By Gabe Rivin
Pregnant women in Durham County have shown high levels of cadmium in their blood, a finding that researchers linked to low birth weights among those women’s children.
The findings, laid out in a new study from researchers at UNC-Chapel Hill and the University of Michigan, also found that nonsmokers tested high for cadmium, a toxic heavy metal that’s found in cigarettes.
The study builds on an earlier analysis of the same data that found cadmium in those women’s blood, but which did not examine any potential health effects of the exposure.
The findings are significant in that underweight babies are at risk of a number of health problems and sicknesses in the first days of their lives, according to the Centers for Disease Control and Prevention. Underweight babies are also at risk of longer-term health problems, such as delayed motor development and learning disabilities, the CDC says.
Low birth weight is also a leading contributor to infant mortality rates in the U.S., which trail those of other developed countries.
The study, published in October in the journal PLOS ONE, tracked a cohort of 1,027 pregnant women in Durham County from 2005 to 2010. Researchers compared blood samples from the women against their electronic medical records, which recorded their children’s birth weight.
Women with the highest blood levels of cadmium, the researchers found, were 71 percent more likely to give birth to babies with low birth weights than women with the lowest blood levels of cadmium.
The high-exposure group was defined as having at least 0.5 micrograms of cadmium per liter of blood, while the low-exposure group’s figure was at most 0.28. The national average for adults is 0.32 micrograms of cadmium per liter of blood.
The study defined babies with low birth weights as those weighing less than 2,500 grams, or about five and a half pounds.
Rebecca Fry, a UNC professor and one of the coauthors of the study, said that cigarettes were partially to blame for the high levels of cadmium in the women’s blood, since about 25 percent of the study participants were smokers.
“We know that cigarette smoke contributes to these metals levels,” she said.
But Fry said the study raised questions about cadmium exposures among nonsmokers, who made up a majority of the high-exposure group.
“One of the things we found was, the relationship between babies being born at lower birth weight was even occurring in those women who are not current, active smokers.”
A toxic heavy metal
Cadmium is hardly a new concern for health researchers. For years, researchers have shown that the heavy metal can cause a variety of health problems.
Cadmium is known to cause lung cancer and kidney damage. It can be fatal if inhaled at large enough levels. And according to some studies, the metal is linked to prostate cancer in men.
Many of those ailments have been concerns among workers in industrial plants, such as refrigeration compressor plants and battery plants.
But, as the new study found, cadmium isn’t just a concern for industrial workers – or smokers, for that matter.
In the study, smokers accounted for 156 of the women with high levels of cadmium in their blood. Yet that number was even higher for nonsmokers. The study tallied 172 women, or about 17 percent of the total cohort, who were nonsmokers but whose blood levels of cadmium were high.
Fry said that it’s unclear why nonsmokers had high cadmium levels, but she noted that the women could be exposed to it through the food they eat. The U.S. Environmental Protection Agency says that people can be exposed to cadmium from food that’s grown with phosphate fertilizers or sewage sludge.
The EPA also says that people can inhale airborne cadmium, which is emitted from coal- and oil-fired power plants, in addition to municipal plants that burn waste.
But one thing is certain: Cadmium is finding its way into women’s uteruses, and it’s worrying health researchers who study heavy metals’ effects on fetuses.
Early and longer exposure poses risks, even at low levels
Researchers haven’t always been so concerned about low levels of heavy metals in humans’ blood. Older academic literature was more focused on industrial accidents and areas in the world where people were highly exposed to metals, according to Carmen Marsit, a professor in Dartmouth College’s school of medicine.
But public-health researchers have increasingly turned their attention to the effects of long-term, low-level exposures. “We might be seeing the same kinds of effects, maybe to a lesser extent,” he said.
Marsit said that this is particularly true for the study of fetal health, a relatively new area for health research. “We’re realizing that that’s such a critical and such a risk period for when these things might have an effect,” he said.
Cells rapidly differentiate while babies are in the womb, he said, and if the normal development process is disrupted, babies can suffer from long-term health problems.
Marsit said that heavy metals like cadmium could harm fetuses in a number of ways. In some cases, they can replace iron in fetuses’ red blood cells, and in other cases they can alter fetuses’ DNA.
As for cadmium and birth weights, the heavy metal could disrupt the transfer of zinc to the fetus, retarding its growth, according to the study.
Still, according to Marsit, researchers are uncertain about some of the exact physiological mechanisms that allow for this harm. That’s especially true, he said, when researchers consider how heavy metals affect fetuses’ neurological development.
One variable to control
The new study doesn’t offer definitive answers about the nonsmokers who have high blood levels of cadmium. But for the roughly 25 percent of pregnant women who did smoke, the source of the heavy metal is easier to pin down.
Tobacco, in fact, can be a potent source of cadmium. One cigarette alone, according to the study, can increase cadmium by 0.1 to 0.2 micrograms per liter of blood.
And smoking’s harms aren’t limited to low birth weights. Smoking increases the risk of cleft lips, miscarriages and sudden infant death syndrome, according to the CDC.
A former state official, when interviewed about Fry’s and others’ previous findings about high blood levels of cadmium, said the state doesn’t have an immediate plan to take action. But one of the state’s highest priorities for prenatal care is to prevent smoking or to help women quit smoking, said Kevin Ryan, who recently stepped down as the head of the women’s and children’s health section in the N.C. Department of Health and Human Services.
Local health departments offer prenatal counseling that encourages pregnant women to quit smoking. That’s also true for clinics that offer family-planning services.
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