The floods that inundated New Orleans five years ago during Hurricane Katrina may have left behind a small silver lining for some families.
The fresh layer of sediment deposited by the floods buried contaminated soils and led to a drop in soil lead levels in some heavily contaminated neighborhoods. That led to a dramatic drop in the blood lead levels of children living there, according to a recent study published in Environmental Science and Technology.
Howard Mielke of Tulane University in New Orleans, and colleagues, was studying the relationship between lead levels in the soil and in children's blood in New Orleans when Katrina hit. After the hurricane, they went back to survey many of the sites and combined the findings with post-hurricane blood samples of more than 2,000 children.
"We looked at the combination of soil lead and blood lead before and after Katrina and what we noticed is that where there was a reduction that took place in soil lead, we expected there would be a reduction in blood lead, and sure enough that's what showed up," Mielke said.
"When kids play outside they are actually putting their hands in their mouths and picking up a lot more soil and a lot more lead dust than we realize," he explained.
Lead entered urban soil from lead-based paint, especially from old exterior paint scraped off of houses, and from leaded gasoline, which was phased out of use in the U.S. beginning in the 1970s.
Soil lead levels in New Orleans declined by an average of nearly 46 percent after Hurricane Katrina, the researchers found, with only six of 46 sampled census tracts exceeding Environmental Protection Agency limits of 400 parts per million of lead in soil post-Katrina, compared to 15 of 46 before.
Meanwhile, the median blood lead level in children dropped an average of 32 percent.
For households where soil lead dropped by more than 50 percent, the average blood level decline was 1.66 micrograms per deciliter. The Centers for Disease Control advised action level is 10 micrograms per deciliter, but studies have found measurable effects of lead in children down to two micrograms per deciliter.
Elevated blood lead levels in children are associated with a long list of behavioral problems, including aggression, attention deficit hyperactivity disorder, and antisocial behavior. High levels of childhood lead exposure increase the likelihood of being convicted of violent crime as an adult and may cause long-term changes to parts of the brain involved in judgement and problem-solving.
"It's a sort of a natural experiment and it really convincingly shows that certainly soil lead is an important contributor to children's blood lead levels," said Kim Dietrich of the University of Cincinnati College of Medicine, who has studied lead levels in children in the Cincinnati area for decades.
"Most research has focused on lead paint residue in the interiors of homes. And indeed it is a very important contributor," he said. "But clearly soil plays a role and this study cleverly, creatively demonstrates that in a convincing way. "
The fact that many families were evacuated after the hurricane -- presumably into less lead-laden housing -- could confound the results, but they would need to have left for a year or more, at least, to explain the observed drop, according to statistical analysis by team member Sammy Zahran of Colorado State University.
"What this study does is demonstrate at least that the possibility exists to reduce children's blood levels by addressing the soil," Meilke said. "We have to now try to improve on that model by doing more work on childcare centers and play areas around homes."
"This is not only a problem for New Orleans," he added. "This is a national problem. It's a global problem."