Wednesday, July 22, 2009

The AP gets a C for their coverage of a study on prenatal pollution exposure

The Associated Press reports on a study of the effects of localized pre-natal pollution on child IQ:
The results are in a study of 249 children of New York City women who wore backpack air monitors for 48 hours during the last few months of pregnancy. They lived in mostly low-income neighborhoods in northern Manhattan and the South Bronx. They had varying levels of exposure to typical kinds of urban air pollution, mostly from car, bus and truck exhaust.

At age 5, before starting school, the children were given IQ tests. Those exposed to the most pollution before birth scored on average four to five points lower than children with less exposure.

The researchers studied pollutants that can cross the placenta and are known scientifically as polycyclic aromatic hydrocarbons. Main sources include vehicle exhaust and factory emissions. Tobacco smoke is another source, but mothers in the study were nonsmokers.

A total of 140 study children, 56%, were in the high exposure group. That means their mothers likely lived close to heavily congested streets, bus depots and other typical sources of city air pollution; the researchers are still examining data to confirm that, Perera said. The mothers were black or Dominican-American; the results likely apply to other groups, researchers said.

The researchers took into account other factors that could influence IQ, including secondhand smoke exposure, the home learning environment and air pollution exposure after birth, and still found a strong influence from prenatal exposure, Perera said.

Dr. Robert Geller, an Emory University pediatrician and toxicologist, said the study can't completely rule out that pollution exposure during early childhood might have contributed. He also noted fewer mothers in the high exposure group had graduated from high school. While that might also have contributed to the high-dose children's lower IQ scores, the study still provides compelling evidence implicating prenatal pollution exposure that should prompt additional studies, Geller said.
The good:

(1) The study is actually forthcoming in a peer reviewed journal.

(2) The article actually mentions some but not all of the conditioning variables and also quotes someone who notes a very important missing conditioning variable, though the article implicitly understates the importance of the issue.

The bad:

(1) The article does not provide a link to the study.

(2) The article does not describe how the sampling was done. Where did these women come from who are willing to wear pollution measuring equipment while already carrying around a soon-to-be-born baby?

(3) The overall tone should be more skeptical.

My take on the study:

(1) Bonus points for the cool measurement methodology.

(2) Given that the authors apparently did not control for either of the parents' IQ scores or their educational attainment, it seems quite unlikely that the selection-on-observed variables holds here. The likely direction of the bias is in the direction of the estimated impact.

(3) Bottom line: this study suggests that it might be worth it to fund a study of the same treatment with a better identification strategy. I don't think you can say much more.

Hat tip: Sue Dynarski