One way to think about these estimates is in terms of underlying heterogeneity in the vulnerability of infants.
Although the average family with a death is different than the average family without one, we are concerned about the impacts of pollution on the infant at the life/death margin.
If the characteristics of the marginal infant who dies because of an increase in pollution is similar to the characteristics of the marginal infant who survives the same increase in pollution, then our results will tell us about the effects of variations in pollution for the range of pollution we observe.