Figure 5 depicts the predicted number of infant and neonatal deaths out of 1,000 live births and marginal effects of drinking water salinity based on the RCS model with fully adjusted covariates (model 3). The predicted values show a U-shaped curve over the entire range of salinity and the marginal effect of salinity on infant death is concave, suggesting that infant and neonatal deaths decreased initially and then increased again with increasing levels of drinking water salinity. As shown in the bottom left figure, with fresh drinking water (<0.7 mS/cm), the marginal effect of salinity on infant death is always negative, which can be calculated by the first derivate of outcome with respect to GWEC shown in the Appendix. Drinking slightly saline water can help to reduce infant mortality because slightly saline water has mostly healthy mineral concentration and limited sodium concentration relative to freshwater with low concentrations of salubrious calcium and magnesium (Naser et al., 2019b) As drinking water becomes mildly saline (0.7 – 2 mS/cm) and moderately saline but below 4.11 mS/cm (calculated in Appendix), the negative marginal effect is decreasing, suggesting that the protective role of drinking saline water is attenuating. As drinking water becomes moderately saline (above 4.11 mS/cm), the marginal effect becomes positive and increasing, with associated high infant mortality and high-level EC contributed by high concentration of sodium but limited concentrations of salubrious calcium and magnesium. Similarly, for neonatal death the bottom right figure presents a diminishing protective role of drinking water salinity as EC increases below 4.38 mS/cm but harmful role as EC increases above 4.38 mS/cm. The restricted cubic spline plots illustrate a U-shaped non-linear association between drinking water EC and infant and neonatal deaths. This is consistent with the results of categorical exposure version which showed harmful effects for infants and neonates exposed to freshwater and severely saline water as well as the protecting effects of mild salinity exposure. Calcium and magnesium peak around 1 mS/cm EC, but their concentrations do not change afterward with increasing level of EC, which explains why there was lower neonatal or infant mortality among the mild-salinity water drinkers (Naser et al., 2019b). In contrast, the relationship between groundwater sodium and EC followed an almost linear upward trend, suggesting severe-saline water drinkers have very high sodium intake. The concentration of these macro-minerals in drinking water is determined by the chemical properties of rocks and sediments through which groundwater percolates and interacts (Hudson, 2018).

The number of infant and neonatal deaths based on the RCS model with fully adjusted covariates (model 3) was predicted with covariates at BDHS cluster means. The first derivative of outcome with respect to drinking water salinity indicated by EC level predicts the probability of infant or neonatal death induced by salinity at each cluster for BDHS 2011 and 2014. To estimate population totals based on BDHS data, the standard weights were de-normalized. The number of live births and predicted infant/neonatal deaths were calculated as summation of de-normalized cluster values at subdivision levels.

Table 6 lists the number of live births, infant, neonatal deaths and their predicted values induced by high salinity exposure in coastal subdivisions in BDHS 2011. During the three years prior to the BDHS 11