Lower IQs found in 9-year-olds whose mothers were exposed to PCBs while pregnant.

Oct 27, 2008

Stewart, PW, E Lonky, J Reihman, J Pagano, BB Gump and T Darvill. The relationship between prenatal PCB exposure and intelligence (IQ) in 9-year-old children. Environmental Health Perspectives 116:1416–1422.

Synopsis by Martha Susiarjo and Wendy Hessler

2008 10-24 fish on platter

Prenatal exposure to polychlorinated biphenyls (PCBs) may lower a child's intelligence at age nine by several points, a study from Oswego, New York, finds. The pre-teen children whose mothers were highly exposed to PCBs during pregnancy scored lower in general and verbal IQ tests than those born to mothers who were mildly exposed. The findings support other human studies that suggest exposure to these persistent chemicals disrupts early childhood mental development.


PCBs are a group of synthetic chemicals used as lubricants and coolants in electrical equipment. Their use has been banned in the US since 1977, but the chemicals still widely persist in the environment due to their resistance to break down. There are 219 varieties of PCBs depending on the number of chlorine atoms in each compound.

The pollutants affect brain development and function and are associated with lowered IQ, motor control and attention. Cell and animal studies suggest PCBs most likely interfere with thyroid hormones that are necessary for proper brain growth (Fritsche et al. 2005). Rats fed PCB-laden fish from Lake Ontario had higher levels of PCBs - especially the more chlorinated varieties- in their brains and were more likely to quit tasks sooner in tests that offered few rewards (Stewart et al. 2000).

A major human source of PCB is fish from contaminated areas, such as the Great Lakes region of the United States. PCBs accumulate in fat and travel up the food chain concentrating in fatty tissue of top-level predators. A recent US Environmental Protection Agency survey looked for 159 PCB types in fish tissue from 486 different lakes across the continental US. The chemicals were found in all the lakes but one, and all but one PCB type were measured (US EPA 2007). In another study of PCBs in Lake Michigan, the highest levels of total PCB were found in its top predator, the lake trout (US EPA 2004).

Fish advisories warn against eating contaminated fish from certain lakes, rivers and streams. To reduce exposure, experts recommend not eating certain fish species and cooking the fish you do eat in ways that minimize PCB levels.

The Intelligence's quotient (IQ) is a measure of a person's ability to think and reason. The most widely used test, Wechsler Intelligence Scale for Children (WISC-III), measures a child's overall intelligence - called full scale IQ, and verbal and nonverbal aspects of mental function. A score of 100 on the three combined tests is considered average while scores less than 100 signify below average and those larger are considered above average. The test scores are generally used as indicators to predict success in school and to diagnose brain disorders, learning disabilities or attention/behavior disorders.

Prenatal exposure to some chemicals during this fast paced developmental period can cause permanent changes in how cells work. Chemicals can alter the actions of hormones that control growth and development. They can also affect genes by changing how and when they are turned on and off through life. Some of these changes can lead to diseases, like cancer, as the body ages.

What did they do?

This paper is the first to report chemical data from the tissues sampled in the Oswego Study, a long-term project meant to follow women and children's health in the Lake Ontario basin in the Northeastern United States. Stewart et al. aimed to replicate in a different Great Lakes population earlier findings from a 12-year study that examined links between chemicals and health in women and children living near Lake Michigan (Jacobson and Jacobson 1996).

In this paper, they examine the relationship between a mother's PCB exposure during pregnancy and IQ test scores in the children when they are nine years old.

Between 1991 and 1994, placental tissue and cord blood samples were collected at birth from mother-children pairs living in Oswego, New York. The tissues and blood, which were frozen for 10 years and then analyzed for PCBs, DDE, HCB, mirex and MeHg, represented the chemical exposure during the pregnancy. Based on their placental PCBs levels, the children were divided into five exposure levels, ranging from lowest (5th percentile) to highest (95th percentile).

A total of 156 children - mostly Caucasian and an equal ratio of boys and girls - were included in the study. At nine years of age, the children's mental abilities were measured through standardized IQ tests that assessed their full scale IQ, verbal IQ, performance IQ and distractibility – the latter is linked to Attention Deficit Hyperactivity Disorder. Verbal IQ measures comprehension, comparison and understanding; all factors in long-term memory and language. Performance IQ measures the ability to discern pictures, objects, designs and sequences. Distractability measures how difficult it is to acquire and remember, which are necessary for concentration, attention and memory.

