Studies of the effects of pollution on infant mortality also yield mixed results. 
For example, Woodruff et al. (1997) report that infants with high exposure to PM10 are more likely to die in the post neonatal period. 
But Lipfert et al. (2000) find that although they can reproduce some earlier results showing effects of county level pollution measures on infant mortality, the results are not robust to including controls for maternal characteristics.