Global mapping of areas with a double burden of poor water quality and unsafe drinking water access
This study introduces the first global subnational dataset (G-HSDW) to quantify the co-exposure of unsafe drinking water access and poor surface water quality, revealing that the majority of people relying on unimproved or surface water sources face high to severe contamination risks.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Safe drinking water is the foundation of public health, yet for hundreds of millions of people, the water they drink is not safe. For decades, global monitors have tracked who has access to water sources, classifying them on a ladder from the most basic, unprotected wells to the safest, managed systems. This tracking has been essential, but it has traditionally focused on whether the water is accessible, often leaving the actual quality of that water unmeasured for the most vulnerable populations. At the same time, scientists have been mapping how rivers and lakes are becoming polluted with bacteria, organic waste, and salt. Until now, these two critical stories—who is drinking what, and how dirty that water is—have rarely been told together on a global scale. This separation has left a blind spot in our understanding of a double burden: the situation where people rely on unsafe sources that are also becoming increasingly contaminated.
A team of researchers from Aalto University has now bridged this gap by creating the first complete global map that combines these two realities. They built a new, detailed dataset covering nearly every region on Earth, tracking how many people rely on surface water like rivers and lakes, or on unimproved sources like unprotected wells, from 1990 through 2023. They then overlaid this with computer simulations of water quality, looking specifically at three major threats: organic pollution, which depletes oxygen and signals waste; fecal contamination, which carries disease-causing germs; and salinity, or the buildup of salt. By merging these datasets, the team could pinpoint exactly where the people with the least safe access are also facing the highest risks from dirty water.
The results reveal a stark and worrying picture. While the number of people relying on the most dangerous water sources has dropped significantly over the last thirty years, the quality of the water they do drink has not kept pace. In 2019, only one in ten people who still relied on surface water or unimproved sources lived in an area where the water quality was considered low risk. The situation is even more dire when looking at fecal contamination alone, where only one in ten faced low risk. Instead, the majority of these populations face high to severe risks. More than half of the people using unimproved sources and sixty percent of those using surface water are exposed to water quality that poses a serious threat to their health. This means that even as the world makes progress in providing access to water, the water itself is often becoming more dangerous for those who have no other choice.
The researchers identified specific regions where this double burden is most acute, calling them "dark spots." These areas, where poor access coincides with poor quality, are concentrated in South and East Asia, the Rift Valley and eastern coast of Africa, and parts of South America. In these regions, tens of millions of people are dependent on water sources that are prone to contamination. For instance, while the percentage of people in China and India relying on surface water is small, the sheer size of their populations means that millions of individuals are still affected. Conversely, the team found "bright spots" where both access and quality have improved, but these areas benefit a much smaller number of people. The most concerning trend is that in many large areas, water quality is deteriorating even as access to safe water improves, leaving a growing number of people in a precarious position.
The study also highlights that water quality is not a static condition; it changes with the seasons. In many parts of the world, the risk of contamination spikes during certain times of the year, creating a seasonal swing in exposure that can be dramatic. For example, in some regions, the number of people facing high-risk conditions can jump by hundreds of millions between the safest and most dangerous months of the year. Furthermore, the analysis shows that while some regions are seeing steady improvements, others are experiencing accelerating declines in water quality, driven by factors like agricultural runoff, population growth, and climate change. The researchers note that their map focuses on surface water and unimproved sources, and while it captures the most immediate dangers, it does not fully account for the complex issues of groundwater contamination, which affects many of the same vulnerable regions.
Ultimately, this work provides a clear, spatially explicit guide for where action is needed most urgently. It moves beyond general statistics to show exactly which communities are facing the combined threat of unsafe access and dirty water. By identifying these dark spots, the study offers a roadmap for policymakers and aid organizations to target investments where they can save the most lives. The findings suggest that improving water security requires a dual approach: continuing to expand access to safe sources while simultaneously protecting the quality of the water bodies that remain. Without addressing both sides of this equation, the progress made in providing water access may be undermined by the very water people are forced to drink.
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