BACKGROUND:
Unsafe drinking water remains a critical global health challenge, posing significant risks to public health. This study aimed to comprehensively evaluate the health risks associated with unsafe drinking water and the effectiveness of various interventions through a global umbrella review.

METHODS:
We synthesized data from 25 systematic reviews and meta-analyses, encompassing 158 independent outcome indicators. The review rigorously adhered to PRISMA guidelines, employed AMSTAR 2 for methodological assessment, Evidence Classification for statistical credibility, and graded evidence quality using the GRADE method. Additionally, a WHO-recommended simplified Evidence-to-Decision framework was utilized to translate findings into actionable policy recommendations.

RESULTS:
The analysis revealed that unsafe drinking water is linked to a broad range of health issues, including infectious diseases, cancers, cardiovascular diseases, respiratory illnesses, and adverse maternal and child health outcomes. However, most evidence was rated as low or very low quality according to GRADE criteria. Drinking water interventions, particularly point-of-use filtration, showed substantial effectiveness in reducing childhood diarrhoea risk (RR = 0.48, GRADE: moderate). Additionally, defluoridation measures provided highly suggestive evidence for reducing fluorosis-related diseases.

CONCLUSIONS:
Although the certainty of evidence attributing health risks to unsafe drinking water remains limited, intervention effectiveness evidence is stronger. These findings support prioritizing proven safe drinking water measures, especially targeting children and high-risk populations, to significantly reduce disease burden and promote global health equity.