Legacies of inequality, legacy lead exposures, and improving population well-being

The findings in McFarland et al. (1) are at once disturbing and reassuring. The bad news is captured in their title: “Half of US population exposed to adverse lead levels in early childhood.” That striking figure, based on a welldesigned and rigorous analysis, will make readers take note, as well it should. We have known for some time now the deleterious impact of lead exposure on a wide range of features in young children’s development, including the disruption of multiple organ systems, cognitive deficits, emotional dysregulation, and impaired self-control. The authors also note the convergent evidence on what they call “legacy lead exposures” which are revealed in multiple outcomes later in life such as educational attainment, income mobility, delinquent behavior, and physical health. The bundle of adversities associated with childhood lead exposure, even if individually subtle, cumulate over time and can generate long-term effects (2–4). How can there be grounds for optimism in a study showing that half the current population of adults has been exposed to the detrimental effects of lead in childhood? The authors’ (1) birth cohort design and population projections throughout the 21st century provide an answer. We know that leaded gasoline declined substantially in the United States starting in the mid-1970s and that there have been subsequent declines in children’s levels of lead. But the innovation of the McFarland et al. study is to estimate cohortspecific blood-lead levels (BLLs) by age in 2015 based on the National Health and Nutrition Examination Survey (NHANES), a representative sample of US children. Constructing serial cross-sectional measures of BLLs across four decades of the NHANES merged with census and mortality data, the authors provide past, present, and estimated future trends in lead exposure, along with consequences for cognitive ability. Although levels of childhood lead exposure among current adults are alarmingly high, and the authors estimate considerable population losses over time in cognitive ability, the trends in lead exposure are nonetheless in the right direction. The decline across successive cohorts is so steep that nearly all of today’s children have BLLs less than 5 μg/ dL, traditionally considered to be low even though no lead level is now considered safe. McFarland et al. project that, as we reach 2100, nearly all adults will have experienced leadfree childhoods if 2015 exposure levels hold. This means that the trajectory of the United States is on the right path, at least with respect to the fading of prior lead exposures among adult populations. That is good news indeed.