July 21, 2009
Pollutants Lower Child IQ?
Pollution is bad.
A mother's exposure to urban air pollutants known as polycyclic aromatic hydrocarbons (PAHs) can adversely affect a child's intelligence quotient or IQ, a study reports. PAHs are chemicals released into the air from the burning of coal, diesel, oil and gas, or other organic substances such as tobacco. In urban areas motor vehicles are a major source of PAHs.
The study, funded by the National Institute of Environmental Health Sciences (NIEHS), a component of the National Institutes of Health, the U.S. Environmental Protection Agency and several private foundations, found that children exposed to high levels of PAHs in New York City had full scale and verbal IQ scores that were 4.31 and 4.67 points lower than those of less exposed children. High PAH levels were defined as above the median of 2.26 nanograms per cubic meter (ng/m3). A difference of four points, which was the average seen in this study, could be educationally meaningful in terms of school success, as reflected, for example, in standardized testing and other measures of academic performance. However, the researchers point out that the effects may vary among individual children.
But one can easily imagine smarter parents figuring out how to reduce their family's exposure to air pollutants. So this could be a selection effect.
But the subjects had common backgrounds and might not have differed all that much in other characteristics. Hard to tell.
The study was conducted by scientists from the Columbia University Center for Children's Environmental Health. It included children who were born to non-smoking black and Dominican-American women age 18 to 35 who resided in Washington Heights, Harlem or the South Bronx in New York. The children were followed from utero to 5 years of age. The mothers wore personal air monitors during pregnancy to measure exposure to PAHs and they responded to questionnaires.
Live in the country and breath cleaner air. If you have to live in a city or near a highway consider getting a HEPA filter in your residence.
Or consider pushing your government to quit spending your money to build roads that poison your children.
When I read a study like this one, I often wonder if the investigators haven't decided on the results in advance. They say:
At 5 years of age, 249 children were given an intelligence test known as the Wechsler Preschool and Primary Scale of the Intelligence, which provides verbal, performance and full-scale IQ scores. The test is regarded as a well validated, reliable and sensitive instrument for assessing intelligence. The researchers developed models to calculate the associations between prenatal PAH exposure and IQ. They accounted for other factors such as second-hand smoke exposure, lead, mother's education and the quality of the home caretaking environment. Study participants exposed to air pollution levels below the average were designated as having low exposure, while those exposed to pollution levels above the median were identified as high exposure.
Now I'm sure no expert in this area, but I wonder about the the SD and precision of measuring IQ in a five year old. But even if that can be done accurately, it's almost impossible to control for every possible confounding variable. The authors mention that they control for some obvious variables, but here are some others that come to mind:
Parental IQ (not just maternal education level - and what about the fathers?)
Maternal drug and alcohol use, either during or after pregnancy
Exposure to heavy metals other than lead and other toxins
Stay-at-home mom vs. daycare
Preschool vs. no preschool
Breast-fed vs. non-breast-fed
Full-time dad vs. part-time dad vs. no dad at all
There could also be selection factors, such as higher IQ mothers having higher incomes and living farther from sources of pollution.
None of this means that the authors' conclusions are wrong - it is certainly plausible that they are right. It's just that this is very difficult to prove. If the effect is real, it will be reproducible and will be detected in other studies. Perhaps it would be wise to look at the same factors in affluent families in both urban and rural environments. Again, if the pollution-IQ relationship is valid, it will show up again and again.
The trouble with studies like this is that they are, at best, very preliminary findings. But pseudo-sophisticated journalists and media hacks, with little to no background in science and statistics, will quote studies such as this one as the last word on the subject and demand all sorts of government controls on "pollution" to "save the children." Now aditional controls on pollution may or may not be a good idea, and such controls are surely not without substantial cost, but they should not be implemented on the basis of studies like this one.
4.67 points of IQ seems rather significant. To the extent of not leaving a whole lot more room for other effects of environment on IQ.
If identical twin studies show a .85 correlation between the IQ of twins, that means of the population IQ varience (15 points squared = 225), only 15% remains after accounting for the IQ of the twin. The remaining varience is 33.75 or a standard deviation of 5.8 points. So their high-end result is 80% of the environmentally determined IQ variation.
1) Air quality has a huge impact on IQ versus other environmental factors, or
2) This study overestimates the effect of air quality, either through bad data or through ignoring confounding effects that were correlated with the air quality readings, or
3) The twin studys underestimate the effect of environment as the twins studied did not enncounter as widly varying environments as these minority New York children.
I would tend to put the most weight on option 2, though it could also be a matter of comparing apples to oranges, as I'm not sure if the twin studies are strictly measuring adult correlations which appear to have much less environmental varience than children. I suppose this air quality study would probably be a lot more useful if they tested the children at 15 or 25 years old instead.