygiene, investments in water supply are undermined by contamination, perpetuating cycles of disease.

As Nobel laureate Michael Kremer emphasizes, the link between water quality, access to safe drinking water, and child survival is especially strong in low- and middle-income countries. Evidence shows that improved water treatment can reduce child mortality by 25 percent, with the greatest benefits for infants and young children.[reference: Kremer]