Exposure to lead during childhood may delay the onset of puberty in young girls, with higher doses increasing the chance for later maturation, according to a new study.

Lead, either alone or in concert with cadmium, might suppress the ovary's production of hormones that prepare a girl's body to ovulate, or release an egg, for the first time, researchers from the National Institutes of Health and other institutions said in a statement.

The researchers analyzed blood drawn from more than 700 girls ages 6 to 11. They found that girls with elevated levels of lead were 75 percent less likely than girls with low levels of lead to have key adolescent hormones at levels that are normally associated with the beginning of puberty. In girls with elevated levels of both lead and cadmium, this pattern was even more pronounced.

Previous studies have shown that exposure to heavy metals such as lead can disrupt normal hormone patterns or, in some cases, reproductive development, according to the researchers.

The Centers for Disease Control and Prevention advise treating people for lead exposure when their levels exceeding 10 micrograms of lead per deciliter of blood, but the study researchers said their findings suggest that lead exposure may have harmful effects at even lower levels. In the study, girls with half that amount were considered to have elevated levels.

"Our findings suggest childhood exposure to lead has worrisome effects as children age and reach adolescence," said study researcher Audra L. Gollenberg, a fellow at the NIH. "These issues are of concern in some parts of the United States as well as in countries where children are exposed to leaded gasoline, paint or industrial pollutants."

The most common sources of lead exposure are deteriorating lead-based paint, lead-contaminated dust and lead-contaminated residential soil, according to the Environmental Protection Agency. Breathing cigarette smoke is a principal source of cadmium exposure, according to the CDC.

The researchers used data on blood and urine samples taken as part of a survey conducted between 1988 and 1994. They compared levels of lead and cadmium to levels of the reproductive hormone inhibin B, which indicates the development of previously dormant egg cells in the ovaries. Inhibin B is known to increase steadily before the start of puberty.

For all age groups, the researchers found that girls with higher blood levels of lead had reduced levels of inhibin B, and so were less likely to reach the threshold level of inhibin B that indicates the onset of puberty.

In addition, the study found the pubertal delay associated with lead was more prevalent in girls with iron deficiencies. Girls with even moderate levels of lead and low iron levels were much less likely to have reached the inhibin B threshold levels indicating puberty than their counterparts with low lead exposure and normal iron levels.

"Iron deficiency appears to be a critical factor in the context of lead exposure," Gollenberg said. "Health care providers may wish to pay particular attention to the importance of screening for iron deficiency among girls at high risk for exposure to lead."

The findings are published in the journal Environmental Health Perspectives.