Despite the dramatic fall in BLLs following the removal of lead from gasoline, elevated childhood lead levels persist as a source of public health concerns. Since 1991, CDC has maintained 10 μg/dL as a guide to excessive exposure. According to the CDC, in 1999 and 2000 2.2% of children in the 1–5-year age group exhibited lead levels above 10 μg/dL (http://www.cdc.gov/nceh/lead/faq/about.htm
). Approximately 20 million children are under 5 years of age, which means that about 440,000 children in the U.S. exceed BLLs of 10 μg/dL. Currently, the CDC states that, “Approximately 310,000 U.S. children aged 1–5 years have blood lead levels greater than the CDC recommended level of 10 μg of lead per deciliter of blood” (CDC, 2005
). But it has yet to establish a strategy for interventions.
No recent data allow us to specify the prevalence of children with BLLs greater than 5 μg/dL. Analysis of the NHANES III data (1988–1994) indicated that 25.6% of 1–5-year olds had BLLs at or above 5 μg/dL (Bernard and McGeehin, 2003
). African American and Mexican American children are more likely to exhibit elevated BLLs than non-Hispanic white children (). Children in homes built before 1946 exhibit a greater likelihood of elevated BLLs (). These data indicate that demographic and socioeconomic variables are important determinants of elevated BLLs. It is an inescapable conclusion that environmental justice questions are a significant issue for lead exposure.
Distribution of children (%) with blood lead levels greater or equal to 5 μg/dL
Beyond costs to the individual, elevated BLLs represent an economic drain on society as well. The direct and indirect costs to society of elevated BLLs were estimated to be $43.4 billion for one age group (Landrigan et al., 2002
). This calculation was based on an average BLL of 2.7 μg/dL for a cohort of children at 5 years of age, a loss of 0.25 IQ point for each 1 μg/dL of blood lead, and the relationship between IQ and lifetime earnings. Over a 20-year period (one generation), the loss amounts to $868 billion. In 1990, the U.S. EPA was asked by Congress to estimate the benefits of the Clean Air Act. , based on the Agency’s response, plots the number of IQ points that would have been lost from 1970 to 1990 had lead remained in gasoline. From this estimate, U.S. EPA calculated that the benefits of lead removal, based on IQ alone, translated into one trillion dollars.
Estimated losses in IQ if lead had remained in gasoline 1970–1990.
Direct costs for children with BLLs greater than 10 μg/dL were estimated in Mahoning County, Ohio (Stefanak et al., 2005
). They estimated “that lead poisoning costs local governments on the order of $0.5 million each year”. These calculations did not include the indirect costs to society of lowered IQ. They concluded that it was cost effective to invest in the reduction of childhood lead exposure. A study in Washington State estimated the total cost of lead exposure as $1.5 billion for 5-year old children in that state for one cohort (Davies, 2005
). Other investigators have found similar costs to society as well as the possible contributions to accelerated neurodegeneration associated with aging (Rice, 1998
). A more profound issue, discussed later, that is not monetized is society’s ethical responsibility for each individual’s loss of potential.
Elevated lead body burdens are also associated with antisocial behaviors such as an elevated risk for adjudicated delinquency (Needleman et al., 2002
), an endpoint not included in analyses focused on IQ scores. But the two criteria are intertwined, and raising IQ offers a number of documented benefits to society and the individual beyond earnings potential. Herrnstein and Murray, in their contentious book THE BELL CURVE: Intelligence and Class Structure in American Life (1996) calculated that a three-point rise in IQ (3%) results in a reduction of the following social indices amounting, on average, to about 20% each:
- males incarcerated in jail;
- reduced poverty rate;
- high school dropouts;
- children not living with parents;
- welfare recipient;
- out-of-wedlock children;
- low birth weight babies;
- bottom decile HOME scores;
- poverty during first 3 years.
Herrnstein and Murray never addressed the question of neurotoxic exposures in disadvantaged populations, the degree to which they contribute to social pathologies, or how their removal or reduction could elevate IQ scores. The list indicates that the calculations by Landrigan et al. (2002)
and U.S. EPA, because they fail to include the costs of social disruption as well as loss of earning power, are gross underestimates of how much wealth lead drains from the economy.
These calculations also underscore the principle that the societal effects of low-level lead exposure on IQ only become apparent when viewed from the standpoint of population-level effects (Weiss, 1988
). For an individual child, the consequences are difficult to discern given that small changes in IQ score occur from one test occasion to the next. Even a five-point IQ drop exerts a significant impact, however, when viewed from the perspective of a population. Assuming a mean IQ of 100 for a large population and a normal distribution, the tails of the curve represent those with superior IQ (greater than 130) and those with lower IQ (less than 70). IQs below 70 require significant societal support such as remedial education. A five-point drop in IQ would significantly change the number of people in the tails of this distribution. For example, in a population of 100 million with a mean IQ of 100 there would be 6 million people with IQs above 130 and an equivalent number with IQs below 70. A shift in the mean of 5 IQ points (5%) would result in only 2.4 million gifted people with IQs above 130 and 9.4 million people with IQs less than 70 who also require remedial assistance. The consequences to society will clearly be enormous ().
Furthermore, all populations are not equal. Disadvantaged populations begin with a handicap. A population with a mean IQ of 85, common among such communities, rather than 100, will suffer disproportionately when exposed to an agent that lowers IQ. (Weiss, 2000
) demonstrates the dramatic increase in the number of children with an IQ below 70 in a community with a mean IQ of 85 compared to a community with a mean IQ of 100 when the mean is reduced by as little as 1%. It is also instructive to consider the loss in high IQ children for a one to five point loss in IQ (). The number of children with an IQ less than 130 increases rapidly with a lowered mean IQ.
Fig. 4 The consequences of a one point (1%) drop in IQ depending on the mean population IQ (Weiss, 2000).
Effect of reductions in mean IQ on the proportion of scores in the superior range.
Disadvantaged populations also suffer from diminished educational opportunities, inflicted by their inability to support the costs of advanced schooling as well as by the reduced educational resources available to them as a result of skewed allocations. Ceci et al. (1997)
argued that cognitive ability (measured by IQ scores) and years of education should be seen as joint determinants of earning potential rather than in isolation. , based on Ceci et al. (1997)
, charts this interaction and offers an additional perspective on the combined influence of elevated lead exposures and educational deficits. Put another way, the adverse effects of lead are multiplied by the adverse effects of curtailed educational opportunities.
Fig. 6 Combined effects on weekly earnings (based on 1997 dollars) of cognitive ability (based on IQ) and educational attainment. Based on Ceci et al. (1997).
Finally, humans are not exposed to lead in isolation from environmental factors such as stress or from other developmental neurotoxicants. The social ecology governing a child’s environment can induce permanent changes in brain structure and function that almost certainly modify its vulnerability to toxic exposures (Weiss and Bellinger, 2006
). Indeed, animal studies indicate that maternal stress is one determinant of the effects of lead (Cory-Slechta et al., 2004
; Virgolini et al., 2005
). And, further, the infant and the fetus are exposed to a broth of chemicals from their in utero environment and their mothers’ breast milk that is poorly accounted for when assessing the effects of lead (Cory-Slechta, 2005