AML is the most common type of leukemia in adults; it is evenly distributed between sexes and occurs in both adults and children, but CLL almost never occurs in children. CML is mostly limited to adults and is the third most common type of leukemia. ALL, the most common form of leukemia in children, accounts for the smallest number of adult cases of leukemia (ACS, 2002a; NCI, 2002x). (Conclusions regarding exposure to insecticides and childhood cancers are discussed at the end of this chapter.) A fifth type of leukemia—hairy cell leukemia (HCL) (ICD-9 202.4)—is similar to CLL in that it is a slowly progressing cancer of the lymphocytes. However, because the cells have a different morphology, HCL is often considered separately. Accounting for an estimated 2% of all leukemia cases diagnosed in adults each year, HCL most often affects adults over 50 years old (ACS, 2002j).
Overall, the 5-year survival rate of leukemia patients is 44%—a rate that has tripled in the last 40 years. The survival rates for the types of leukemia differ markedly: 14% for AML, 32% for CML, 58% for ALL, and 71% for CLL. Leukemia is known to be associated with exposure to benzene, high doses of radiation, chemical drugs used to treat other cancers, and smoking. One factor currently under investigation is exposure to electromagnetic fields. However, none of those exposures can explain the majority of leukemia cases diagnosed each year. Rare inherited diseases—such as Fanconi’s anemia, Wiskott-Aldrich syndrome, Bloom’s syndrome, Li-Fraumeni syndrome, Down’s syndrome, and ataxia telangiectasia—also increase the risk of acute leukemia (ACS, 2002j; NCI, 2002x).
Several studies examined the relationship between exposure to specific insecticides, classes of insecticides, and insecticides in general and the risk of adult leukemia. The discussion below describes the key studies evaluated by the committee in making its conclusion of associaiton, and results of these studies are found in Table 5.12.
Of the studies that examined the relationship between exposure to insecticides and the risk of adult leukemia, only one evaluated the role of specific insecticides—a case-control study by Brown and colleagues (1990) of confirmed leukemias in 578 men in Minnesota (1980–1982) and Iowa (1981–1983). The authors interviewed cases or close relatives to determine exposures and the use of 112 pesticides, including 58 insecticides. Risk of leukemia with ever handling specific animal insecticides was evaluated for carbaryl (OR=1.3, 95% CI=0.5–3.2), dichlorvos (OR=2.0, 95% CI=1.2–3.5), lindane (OR=1.1, 95% CI=0.7–1.7), and malathion (OR=1.2, 95% CI=0.8–2.0). Risk of leukemia with ever handling crop insecticides was evaluated for carbaryl (OR=0.9, 95% CI=0.4–2.1), diazinon (OR=1.2, 95% CI=0.6–2.1), lindane (OR=1.6, 95% CI=0.8–3.2), and malathion (OR=0.9, 95% CI=0.4–1.9). The authors also conducted supplemental interviews of self-identified Iowa pesticide users to obtain information about the usual number of days per year that insecticides were handled. No clear exposure-response patterns were found between leukemia risk and increasing number of days per year of insecticide use. The study authors acknowledge the role of nondifferential exposure misclassification (including difficulty in recalling information by self-respondents and next of kin) and recall bias in the findings and note that the multiple statistical comparisons make it likely that some findings may be due to chance alone. Uncontrolled confounding by exposure to other insecticides or pesticides, such as herbicides, is also a limitation of the study.