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Cancer and Immunological Effects

What do we know about childhood cancer?

Cancer is the second most common cause of death among children beyond their first year. An annual average of over 1,260 Canadian children was diagnosed with cancer in the mid-1990s, leukemia being the most common type.

The debate about whether childhood cancer is increasing in North America is currently unresolved. Some sources suggest there has been a 25% increase in incidence in the last 25 years, and there is a sense among health professionals that this figure is accurate. However, others suggest that any increase was related to improved detection methods and has leveled off since the 1980s. Canadian data on childhood cancer provide an equivocal picture. We do know that the childhood cancer incidence is apparently not declining. There are now more survivors of childhood cancer; however, this is tempered with concern that radiation therapy predisposes these children to further cancers as adults.

Childhood cancers are typically of a different variety from those observed in adults, and different mechanisms and exposures may cause them. Children donít exhibit the lifestyle choices that increase risk of cancer in adults; therefore, most childhood cancer is likely explained by involuntary exposure to environmental agents. It is now believed that exposures in utero or during infancy represent a substantial, largely preventable, cause of cancer in the young.

How does the environment influence cancer in childhood?

The precise causes of childhood cancers are still insufficiently known. The multifactorial nature of cancer etiology means it is difficult to clearly determine cause for individual cases. The vast majority of cancers are still of undetermined cause. It is also highly difficult to study cancer in children because the numbers afflicted are relatively small in epidemiological terms.

Known or suspected substances linked to cancer in children include:

  • Some medications (such as diethylstilbestrol)
  • Chemotherapeutic drugs
  • Ionizing radiation
  • Certain industrial and agricultural chemicals (e.g., organic solvents, nitrosamines, some pesticides)

Childhood exposure to the following substances is likely to increase the risk of developing certain cancers in adulthood:

  • Environmental tobacco smoke (ETS)
  • Asbestos
  • Ultraviolet radiation
  • Perhaps persistent organic pollutants

What is known about environmentally linked immunological effects?

There is a great deal less information surrounding the relationship between environmental exposures and immunocompetence. A number of exposures may cause immunosuppression in humans, as shown by changes in antibody levels or immune cell numbers. While the links to compromised disease resistance are not entirely certain, there are implications for cancer effects from such changes in immune system functioning. Animal and epidemiological studies indicate that the developing immune system is vulnerable to the toxic effects of environmental contaminants. Perinatal exposure can produce persistent effects at doses much lower than those necessary to affect adult immune functioning.

What substances are immunotoxic?

Because research testing for immunotoxicity has been limited, the number of substances known to be immunotoxic is relatively small, but may only represent the proverbial "tip of the iceberg."

Known or presumed developmental immunotoxins include:

  • Organochlorine contaminants (such as PCBs, the pesticide chlordane, and the dioxin TCDD)
  • Metals (such as chromium, cadmium, lead and mercury)
  • The polycyclic aromatic hydrocarbon benzo[a]pyrene
  • UV-B radiation
  • Air pollutants (such as, ozone, nitrous oxides and environmental tobacco smoke)

Despite the fact that human data to support immune system effects are incomplete, the majority of researchers conclude that this is an important area for further research and critical evaluation of scientific evidence nonetheless.

How are children exposed to carcinogens and immunotoxic substances?

Carcinogens and immunotoxins abound in our environment, and are present naturally and from human activities. Most of our intake is through diet, but carcinogens can be inhaled and dermally absorbed as well. For children, the most likely route of exposure to carcinogens or immunotoxins is transplacentally or in the case of some carcinogens, via direct exposure in utero. Prenatal exposure to X-rays, ionizing radiation, some pesticides, nitroso compounds in cured meats and solvents have all been linked to higher risk of cancer in children. Paternal occupational exposure to organic solvents such as benzene may also cause cancer in offspring. It is speculated that in utero exposure to persistent organic pollutants (POPs) may be responsible for observed increases in certain reproductive cancers among young adults.

Early childhood exposure to pesticides used in the home appears to increase cancer risk in those children. Early exposures to environmental tobacco smoke, asbestos and ultraviolet radiation increase the chances that those children will develop specific cancers as adults.

Health depends largely on proper functioning of the immune system, the foundations for which are established early in development. It appears that immune system impairment is a potentially long-term result of altered developmental processes, the effects of which may not manifest or be recognized until later in life, long after exposure. Despite the fact that human data to support immune system effects are incomplete, the majority of researchers conclude that this is an important area for further research and critical evaluation of scientific evidence nonetheless

How does one prevent such exposures on a personal level?

Preventing childhood cancer and immunotoxicity represents immense challenges given the gaps in scientific information. The most fundamental preventive advice clinicians can offer patients to reduce the risk of such health effects in their children is to avoid exposure to substances that are known or suspected carcinogens or immunotoxins. Such advice should come before a couple is considering having a child but also holds during pregnancy and at all stages of life. There are a number of specific preventive actions outlined below.

Behaviours that might enhance protection against cancer in offspring include taking multivitamin supplements during pregnancy, breastfeeding as long as possible and optimizing the childís dietary intake of fiber, fruits and vegetables. Instilling other healthy lifestyle choices such as exercise and dietary moderation during childhood lays an important foundation for long-term cancer prevention. Breastfeeding is critical for transferring immunoglobulins and establishing full functionality of the infant immune system.

What do we do as a society to prevent these exposures?

Only a small fraction of chemicals and substances has been adequately tested for carcinogenicity and even fewer are confirmed for immunotoxic effects. There is a need for further research to determine the precise mechanisms of effects from carcinogens and immunotoxins and to better identify such substances in the environment. However, a call for further research should not delay instilling greater precaution in environmental regulations to prevent harm to childrenís health from carcinogens and immunotoxins.

 
Copyright © 2000 Canadian Association of Physicians for the Environment
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