Pollution levels are orders of magnitude higher in lower-income countries than in the developed world. This means that studies of the health effects of pollution based on data from the latter will not necessarily be relevant to the former. This column reports on the effect of air pollution on infant mortality in Mexico City. Significant effects are found that are much larger than found in earlier work based on US data. These findings highlight the potential pitfalls of naively extrapolating findings from high-income to developing countries.