The study rests on an extraordinary evidence base: more than 4.2 million reported cases of bacillary dysentery recorded across 338 Chinese cities between 2005 and 2020. That sixteen-year window captures both the period before the sponge city initiative launched in 2015 and the years of its early rollout, allowing the researchers to observe how disease dynamics shifted as urban landscapes were transformed. The surveillance data, drawn from China’s national infectious disease reporting system, were linked with high-resolution meteorological reanalysis data from the ERA5-Land dataset produced by the European Centre for Medium-Range Weather Forecasts, enabling the team to pinpoint precisely when and where flood conditions occurred relative to each reported case.

To isolate the acute effect of flooding on disease risk, the researchers employed a case-crossover design, an epidemiological technique in which each case of dysentery effectively serves as its own control. By comparing each patient’s flood exposure in the days immediately before illness onset with their exposure during comparable control periods, the design automatically strips away confounding from individual characteristics, seasonal patterns, and stable city-level factors. The team modeled the lagged relationship using distributed lag nonlinear models, allowing the effect of a flood to accumulate and decay over time rather than assuming a single-day impact. Their findings were unambiguous: flood exposure elevated bacillary dysentery risk over a lag period extending to 28 days, with severe floods producing the strongest effects.

The vulnerability was not evenly distributed across the population. School-aged children emerged as the group most sensitive to floods overall, a pattern consistent with their greater exposure to contaminated water and surfaces and their still-developing immune defenses. For severe floods, however, the burden shifted toward the elderly, whose physiological resilience and access to care can be compromised during disaster conditions. These age-specific patterns echo a broader literature linking hydrometeorological hazards to child diarrhea and enteric infections, and they underscore that flood adaptation policies carry direct consequences for the most demographically sensitive members of society.

The second and arguably more consequential half of the analysis addressed whether the sponge city initiative actually changed the flood-disease relationship. Launched by the Chinese government in 2015, the program promotes green infrastructure—permeable pavements, rain gardens, constructed wetlands, green roofs, and restored urban waterways—designed to absorb, store, and purify stormwater at its source rather than channeling it through overburdened pipe networks. Because participation was not random, the researchers used a quasi-experimental difference-in-differences framework, comparing 27 sponge cities against 27 carefully matched non-sponge cities over the same period. This approach, adapted from modern econometric methods for staggered policy adoption, estimates what would have happened to disease risk in the absence of the intervention.

The results point to a genuine protective effect. Cities that adopted sponge city development showed lower flood-related bacillary dysentery risk than their matched counterparts, and the benefit deepened over time. The greatest reduction—12.2 percent—was observed by the third year after the initiative, a trajectory consistent with the gradual build-out of green infrastructure and the time required for absorption capacity, drainage performance, and water quality improvements to mature. Event-study estimates confirmed that the divergence between sponge and non-sponge cities emerged only after policy implementation, strengthening the causal interpretation of the findings.

Perhaps the most policy-relevant discovery concerns where the benefits were largest. The protective effect of sponge city development was strongest in cities that started with weaker baseline infrastructure—places with fewer public toilets, lower water pipeline density, and less favorable industrial profiles. In other words, nature-based adaptation delivered its greatest health dividends precisely where conventional sanitation and water systems were least able to buffer flood impacts. This finding challenges the assumption that green infrastructure is a luxury of wealthy cities and instead positions it as an equity tool, capable of narrowing health gaps between developed and less-developed urban settings.

The mechanistic logic behind these results is straightforward. Floods overwhelm combined sewer systems, causing untreated wastewater to mix with floodwater and contaminate surface water, wells, and distribution networks. Green infrastructure interrupts this chain by reducing the volume and velocity of stormwater runoff, limiting sewer overflows, and filtering pollutants before they reach water bodies. By dampening the hydrological shock of heavy rainfall, sponge city features reduce both the probability that pathogens enter the water supply and the duration of human exposure to contaminated environments. The epidemiological signal observed in this study is thus a plausible downstream consequence of well-documented hydrological and water-quality mechanisms.

The study’s scale and methodological rigor distinguish it from earlier work on green infrastructure and health. Previous systematic reviews had catalogued the social and well-being benefits of stormwater-focused green infrastructure but offered little quantitative evidence on infectious disease outcomes, and prior Chinese studies of floods and dysentery were typically confined to single provinces or short time frames. By combining a nationwide case-crossover analysis with a quasi-experimental policy evaluation across matched city pairs, the researchers triangulated the flood-disease association from two independent analytical directions, each with different vulnerability to bias. The convergence of the two approaches lends considerable weight to the conclusion that the observed risk reductions reflect a real intervention effect rather than pre-existing differences between city types.

The implications extend well beyond China. Rapid urbanization and climate change are intensifying pluvial flooding in cities across Asia, Africa, and Latin America, where aging drainage systems and dense populations create fertile conditions for waterborne disease transmission. The Chinese experience suggests that climate adaptation and public health protection should not be treated as separate policy silos: the same permeable surfaces and urban wetlands that mitigate flood peaks also appear to suppress the infectious disease burden that follows in floods’ wake. For governments weighing the costs of green infrastructure investment, the study provides a rare quantified health co-benefit—one that accrues fastest and largest in the communities least equipped to cope with flooding. As climate volatility grows, the sponge city model offers a template for building cities that are not only drier, but measurably healthier.

Subject of Research: The effect of China's sponge city development initiative on flood-related bacillary dysentery risk

Article Title: Reduced flood-related bacillary dysentery risk with sponge city development in China

Article References: Zhang, L., Zha, Q., Zhao, H., Liu, C., Wang, Y., Ji, J. S., Huang, C., & Luo, B. (2026). Reduced flood-related bacillary dysentery risk with sponge city development in China. Nature Cities. https://doi.org/10.1038/s44284-026-00514-z

Image Credits: AI Generated

DOI: 10.1038/s44284-026-00514-z

Keywords: sponge city, bacillary dysentery, flooding, green infrastructure, waterborne disease, China, public health, climate adaptation, case-crossover design, urban resilience, quasi-experimental study, Nature Cities