Children, Cancer, and the Environment
Leukemia is a cancer of the tissues that produce blood cells, resulting in abnormal blood cells. Leukemia appears to be related to damage to chromosomes or genes (1). The damage disrupts the process by which blood cells achieve their final and functional form.
Leukemia is the most common kind of cancer in children
Leukemia also causes more deaths than any other form of cancer in children. Fortunately, improved treatment methods have greatly reduced deaths from leukemia.
Leukemia rates are higher for white children than black children.
There are several forms of leukemia, two of which are particularly important in children. These are usually known as ALL and AML.
Acute lymphoblastic leukemia (ALL) is the most common form in children and represents 78% of cases of leukemia (2). (ALL is also called acute lymphocytic leukemia.) From 1991 to 1994, the incidence rate was 59 cases per million children under five. ALL reaches its greatest frequency in children between 2 and 6, with a peak of more than 80 cases per million children per year at ages 3 to 4 (2). Rates then decline to age 20. Scientists believe that this pattern means that two genetic changes are needed to cause the disease and that one occurs before a child is born.
ALL increased about 1% per year between 1977 and 1995, though some of this change may result from changes in the groups that are tracked for cancer (2). However, many scientists believe that this change represents a genuine increase in the frequency of the disease that could be caused by environmental factors.
Acute non-lymphocytic leukemia is the second most common form of leukemia in children and represents 19% of cases. It is also called acute myelogenous leukemia (AML). It is the form of leukemia most commonly diagnosed in children less than one year old. Rates are higher at ages 1-3 and in late adolescence (2).
Unlike ALL, the rates for AML do not appear to have increased since 1975 (2).
Known and Suspected Causes of Leukemia
Both pre-natal and post-natal exposure to ionizing radiation (particularly X rays) can cause leukemia in children. Pre-natal exposure to X rays has been greatly reduced with the adoption of ultrasound for screening in pregnant women.
Several studies link pesticide exposure by both parents and children to leukemia. The pattern of disease suggests that some damage to chromosomes may occur before the child is born (3). Children born to parents employed in certain occupations that have chemical exposures are more likely to have leukemia (4). Chemicals, specifically including benzene, have been shown to cause leukemia in adults.
A recent review of 48 epidemiological studies concluded that the strongest evidence for a relationship between a parent's exposure to chemicals other than pesticides and childhood leukemia was for solvents, paints, and employment in motor vehicle-related occupations (5).
These studies tend to look at the occupation of fathers more often than those of mothers, despite the fact that exposures of mothers are likely to be at least as important. For occupations of the mothers, the review concluded that the most significant were employment in personal services industries, in metal processing, and in textiles. All three categories had significantly elevated risks. For occupations of fathers, employment in painting led to increased risk of leukemia in a child..
Some studies show that exposure to electric and magnetic fields (EMFs) is associated with increased risk of leukemia.
There has been some evidence for an association between leukemia and smoking by parents, though the largest study performed to date did not find that smoking by parents, either before birth or afterwards, increased risk of ALL or AML in children (6).
IONIZING RADIATION
Ionizing radiation is considered a "known" cause of childhood leukemia. Follow-up studies of people who survived the detonation of atomic bombs at Hiroshima and Nagasaki found that the risk of leukemia was higher for those exposed to radiation. The risk is also higher for those exposed at an earlier age (7). Radiation from nuclear power plants is a known cause for both kinds of leukemia (8). A recent study found that exposure to X rays after birth also increased the risk of leukemia. Infants receiving diagnostic X rays had 60% more leukemia than other children (9).
PESTICIDES
Several studies have linked leukemia to pesticides. Two recent reviews concluded that pesticide exposure may be a cause of leukemia (10)(11). These reviews report that most, though not all, studies find leukemia was more likely in children whose fathers were exposed to pesticides at work than other children. Risks for children are often reported to be greater than risks for adults (12).
· One large recent study of 491 children with ALL found that risk was increased by home use of some kinds of pesticides and by use of multiple different pesticides. Herbicide use during pregnancy was associated with a 50% increase in risk. Use of insecticides in the home was associated with increased risk of ALL, and frequent use was associated with higher risk. Use of some garden products also seemed to increase risk. The heightened risk was associated with use of multiple products (13).
· A study of childhood leukemia cases in Shanghai found a more than threefold increase in risk for children whose mothers were exposed to pesticides at work (14).
· A study of children under 15 in the Denver area reported that use of pest strips was associated with higher risks of leukemia (15).
· A study of children in the US found that AML risk was increased when either parent was exposed to pesticides or when the child was exposed to pesticides after birth (16).
· In 1989, the Childrens Cancer Study Group reported that, among families of 204 children with AML, children whose fathers worked with pesticides for more than 1000 days had nearly three times the risk of other children. The risk was greater for children under the age of 6. Children regularly exposed to pesticides in the household had 3.5 times greater risk of leukemia than those not exposed (17).
