Author’s Response

Thank you for sharing your epiphany on the impact of low levels of lead on children’s intellectual abilities and the contribution of lead from water, as well as your question about seasonal variation in blood lead concentrations. It is difficult to appreciate how exceedingly small amounts of lead can impact brain function, especially for pediatricians who can recall when blood lead concentrations <30 μg/dL (<300 ppb) were considered “acceptable.” The Canfield et al1 study, along with dozens of other studies from around the world,2,3 have confirmed that blood lead concentrations <5 μg/dL (<50 ppb) can adversely impact brain development. This shouldn’t be surprising; the concentrations of toxic chemicals in children’s blood that are harmful, like lead, polybrominated diphenyl ethers, and polychlorinated biphenyls, are comparable to the therapeutic range of chemicals administered as drugs to alter behaviors, like methylphenidate.3 Moreover, on an evolutionary scale, the levels found in contemporary children aren’t small; they are 10 to 100 times higher than in our preindustrial ancestors.4Seasonal variation in lead poisoning is one of the enduring mysteries of childhood lead poisoning. Cases of lead poisoning and children’s blood lead concentrations appear to increase during summer months for various reasons. As Dr Marcus noted, warmer temperatures enhance water’s ability to leach lead from lead service lines and lead solder.5 The amount of lead in house dust also increases during summer months.6 Windows are frequently opened; dust trapped in the window troughs is blown indoors or accessible to children’s curious hands. Soil that was contaminated by lead from past use of leaded gasoline or lead-based paints can also be “tracked in” from outdoors or become resuspended and settle in house dust, which is then readily accessible to children.7 Children also spend more time outdoors in the summer6; a sizable fraction of children (∼30%) are reported to put soil or dirt in their mouths, especially during the second year of life, when blood lead levels tend to peak.8 The original description of overt lead poisoning among children in 1904 included paint from porch railings as an important source of lead intake.9 Many toddlers in Rochester reportedly played on weathered porches that were covered with leaded paint during summer months; exterior paints typically contained higher concentrations of lead than interior paints. Finally, calcium absorption is increased by greater sun exposure (vitamin D activation) during summer months and, by its mimicry of calcium, lead absorption may also increase.

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