Animal feeding operations and infant mortality: No evidence of causality

Sneeringer (1) raises the important public health issue of whether air pollution emissions from livestock operations increase infant mortality rate (IMR). She compares changes in IMR against changes in aggregate “animal units” (a weighted sum of cattle, pig, and poultry numbers) over time, for counties throughout the United States, while attempting to control for other relevant variables in a reduced-form, fixed-effect regression model. She concludes that “a doubling of production leads to a 7.4% increase in infant mortality.” We note that causal interpretation of reduced-form model coefficients is not justified. Reexamining the livestock production-IMR association using specific components of “animal units” (cattle, pigs, and broilers) at the state level, we find no statistically significant positive relations between livestock variables and IMR variables, after controlling for poverty and percent Black (not included in Sneeringer's models) and other variables. To the contrary, we observe statistically significant negative associations between changes in livestock production (especially, cattle production) and changes in IMR. We conclude that statistical associations between livestock variables and IMR variables are very sensitive to modeling choices (e.g., selection of explanatory variables, and use of specific animal types vs. aggregate “animal units) and that such associations do not warrant causal interpretation. We find no evidence of a valid causal relation between livestock production and any increase in infant mortality.

(1) Sneeringer S. Does animal feeding operation pollution hurt public health? A national longitudinal study of health externalities identified by geographic shifts in livestock production. Amer. J. Agr. Econ. 91(1) (February 2009): 124–137