Several neurotoxic chemicals can disrupt brain development, which contributes to neurodevelopmental and psychiatric disorders—including ADHD. Lead is among the most studied neurotoxicants relevant to mental disorders. Because lead is stable and inert, the total amount on earth never changes. Over the past 6000 years, people have mined about 300 million tons of lead; some 150 million tons are still dispersed in the environment in one form or another.
Most exposure in children in the US (about 70%) occurs through lead paint in older houses, schools, and other buildings; or in surrounding soil and dust, which has accumulated and bound lead over the decades from airborne pollution. Other sources of exposure include water (leaching from lead in pipes, as in the recent Flint water crisis); and lead in imported toys, jewelry, candles, canned foods, candy wrappers, cosmetics, and poorly regulated dietary supplements. For those who live near airports, air pollution is a further source—airplane gasoline is still leaded. Pica is an obvious potential contributor when present in the child or mother.
Lead’s effects on childhood IQ, ADHD, and conduct problems as well as physical health have been of concern for decades. We now know a great deal about how lead affects the brain, including disruption of signaling in the prefrontal cortex and striatum.1
During the evolutionary period, from one million years ago until 10,000 years ago, humans may have had a typical blood lead level of about 0.01 to 0.02 µg/dL, based on limited fossil data. This may be the best estimate of a “normal” level of lead for humans. Lead is fatal to children at 100 to 150 µg/dL—or 10,000 times the “normal” level. During the 19th and 20th century, the levels among American children spiked to around 30 to 50 µg/dL on average, with thousands of children killed and many others sickened and permanently injured by lead poisoning. This was the direct result of unrestrained commercial and industrial use of lead in gasoline, house paint, and other products.1
Lead’s poisonous properties were already known in antiquity. However, harm to children in modern times as a result of routine exposure to commercial products was first documented in the 1890s.2 By the 1920s lead’s harmful effect on child development was medically established, and several nations had begun to restrict or phase out lead paint. In parallel with subsequent industry activity related to health and societal threats from cigarettes, asbestos, aerosols and ozone, and climate change, industry groups and politicians in the US resisted restrictions on lead use in paint and other consumer products—ignoring or distorting the science—for much of the 20th century.3
Lead use was finally restricted in the US in the 1970s and phased out of gasoline and paint by 1986, which reduced the average lead level among children to about 1.0 µg/dL by the 2000s. This improved level, however, is still about 100 times the “normal” background level if the prehistoric fossil studies are close to accurate. At this level, acute toxicity is not observed. Instead, there are subtle effects on IQ and attention caused by alterations in neurodevelopment.
Crucially for clinical reflection, this average still masks wide variation. Low income and racial and ethnic minority children can have levels much higher than average. Children in nations that do not regulate lead have exposure and blood lead levels higher than in the US as well.
Association with ADHD
The correlational association of lead with conduct problems, IQ, and ADHD is well established. Goodlad and colleagues4 concluded from their comprehensive meta-analysis that the association of lead with symptoms of inattention was r = .03 to .25 with a point estimate of r = .16. This effect holds even at low, previously safe levels. While this is a small statistical effect, small effects have large public health consequences when exposures are widespread. The effect on ADHD and IQ results in part from lead’s disruption of executive functions.
While many studies in the literature and the meta-analysis by Goodlad and colleagues assayed lead levels that were higher than are now common among the US population, several studies using varying methodology from 2005 to 2015 confirmed that blood lead level was associated with ADHD even at levels in the 0.5 to 3.0 µg/dL range, after control for many covariates.5-9 If there exists a “safe” level of lead for children, it is below the detection limit of the best mass spectroscopy instruments.
Dr. Nigg is Director, Division of Psychology, and Professor of Psychiatry, Pediatrics, and Behavioral Neuroscience, Department of Psychiatry, Oregon Health and Science University, Portland, OR. He reports that this work was supported by MH R37-59105.
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