"In almost all situations where the ferritin is low, the blood lead level is significantly higher," said Bradman. "The difference becomes even more significant in more (lead) contaminated environments."

Overall, the authors found that iron-deficient children had an average blood lead level of 5.6 micrograms per deciliter, or 1 microgram per deciliter higher than children with normal blood levels of iron. But in highly contaminated environments, iron-deficient children had average blood lead levels that were 2.8 micrograms per deciliter higher than children with sufficient iron in their blood. Lead is considered toxic at blood levels above 10 micrograms per deciliter.

Iron is a component of hemoglobin, a protein that helps the red blood cells carry oxygen throughout the body. Iron levels that drop low enough can lead to anemia, a condition where not enough red blood cells are produced. Symptoms of iron deficiency include decreased physical activity, inattentiveness, and delays in cognitive development.

One theory for the association between iron and lead levels in the blood comes from the fact that the two are biochemically similar. Many symptoms of severe iron deficiency even mimic those of lead poisoning. The absence of iron creates a nutrient deficit in the body, which responds by grabbing more of the lead that is ingested by the child, or hanging onto the lead more strongly once it is in the body, explained Bradman.