Institute Of Medicine of the National Academies
23 January 2003

New Report Supports Association Between Agent Orange and One Form of Chronic Leukemia

Institute of Medicine Press Release:

WASHINGTON -- A re-evaluation of evidence now supports an association between exposure to herbicides used during the Vietnam War and the development of a specific form of leukemia in veterans, says a new report from the Institute of Medicine (IOM) of the National Academies. The report is the latest update in a series examining the health effects of defoliants -- including Agent Orange -- and chemicals that contaminate them.

As part of its biennial update, the committee that wrote the report reassessed six studies of herbicide exposure that provided information on chronic lymphocytic leukemia (CLL) among other health effects. The re-examination revealed sufficient evidence of an association between exposure to chemicals sprayed in Vietnam and risk of developing CLL.

In previous updates on the health risk to veterans posed by exposure to Agent Orange and other chemicals used in Vietnam, IOM had considered all forms of leukemia collectively when examining research on links between herbicide exposure and risk of cancer. The combined evidence was found to be inadequate or insufficient to determine whether any association exists between leukemia and exposure to the herbicides or their contaminants. However, although classified as a form of leukemia, CLL shares many traits with Hodgkin's disease and non-Hodgkin's lymphoma, both of which previously have been found to be associated with herbicide exposure. Both CLL and lymphomas originate from malignant B-cells, and CLL can transform into an aggressive non-Hodgkin's lymphoma known as Richter's Syndrome.

"The similarities between CLL and lymphomas -- which we have long known to be associated with exposure to the types of chemicals used in Agent Orange and other defoliants -- began to raise questions about whether CLL should be considered separately from other forms of leukemia," said committee chair Irva Hertz-Picciotto, professor of epidemiology, University of North Carolina, Chapel Hill, and University of California, Davis. "At the request of the Department of Veterans Affairs, we looked into the matter, and our reassessment indicates that CLL is indeed a special case. The data are sufficient to support a link between herbicide exposure and this type of cancer."

The committee's new assessment of CLL is based on evidence from six studies that looked at cancer rates, including specific forms of leukemia, and other health effects among agricultural workers exposed to herbicides, as well as individuals who resided in agrarian settings. The risk for CLL was found to be elevated in those whose occupations involved handling of or exposure to the types of herbicidal chemicals also used during the Vietnam War.

The ability of researchers to pinpoint the health risks faced by individual veterans is hampered by inadequate information about exposure levels of troops in Vietnam. Most information comes from studies of civilians who have been exposed on the job or in industrial accidents to herbicides or their contaminants. However, most veterans probably experienced lower levels of exposure than people who have worked with these chemicals over long periods in occupational or agricultural settings, and it is difficult to say precisely which troops may have been exposed to larger amounts.

CLL is the most common form of leukemia, with roughly 7,000 new cases diagnosed in the United States last year. However, it is among the rarer forms of cancer, making it difficult to do large-scale studies to determine causes. There are no accurate estimates of how many Vietnam veterans have been diagnosed with CLL.

The committee's congressionally mandated report also reaffirms findings from previous IOM updates. In addition to non-Hodgkin's lymphoma, Hodgkin's disease, and now CLL, there is sufficient evidence of a link between exposure to chemical defoliants or their contaminants and the development of soft-tissue sarcoma and chloracne in veterans. Also, scientific studies continue to offer limited or suggestive evidence of an association with other diseases in veterans -- including Type 2 diabetes, respiratory cancers, prostate cancer, and multiple myeloma -- as well as the congenital birth defect spina bifida in veterans' children.

U.S. forces sprayed Agent Orange and other defoliants over parts of south Vietnam and Cambodia beginning in 1962. Most large-scale sprayings were conducted from airplanes and helicopters, but considerable quantities of herbicides were dispersed from boats and ground vehicles or by soldiers wearing back-mounted equipment. A 1969 scientific report concluded that one of the primary chemicals used in Agent Orange could cause birth defects in laboratory animals. The U.S. military therefore suspended the use of Agent Orange in 1970 and halted all herbicide spraying in Vietnam the following year.

The committee's work is sponsored by the U.S. Department of Veterans Affairs. The Institute of Medicine is a private, nonprofit institution that provides health policy advice under a congressional charter granted to the National Academy of Sciences. A committee roster follows.