"The general rule of thumb is that there is no 'safe' level of lead when it comes to children," said Bryan L. Williams, associate professor of pediatrics at Le Bonheur Children's Medical Center in Memphis, Tenn. "Trace amounts of lead can be harmful to a child over time."
Such concerns have provoked massive recall of toys by Mattel, a leading US toy company and the suicide of a Chinese manufacturer whose supplies were declared tainted.
But many pediatric toxicology specialists noted that the recall should not cause undue panic among parents.
"Say the child has one of these toys and has played with it for four to five months," said LuAnn White, director of the Center for Applied Environmental Public Health at Tulane University in New Orleans.
"The actual dose of lead they receive is probably very low. They may have gotten a little lead from it, but if you look at lead poisoning, it's more of a chronic thing, a low level of exposure over a long period of time," she said. "If the parents did buy this toy and their children were playing with it, they would probably want to take it away, but they don't need to be unduly concerned that the child's lead levels pose a significant threat."
"Usually, at the levels lead is found in toys, a single bite or fleck of paint would not cause any problems," said Dr. John G. Benitez, associate professor of environmental medicine and pediatrics at the University of Rochester Medical Center in New York.
"Just like old lead paint chips from older homes, a single chip or a few chips would not hurt a child. It is the continual exposure over time that would cause elevated blood lead levels and their consequent health effects on the child," said Benitez.
Despite the fact that short-term exposure to the toys likely posed little health risk to children, the notion that any lead was present in the products at all is an issue of concern for doctors and parents alike.
This is partly due to the fact that consistent exposure, even to very small amounts of lead, can have major health impacts among them, a lowered IQ for children in the long run.
But researchers are still split over exactly how much lead must be present in the body for these health effects to be seen.
Though it is generally agreed that the clinical threshold for blood lead levels in children is 10 micrograms per deciliter, establishing a "danger level" for lead in a child's body is a daunting task, since body weight, metabolism, and overall development can vary dramatically from one kid to the next.
But, if one thing is clear, it is that lead exposure seems to have a much graver effect on younger children.
"Young children and infants are much more susceptible to the effects of lead on the developing brain than adults are," said David Eaton, associate vice provost for research at the University of Washington School of Public Health and Community Medicine. Eaton added that though he does not know how much lead the toys would have exposed the children to, "There obviously shouldn't be any lead in the paint on a child's toy."
"The lead levels found in plastics from China so far are extremely high by American standards, and could theoretically pose a risk of acute toxicity in a child," Williams said.
But while there is some debate over the lower threshold of lead poisoning, the symptoms of acute lead toxicity levels of lead in the blood that are seven or more times higher than the 10-microgram benchmark are noticeable and frightening.
"The symptoms of overt lead poisoning, which rarely occur today, includes seizures, vomiting, abdominal discomfort, imbalance and coma," said Bruce Lanphear, director of the Cincinnati Children's Environmental Health Center.
Such poisonings are most often seen when children, whose families live in older houses, ingest chips of peeling paint that contain high levels of lead.
The less acute but, nonetheless, devastating, mental effects seen with long-term exposure to lower levels of lead are often less readily apparent typically, because parents may not be aware that their children have high levels of the toxin until the damage has been done.
"For an individual child, it is difficult to measure the effects of low-level lead exposure," said Lanphear, who is currently the principal investigator for a study examining fetal and early childhood exposures to prevalent environmental neurotoxins, including lead.
"But, in large groups of children, there is considerable evidence that lead exposure, at these levels, is associated with deficits in reading, intellectual abilities and an increased risk for ADHD [attention deficit hyperactivity disorder]."
Dr. Adam Spanier, associate director of the Lead Clinic at Cincinnati Children's Hospital. said these deficits can even be seen in children with lower blood lead levels.
"Another recent study from Cincinnati Children's found that lead exposure defined as having a blood lead level greater than 2 micrograms per deciliter in 4- to 15-year-old children accounted for 20 percent of ADHD in U.S. children," he said.
While the toys involved in the Mattel recall are unlikely to harm children in this case, pediatricians said the episode could be a wake-up call for parents who otherwise would pay little attention to the risks lead exposure poses to their children.
"Parents should provide their children with age-appropriate toys, and supervise their children appropriately, to be sure that the child is not chewing on toys not intended to be put in the mouth," said Dr. Robert Geller, director of the Emory Southeast Pediatric Environmental Health Specialty Unit in Atlanta.
"Toys that appear to be poorly made, or that are starting to fall apart, should be avoided and taken away from the child," Geller said.
Spanier said some parents who bought the recalled toys may still want to consult their family physicians to ensure that their children's exposure was not severe.
"If a young child has been frequently playing with a recalled toy, or has put the toy in his or her mouth a lot, then the parent should discuss this with their child's physician," he said, adding that the physician may want to order a blood test to determine the child's lead level.