Comparing the population neurodevelopmental burdens associated with children's exposures to environmental chemicals and other risk factors.

To estimate the population burden of an exposure that is associated with neurodevelopmental impairment, it is necessary to consider both the effect size associated with the exposure (i.e., the decrease in function per unit increase in biomarker level) and the prevalence of the exposure. An exposure with a modest effect size might, nevertheless, be associated with a substantial population burden if many children are exposed at levels at which the exposure is known to have a detrimental impact. This illustrates the important distinction between individual risk and population risk. A method is described that can be used to compare different risk factors in terms of their contributions to the population burden of neurodevelopmental impairment. Combining estimates of the incidence/prevalence/distribution of different conditions or exposures with estimates, derived from meta-analyses, for the impact of different risk factors on children's Full-Scale IQ scores (FSIQ), the total FSIQ losses associated with each were calculated for the U.S. population of children less than 5 years of age. The losses associated with non-chemical risk factors ranged widely: 34,000,000 FSIQ points for preterm birth, 17,000,000 for Attention Deficit Hyperactivity Disorder, 9,000,000 for iron deficiency, 136,000 for acute lymphocytic leukemia, and 37,000 for brain tumors. The FSIQ losses could be estimated for three chemicals: lead, 23,000,000 points; methylmercury, 285,000 points; and organophosphate pesticides, 17,000,000 points. Many caveats attend these calculations, but the findings suggest that in continuing to apply standards appropriate to evaluating the impact of chemical exposures on an individual child rather than on the population as a whole, we risk underestimating the population burdens associated with them.