Children, Cancer, and the Environment
Environmental factors may be important for childhood cancer. There are several reasons why we believe that this may be the case.
First, some chemicals or other environmental agents are known to cause cancer in children -- exposure to radiation, some agents used for chemotherapy, and the synthetic hormone DES (diethylstilbestrol).
Children exposed to X rays used for medical purposes on their mothers before birth are about 40% more likely to have cancer, particularly leukemia, before they reach 10 years of age, than other children. A fetus may be more sensitive than an infant to radiation (1). Because of these findings, ultrasound is now used on pregnant women instead of X-rays. Moreover, use of radiation therapy on children is linked to increases in breast and thyroid cancer later in life (2).
Second, there are differences in how common childhood cancers are in different parts of the world, which suggests that there may be environmental factors that contribute to the disease (3).
Third, studies of identical twins suggest that genetic causes do not cause most childhood cancer. Pairs of identical twins have exactly the same genetic makeup. If cancer a person's genetic makeup determines their cancer experience, then we would expect that pairs of identical twins would have the same experience for cancer.
For childhood cancer, studies of twins show that there is little similarity in the experience of pairs of twins. In one study, for leukemia, both twins got the disease only 5% of the time. The percent was even lower for non-retinoblastoma solid tumors (4). The authors concluded that inherited genetic factors were not the cause of most forms of childhood cancer, apart from retinoblastoma.
Certain environmental agents are suspected of causing of cancer in children. These include second-hand tobacco smoke, electromagnetic fields, pesticides, solvents, paints, metals, and chemicals associated with use of vehicles (5, 6, 7).
Epidemiological studies provide clues about environmental factors that may lead to cancer. These kinds of studies look at patterns of disease in people and consider the environmental exposures that people experience. Though the results of epidemiological studies are not always conclusive, they do provide important evidence.
Children with cancers, particularly brain tumors and leukemia, are more likely to have been exposed to pesticides or solvents than other children. Solvents have been linked to brain cancer and leukemia, generally at relatively high occupational exposures in adults. Similar results have been reported with parental exposures to solvents during gestation or just after birth. Solvents are commonly in paints, non-water-based glues, de-greasers, varnish strippers, and gasoline.
Another class of products that may cause cancer is combustion by-products. These include chemical such as dioxins, poly aromatic hydrocarbons (PAHs), and sooty particles. These come from second hand cigarette smoke, diesel exhaust from buses and other vehicles, and industrial pollution.
When thinking about possible environmental causes of cancer in children, it is important to identify the several different time periods when critical exposures could occur. These include the time period before conception (when environmental agents could affect the sperm or egg of the parents), during pregnancy (when the mother's exposure would be critical), and after birth, when the child's direct exposure would be important.
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1. Doll R, Wakeford R. Risk of childhood cancer from fetal irradiation. British Journal of Radiology 1997g; 70:130-9.
2. Boice JD, Jr., Miller RW. Childhood and adult cancer after intrauterine exposure to ionizing radiation. Teratology 1999; 59:227-33.
3. McBride ML. Childhood cancer and environmental contaminants. Canadian Journal of Public Health. Revue Canadienne de Sante Publique 1998; 89 Suppl 1:S53-62, S58-68.
4. Buckley JD, Buckley CM, Breslow NE, Draper GJ, Roberson PK, Mack TM. Concordance for childhood cancer in twins. Med Pediatric Oncology 1996; 26:223-9.
5. Zahm SH, Devesa SS. Childhood cancer: overview of incidence trends and environmental carcinogens. Environmental Health Perspectives 1995; 103 Suppl 6:177-84.
6. Zahm SH, Ward MH. Pesticides and childhood cancer. Environmental Health Perspectives 1998; 106 Suppl 3:893-908.
7. Zahm SH, Ward MH, Blair A. Pesticides and cancer. Occupational Medicine 1997; 12:269-89.
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