Fifty-two other factors and chemicals, such as mercury, that may influence intelligence and bias the data were statistically controlled. Some of these included maternal influence (intelligence, smoking history, marital status, etc), child factors (birth weight/height, birth head circumference, etc) and socioeconomic status.

Statistical analysis determined if total levels and individual levels of four PCBs predicted IQ scores in the children. In other words, they asked if children with higher PCB exposure had lower IQ scores.

What did they find?

The total level of PCBs measured in the placentas was significantly associated with lower general intelligence, verbal scores and higher levels of distraction. This means that as PCB levels in the placenta rose, certain IQ scores and attention span measures decreased. Two different analyses yielded very similar results, which adds validity to the findings.

PCBs in the placental tissue ranged from 0.54 ng/g (5th percentile) to 3.21 ng/g (95th percentile). Full scale IQ scores ranged from 62 to 135.

The children with higher placental PCB levels performed worse on three of the IQ tests than children with lower PCB exposures. For each 1 ng/g increase in PCBs level, test scores dropped by 3, 4 and 4 points in full scale IQ, verbal IQ and freedom from distractibility, respectively. The difference between the most and least exposed groups could be as high as 7 and 9 points in the full scale and verbal IQs.

Four PCBs were analyzed individually. Although none was statistically associated with IQ, those compounds with the most chlorine atoms had the highest impact on lowering IQ scores.

As in other studies, the PCBs in cord blood, whether analyzed as total or individuals, were not related to IQ scores and cannot be used to predict IQ from prenatal exposures.

Results of this study were still statistically significant even when other potential confounding variables were taken into account. However, mercury levels were found to be associated with poorer performance on freedom from distraction, but not on full scale and verbal IQs.

What does it mean?

Children exposed to PCBs during gestation have an increased risk of not developing to their full cognitive potentials. Prebirth exposure to the chlorinated compounds lowered by a few points the general IQ, verbal capacities and attention of the school aged children.

Even these seemingly slight reductions in intelligence can affect lifelong ability to learn, perform and be productive adults. Lower intelligence also affects society in terms of economics, health care and educational costs. A drop of even one IQ point lowers worker productivity and affects life-long earnings potential (Grosse et al. 2002). These subtle differences are often hard to detect through testing but may affect a generation of children. For the individual and society, this could translate into lower quality of life, lost future income, higher lifetime health care costs and expanded special education needs.

Two other similar studies give conflicting results. One found a relationship between exposure and cognitive behavior in 11-year-old children who live in the Great Lakes region (Jacobson and Jacobson 1996). Another used PCB exposure in the general population and found no links between PCB and intelligence (Gray et al. 2005).

Stewart et al.'s findings, which support the previous Great Lakes study, but at PCB levels almost half the levels as those reported then, suggest that geography plays a role in how exposures affect mental development. More highly chlorinated types and mixes of PCBs pollute this region than other areas. The fish in the Great Lakes region are heavily contaminated with these particularly harmful varieties of persistent chemicals. Thus, children from this region may be exposed to a more harmful mix and possibly higher "pulse" levels during the time when their mothers ate fish during pregnancy. These factors could increase prenatal exposure in this regional population when compared to children who live in other parts of the country.

Although the authors found a strong association that held in two different statistical analyses, the lower IQ observed in PCB-exposed children may not be directly caused by the exposure. An unknown factor(s) associated with the exposure may be responsible. Stewart et al. acknowledges that “establishing causality in non-experimental studies is incredibly difficult.”

Among the study's strengths was finding a significant association that was independent of 50 other potential confounding factors. Further, the results followed a similar pattern as found in previous PCB-exposed children in the Lake Michigan area.

Even though the study cannot prove a direct and causal relationship, it still suggests lower intelligence performance in children with higher prenatal PCB exposure level. The study, combined with previous findings, suggests that although banned three decades ago, PCBs continue to contaminate the environment and influence human health and welfare. The strong association between exposure and reduced intelligence is a troublesome issue.



California's Office of Environmental Health and Assessment. PCBs in sport fish: Answers to questions on health effects.

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