There is growing concern that early exposure to POPs impairs neurological development and can lead to developmental deficiencies and learning disabilities in the young.

The issue of exposure to contaminants in sport fish is one of balancing both risks and benefits to health.

Exposure to POPs such as PCBs comes mainly from diet, particularly consumption of breast milk, fish, fatty meats and dairy items.

Persistent Organic Pollutants

Although the neurotoxic effects from heavy metals such as lead and methylmercury have been well characterized, fewer data exist on the potential of neurotoxic effects from other environmental contaminants. There is, however, growing concern, based on epidemiological and laboratory data, that early exposure to compounds known as persistent organic pollutants (POPs) impairs neurological development and can lead to developmental deficiencies and learning disabilities in the young.32,33

What Are POPs?

POPs are a class of contaminants that includes many industrial chemicals and some pesticides. These chemicals are of major health significance because they are not easily degraded and therefore remain in the environment for a long time (hence the term persistent). They are not water-soluble but are soluble in fat (i.e., they are lipophilic), so they become stored in fatty tissues of organisms that ingest them. Because they persist and are stored in fat, they become concentrated in organisms at increasingly higher levels of the food chain (i.e., these chemicals biomagnify).

Specific industrial chemicals that are persistent organic pollutants include PCBs, dioxins and furans. Pesticides of the organochlorine type, such as DDT, toxaphene, hexachlorobenzene and lindane among others, also represent POPs. Although most are not currently in use in North America, they continue to be measured in the global environment because of their chemical stability and persistence and because they are still used in several developing countries. Until recently, lindane was the active ingredient found in common treatments for lice.

Humans and other mammals efficiently absorb POPs, and since they remain virtually unmetabolized and are only minimally excreted, they are stored in fatty tissues throughout the body.34 The only normal route of elimination is by liberation from fat stores and excretion during lactation.

How Are Children Exposed to POPs?

The fetus can be exposed by the mother’s exposures during pregnancy; however, these persistent fat-soluble compounds, such as PCBs, DDT and dioxins, may also be mobilized from maternal fat stores (i.e., representing preconceptional exposures) and can cross the placenta to reach the developing fetus.

The other concern regarding POPs is that, when maternal fat stores are mobilized to supply fat for lactation, breast milk will then transfer those contaminants to the breastfeeding infant. Depending on the parity and age of the mother, the breastfed infant may then receive close to an adult body burden at the earliest stage of life. Health Canada estimates for the average total daily intake of various POPs from all media indicate that adolescents, children and especially breastfed infants have relatively much greater exposure to these contaminants than do adults. In some cases (e.g., PCBs), this is almost sixty times the amount that adults receive and in other cases (e.g., dioxins and furans), the amount far exceeds the Health Canada guidelines for tolerable daily intake.35

While the exposure through breast milk is absolutely greater than during development in the womb, in utero exposure is relatively more significant due to the greater vulnerability of the fetal brain and CNS.36

Neurodevelopmental Effects from Exposure to POPs

In a 1996 paper, researchers who studied a cohort of children exposed prenatally to levels of PCBs slightly higher than that of the general population concluded that "in utero exposure to polychlorinated biphenyls in concentrations slightly higher than those in the general population can have a long-term impact on intellectual function."37 Jacobson and Jacobson also observed reductions in birth weight and head circumference when these children were newborns and some developmental delays at certain periods. They concluded, however, that such effects were largely due to the exposures in utero, rather than through breast milk.38

Studies of newborns whose mothers’ body burden of PCBs and dioxins more nearly reflects that of the general population than fish eaters, suggest that while the neurological effects are not severe, higher exposure is associated with hypotonia and increased incidence of abnormally weak reflexes.39,40 These studies found also that transplacental PCB exposure had a small negative effect on the neurological condition of these children at about 18 to 24 months of age but not at older ages.41,42

Many of the POPs are also endocrine disruptors, which are being shown to affect thyroid function.43,44 This is of concern since thyroid hormones are vital to proper development of various brain functions involved in learning and memory.45

Questions to Ask

Exposure to POPs such as PCBs comes mainly from diet, particularly consumption of breast milk, fish, fatty meats and dairy items.46 We all carry a body burden of such contaminants because of their presence in the food chain; the most heavily exposed people, however, are those who consume sport fish and wild game that are unregulated by government testing.

• Is sport fish or wild game consumed in the household? Did the mother consume sport fish during her pregnancy?

Researchers at Wayne State University have examined the intellectual and behavioural development of a cohort of children whose mothers ate significant amounts of contaminated fish from Lake Michigan during their pregnancy. These children were exposed to higher than average levels of PCBs and in the most recent published reports of the study, the children, who were then age 11, demonstrated permanent reductions in IQ compared to controls.49

Assessing Exposure to POPs

• Thyroid function – Thyroid hormone (thyroxine) is among several factors essential for normal neurological development. Substances that alter thyroxine levels or functioning can have profound effects on IQ and brain activity. "Even transient decreases in thyroxine in the CNS during critical developmental periods may produce alteration in neuronal branching and cellular architecture in the brain".50
• Breast milk testing is NOT recommended for the individual clinical case unless the physician suspects there has been an unusually high exposure. The clinician must be aware that there are potential detrimental psychosocial implications from a mother learning that her breast milk is "contaminated."

Personal Prevention of Exposure to POPs

Physicians can counsel patients on a number of different choices in their diet and regarding personal activities, particularly if they are considering having children or are pregnant.

• The advice for women of childbearing age and children under age 15, to avoid or limit meals of sport fish and wild game, also stands for preventing undue exposure to POPs.
• There are recommended cleaning and cooking methods that will reduce the amount of contaminants ingested. In particular, skin and fat of fish should be avoided as prime depots of stored contaminants.
• In Ontario, the Guide to Eating Ontario Sport Fish is published annually in print (distributed free in beer stores and at fishing licence agencies) and online. The guide gives detailed recommendations for reducing exposure to contaminants in sport fish.
• Other provinces have similar resources. For example, in Quebec, Le Fleuve, the online newsletter for the St. Lawrence Vision 2000 Web site presents reports on risks from eating St. Lawrence marine life. See, for example, Monitoring poisonings linked to the consumption of shellfish from the St. Lawrence.
• Clearly, patients should not be advised to avoid fish consumption altogether as eating fish provides an important nutritional benefit.

Further Recommended Resources

Health Canada, It’s Your Health Series: Dioxins & Furans

World Wildlife Fund Canada’s, Reducing Your Risk: A Guide to Avoiding Hormone-Disrupting Chemicals

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