Mortality from waterborne infectious diseases remains a serious issue
globally. Investigating the efficient laying plan of waterworks to mitigate the risk
factors for such diseases has been an important research avenue for industrializing
countries. While a growing body of the literature has revealed the mitigating effects
of water-purification facilities on diseases, the heterogeneous treatment effects of
clean water have been understudied. The present study thus focuses on the treatment
effect heterogeneity of piped water with respect to the external meteorological
environment of cities in industrializing Japan. To estimate the varying effects, we
implement fixed-effects semivarying coefficient models to deal with the unobservable
confounding factors, using a nationwide city-level panel dataset between
1922 and 1940. We find evidence that the magnitude of safe water on the reduction
in the typhoid death rate is larger in cities with a higher temperature, which is
consistent with recent epidemiological evidence. These findings underscore the
importance of the variations in the external meteorological conditions of the
municipalities that install water-purification facilities in developing countries.

• Heterogeneous treatment effects of safe water on infectious disease: Do meteorological factors matter?