These families are also likely to provide other investments in their children, so that fetuses and infants exposed to lower levels of pollution also receive more family inputs, such as better quality prenatal care.
If these factors are unaccounted for, this would lead to an upward bias in estimates. Alternatively, pollution emission sources tend to be located in urban areas, and individuals in urban areas may be more educated and have better access to health care, factors that may improve health.
Omitting these factors would lead to a downward bias, suggesting the overall direction of bias from confounding is unclear.