· In a 1987 National Cancer Institute study, the risk of childhood leukemia increased nearly four times when pesticides were used within the house at least once per week. The risk increased more than six times when garden pesticides were used at least once per month (18).
· Children of fathers with jobs including pesticide exposure had a 2.7 times higher risk of leukemia when compared to controls (17).
· A small study in the Netherlands reported increased risk of leukemia in children who were exposed to pesticides directly or whose fathers were exposed at work (19).
· An increased risk was found for children whose parents used pesticides in the home (OR = 3.8, P = .004) or garden (OR = 6.5, P = .007) or who burned incense in the home (OR = 2.7, P = .007). The risk was greater for frequent use (18).
SOLVENTS
· A 1998 review concluded that the evidence for an association between childhood leukemia and paternal exposure to solvents is "quite strong." Chemicals where risks are elevated include solvents in general, chlorinated solvents, benzene, carbon tetrachloride, and trichloroethylene (TCE) (5).
· A study of nearly 2,000 children found that the risk of acute lymphoblastic leukemia (ALL) was increased if the children's mothers were exposed to solvents, paints, or thinners before conception or during pregnancy or to plastics after birth. The father's exposure to plastics before conception was associated with greater risk. This study reported that the timing of exposure was an important factor (20).
· A study of children in the US found that the father's exposure to petroleum products increased the risk of AML (16). Petroleum products usually contain benzene.
· An earlier study had found that children born to fathers with exposure to solvents, petroleum products, plastics or lead were more likely to have AML than other children (17). For solvents, the excess risk was substantial. The same study found that children born to mothers with exposure to metal dusts, pigments and paints at work were more likely to have AML.
· A case-control study of 123 children 10 and under in Los Angeles County looked at specific occupational and home exposures and found an increased risk of leukemia for children whose fathers were exposed at work after the birth of the child to chlorinated solvents {odds ratio (OR) = 3.5, p = .01}. Risk of leukemia was related to mothers' employment in personal service industries (OR = 2.7, p = .04) but not to specified occupational exposures (18).
· A Scottish study found greatly increased risk of leukemia for children born to fathers who had exposure to benzene (21).
· A study of children in Shanghai reported increased ALL in children whose mothers were employed in the chemical industry during pregnancy, increased AML in children whose mothers were exposed to benzene and increased AML and ALL to children whose mothers were exposed to gasoline (14). Also children born to women employed in metal processing were more likely to have AML.
· A study in the Netherlands found that mothers who were exposed to hydrocarbons at work or certain other chemicals in the year before the birth of their child were more likely to have a child with ALL (22).
· A study in New York found that children born to fathers who worked in motor vehicle-related jobs were more than twice as likely to have leukemia (23).
EMFs - Electric and Magnetic Fields
Evidence about whether Electric and Magnetic Fields (EMFs)contribute to leukemia in children is contradictory, with some studies finding an effect and others not finding an effect. However, there are many ways to measure these fields (24), and the various approaches do not correlate in all cases (25). Some of the contradictory results could be due to differences in methods of measuring the EMFs. Also, as is true with other possible causes, there may be a specific time period when children are most susceptible, studies do not necessarily identify the relevant time period.
· More positive results are found when EMFs are measured according to "wire codes," which are classifications based on how wiring is configured (26) than when EMFs are measured directly (27).
· Several studies have found that exposure to EMFs increases risk of leukemia for children (28). One study found that children living near high voltage power installations were more likely to be found to have leukemia than other children (29). However, other studies have failed to find any link (30)(37).
· A 1995 review of several studies reported a link between EMF exposure and increased leukemia that cannot be readily explained by errors in the studies (31).
· One recent study found that risk of leukemia was elevated when exposure to EMFs was consistent over the term of the pregnancy and in cases where the design of the water system in the home led to "ground currents" from connections between plumbing pipes and the grounding for the electricity (32).
· A large study in Britain found no association between exposure to EMFs and leukemia in 3,838 children (33). A study in Los Angeles that looked at combined measures of EMF found increased risk of leukemia to be associated with increased EMF exposure (34).
· A first study of exposures of parents to EMFs at work before their children were born found that fathers with high exposures were more likely to have children who got leukemia (35). This study had several good design elements that avoided problems with recall of exposures by parents.
Radon
Radon is a naturally occurring radioactive gas that gets into homes from materials underneath houses, such as soil or rocks, or from water piped into the houses. Two studies have investigated whether radon exposure is related to ALL but have not found any relationship (38, 39). Both of these studies were limited in the conclusions that they could draw because participation rates were low. One earlier study reported an association between levels of radon in geographic areas and increased risk of childhood leukemia (40).
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