Parents around the country have voiced alarm since the 2015 revelations of staggeringly high lead levels in the drinking water of Flint, Mich., and more recent reports of spikes in other cities and states. It is easy to understand why people are upset. “Children are particularly vulnerable to the neurotoxic effects of lead,” states the World Health Organization on its Web site. It goes on: “Even relatively low levels of exposure can cause serious and in some cases irreversible neurological damage.” After learning that Flint's water was tainted, some medical professionals have said the lead would permanently harm young brains and possibly other organs. Personal injury attorneys, pressing lawsuits, use phrases such as “irreparable damage” and “lead poisoning.”
Yet the Flint children and others like them are not doomed. The WHO statement uses words like “relatively” and “can,” but it does not say damage is certain. Water lead levels in Flint were extremely high, and blood lead levels did rise after the exposure. But experts involved in setting lead-exposure standards say the blood levels linked to Flint—thought to have peaked between five and 10 micrograms per deciliter (μg/dL) in most cases, although exact numbers have not been made public—generally do not result in permanent neurological deficits. There is even evidence that an anatomical feature called the blood-brain barrier may keep low levels of lead from entering brain cells.
Such insights, scientists emphasize, are no excuse for inaction. To the contrary, researchers argue that public officials must not only eliminate lead sources but improve nutrition and education because studies indicate these measures could reduce lasting ill effects of environmental insults. Still, the common belief that children with low blood lead levels are poisoned or mentally deficient is not only untrue, it can result in hurtful stigma and stress for children and their families. “Poisoning is obviously a loaded word,” says Marc Edwards, the Virginia Polytechnic Institute environmental engineer who documented the shocking levels of lead in the Flint water supply. “I've spoken to many parents in Flint, and I'm concerned because I don't want children there defined by what happened to them.”
Much of the worry over lead in Flint stems from the discovery that between 2013 and 2015 the percentage of children with blood lead levels at or above 5 μg/dL doubled, rising from 2.4 to 4.9 percent. Alert parents, scientists and advocates forced public officials to stop denying the problem and remove the cause: water from the Flint River so corrosive that it leached lead from aged city water pipes.
What does a blood level of 5 μg/dL actually mean? In 2012 neuropsychologist Kim Dietrich of the University of Cincinnati College of Medicine helped the Centers for Disease Control and Prevention set that level as a threshold for official action. He explains that the number was based on what public health experts call the “precautionary principle”—the idea that when an activity raises threats of harm, we should take measures to stop it, even if some cause-and-effect relations are not fully established scientifically. “This does not mean that children at this level are poisoned,” Dietrich says. “There are very few studies of low-level lead exposure, but there is nothing in the data that suggests that children will have negative impacts of short-term low-level exposure” over their lives. In fact, he notes, the 5 μg/dL figure was set because 97.5 percent of young children fall below it, not because blood lead levels at that threshold result in permanent harm.
Although some studies have found an association between low blood lead levels and cognitive deficits, none has established a causative link. Robert Fischer, a social scientist at Case Western Reserve University, is an expert on study evaluation who has monitored the lead problem for decades. He says these studies have been repeatedly confounded by other factors in the environment that affect cognitive performance. “Lead exposure also correlates with extreme poverty, low resource availability and poor schools,” he says, noting that there are so many intertwined factors that it is nearly impossible to tease them apart. An exhaustive study of children with blood lead levels averaging more than 17 μg/dL, published in 2013 in NeuroToxicology, concluded: “It is unclear whether lead exposure or early childhood confounders were driving these associations” between lead and long-term cognitive impacts. Norman Paradis, a physician who studies clinical trial design at Dartmouth-Hitchcock Medical Center in New Hampshire, adds that at low lead levels it is difficult to get a reliable statistical signal with so many other variables in play.
How different levels of lead in the blood affect the brain may also depend on the effectiveness of the blood-brain barrier, a network of specially lined blood vessels that blocks many toxic substances from entering the brain. Prolonged exposure to lead at high levels—probably well in excess of 5 or 10 μg/dL—does subvert this system. But for the blood levels seen in Flint, it is not clear that the same can be said, according to endocrinologist William Pardridge. Author of five books and more than 400 journal articles on the blood-brain barrier, Pardridge is a distinguished professor emeritus of medicine and member of the blood-brain barrier research laboratory at the University of California, Los Angeles. He says that most lead in the blood is carried within red blood cells and that red blood cells do not cross the barrier. Thus, there is little lead available for transport from blood to brain, and it comes from blood plasma, a much smaller source. Indeed, a 1993 study in NeuroToxicology found that although severe lead poisoning in rat pups and in young children may damage the blood-brain barrier, “there is little evidence that there is either damage or even disturbance” when levels are below 80 μg/dl. At lower thresholds, lead may find its way beyond the barrier but most likely in only a small fraction of the concentration found in the blood.
Still, what worries Mona Hanna-Attisha, a pediatrician and director of the Pediatric Public Health Initiative at Hurley Children's Hospital in Flint, is that any cognitive deficits associated with lead exposure—at whatever level—seem to be made worse by poverty, and poverty is rampant in Flint. “No one is saying that these children are all going to have problems. Most should be fine,” she wrote me in an e-mail. “But we are not going to wait to see who is fine and not fine. We are trying to build robust wraparound services in nutrition, education and health to mitigate the potential impact of this exposure.”
Howard Hu, a physician and dean of the Dalla Lana School of Public Health at the University of Toronto who has published widely on lead's effects, agrees that the interlaced issues of lead exposure and social inequality point to strategies for intervention. “Certainly some children are more susceptible to lead than others—we are just beginning to look at this variance,” he says. But a reason for hope, he adds, is that “low-level lead exposure can be mitigated by good parenting, good schooling and good nutrition.”
Hu is among several scientists who have found that in children, nutritional deficiencies in iron, calcium or zinc increase the danger of lead exposure by encouraging lead absorption. Making certain that children have sufficient amounts of these essential nutrients can reduce this risk, he says. But diet is only one part of a very complex problem. A lack of mental stimulation—as can occur when children get little adult attention and when schools lack resources—also appears to exacerbate lead's effects, although again scientists are not sure how much to blame on lead and how much to blame on the surroundings.
Although there are no controlled human studies, research in animals suggests that an arousing environment might begin to compensate for lead-induced brain damage. Bruce Lanphear, a public health expert at Simon Fraser University in British Columbia, is principal investigator for a study examining fetal and early-age exposure to lead and other neurotoxic chemicals. “Studies in rats have shown that the effects of lead exposure can be attenuated by environmental stimulation,” Lanphear says. Early research showed that animals exposed to lead yet provided with enriched environments (cages with other rats as well as water mazes, exercise wheels and other stimuli) showed fewer deficits than did those from deprived situations.
The best approach is to minimize lead exposure while improving surroundings, says Stuart Shalat, director of the division of environmental health at Georgia State University's School of Public Health. Shalat says that poor children are both most likely to be exposed to lead—from factory smelters, dust, soil and paint—and to suffer the worst effects of that exposure because of inadequate access to health care, proper nutrition and high-quality schools. “There should be a sense of urgency to evaluate and minimize exposure,” he says. But when kids do meet lead, “it is increasingly clear that some of the toxic damage can be mitigated by commonsense practices. What we need to focus on is doing everything we can to see that every child has the opportunity to develop his or her potential.”