PCBs: measuring the danger. (Technical Briefs).

Health Effects--The Known and the Unknown

Polychlorinated biphenyls (PCBs) are known to cause numerous adverse health effects in animals. Some of the human studies, however, have been inconclusive. They have failed to show a link between PCB exposure and diseases, largely because epidemiologic studies have limitations that affect their ability to find health effects. Those limitations include the small numbers of persons often involved in studies (making it hard to discern a pattern), difficulty in determining actual exposure levels, and multiple confounding factors (e.g., smoking, drinking, and exposure to other contaminants). The inconclusiveness of such studies does not mean that PCBs are safe. In fact, other studies in humans suggest that PCBs have both carcinogenic and noncarcinogenic health effects. Therefore, the U.S. Environmental Protection Agency (U.S. EPA) classifies PCBs as probable human carcinogens of medium carcinogenic hazard.

PCBs have a long life and tend to accumulate in humans. All people in the industrialized world have PCBs in their bodies. People are exposed primarily through ingestion of contaminated foods--especially fish, meat, and poultry--and through inhalation. Studies show, however, that the levels of PCBs in humans have been decreasing steadily over the years.

The composition of PCB mixtures changes when they are released into the environment. The most carcinogenic components of PCB mixtures bio-accumulate in fish and other animals and bind to sediments. In fact, the buildup of PCBs in fish can reach levels thousands of times higher than the level of PCBs in water. This means that eating fish or other animal products contaminated with PCBs or coming into contact with PCB-contaminated sediments is more harmful than being exposed to PCBs through occupational exposure or through PCB releases into the environment.

Acute oral exposure to PCBs is associated in animals with damage to the liver, the kidneys, and the central nervous system. Chronic oral exposure is associated in humans with cardiovascular effects (hypertension), mild liver effects, and skin effects (acne and pigmentation). Chronic human exposure through inhalation produces respiratory-tract symptoms (cough and tightness in the chest); gastrointestinal effects (nausea, vomiting, abdominal pain, weight loss, and anorexia); mild liver effects; skin rashes; and eye irritation.

Numerous health studies have focused on the carcinogenic effects of PCBs and have concluded that PCBs cause liver cancer in animals. Studies of workers occupationally exposed to PCBs lead to the conclusion that PCBs are probable human carcinogens.

In animals, PCBs also cause a variety of noncancerous effects in the immune, central nervous, endocrine, and reproductive systems, and humans could likewise be affected. Following is a synopsis of the noncancerous health effects of PCBs:

* Immune effects--Studies of animals have shown that exposure to PCBs has significant effects on the immune system, including decreases in the size of the thymus gland and reductions in resistance to Epstein-Barr virus and other infections. Studies of humans also have found that exposure to PCBs affects the immune system.

* Neurologic effects--Animal studies have found that PCB exposure in newborn monkeys negatively affects visual recognition, short-term memory, and cognitive skills. Studies of humans suggest similar effects, including learning deficits among children.

* Endocrine effects--PCBs decrease thyroid hormone levels in both animals and humans. Thyroid hormones are critical to growth and development.

* Reproductive effects--Animals exposed to PCBs suffer serious and long-lasting reproductive effects, such as longer menstruation and decreases in fertility, conception, and sperm rates. Women occupationally exposed to PCBs and women eating PCB-contaminated fish had shorter gestational periods, and their newborns had lower birth weights. Furthermore, PCBs can reach a fetus through the placenta and can be transferred to a newborn via breast milk. Studies of the effects PCBs have on the male reproductive system remain inconclusive. One study of men occupationally exposed to PCBs indicated no fertility problems, while another showed an association between low sperm count and high levels of PCBs in the blood.

* Other effects--A number of other health effects have been reported, including dermal and ocular effects in monkeys and humans, and liver toxicity in rodents. Humans exposed to PCBs also experience high blood pressure and elevated levels of serum triglycerides and serum cholesterol.

Children and PCBs

Major sources of PCB exposure in children include contaminated food, contaminated water, and contaminated air near some hazardous waste sites or in buildings with old electrical appliances that use PCBs. PCBs accumulate in pregnant women's bodies and are released during pregnancy, passing through the placenta and thus exposing fetuses. Infants may also be exposed through nursing; because PCBs dissolve in fat, they can accumulate in the mother's breast milk.

Adverse effects in infants and children include

* low birth weight;

* shorter gestational period;

* smaller head size;

* abnormal neurodevelopment (e.g., abnormal reflexes, motor immaturity permanent learning disabilities, mental retardation, impaired cognitive skills, problems with memory and depressed responsiveness);

* neurobehavioral deficits (e.g., increased hyperactivity);

* immunologic effects (e.g., less wheezing, fewer allergic reactions, higher prevalence of recurrent middle-ear infections, and higher prevalence of chicken pox);

* problems with thyroid function;

* facial abnormalities; and

* birth defects.

Measurement of Behavioral Effects in Infants and Children

The following tests measure behavioral effects in infants and children:

* The Fagan test shows an infant or child two identical photos of human faces for about 20 seconds, then replaces one of the photos with a new one and presents the new combination of photos. Normal babies spend more time looking at the photo of the new face. Children with memory problems cannot remember the first two photos well enough to recognize that the second two photos are different.

* The Bayley test is a standardized test gauging the development of small children (one month to 3.5 years of age). It includes a mental-development index (MDI) and a psychomotor-development index (PDI). The MDI measures memory, the ability to solve simple problems, and language capabilities. The PDI measures body control, coordination, and fine motor movement. Both indexes are scaled like a standard IQ test.

* The Wechsler Intelligence Scale for Children--Revised measures verbal and nonverbal performance skills.

* The Neonatal Behavioral Assessment Scale measures behavioral development. The scale evaluates 28 behavioral and 18 reflex items (e.g., reactions to a variety of stimuli, such as a light in the eyes, a rattle, or a moving ball).

(Adapted from Hazardous Substances and Public Health, Vol. 11, No. 2, 2001, a publication of the Agency for Toxic Substances and Disease Registry, in the public domain.)