Newswise — Even Vietnam veterans who were not involved in spraying Agent Orange experience higher levels of dioxin contamination, which is linked to an increased overall risk of cancer, reports a study in the April Journal of Occupational and Environmental Medicine, official publication of the American College of Occupational and Environmental Medicine (ACOEM).

The study, conducted by a group of U.S. Air Force and other researchers, analyzed cancer rates among nearly 1,500 Air Force veterans who served in Southeast Asia during the Vietnam War, but did not actually spray Agent Orange or other herbicides. The men served as a comparison group in a previous study of cancer risk in veterans of Operation Ranch Hand, the unit responsible for spraying Agent Orange.

Even though they didn't work with Agent Orange, veterans in the comparison group had significant blood levels of TCDD, the highly toxic dioxin contaminant of Agent Orange. The current study was designed to assess whether low-level exposure to TCDD affected the later risk of developing cancer.

Higher blood levels of TCDD were associated with higher rates of cancer in the years after serving in Southeast Asia. For veterans with blood TCDD levels above the median, cancer risk was 60 percent higher than for veterans with lower levels. The increased risk wasn't limited to any specific type of cancer, although much of it was related to digestive and respiratory cancers and of the skin cancer melanoma.

The study also looked at how length of service in Southeast Asia affected cancer risk. Veterans whose time in Southeast Asia was above the median were more than twice as likely to develop prostate cancer, compared to those with shorter tours of duty. Prostate cancer risk was unrelated to TCDD level.

There was also a significant interaction between duration of service and dioxin contamination—veterans who served longer in Southeast Asia tended to have higher blood levels of TCDD. Cancer risk was highest for vets who spent more than 2 years in Southeast Asia and had TCDD levels above the median.

TCDD and other dioxins have been linked to cancer and a wide range of other health problems. A recent study found increased rates of cancer—specifically prostate cancer and melanoma—in Air Force veterans who sprayed Agent Orange during the Vietnam War. That study also suggested possible increases in cancer risk in the comparison group of Air Force veterans who served in Southeast Asia but did not spray Agent Orange.

The new results support the finding of increased cancer rates for veterans with higher TCDD levels, even though they weren't directly exposed to Agent Orange. TCDD may promote the development of cancer even at very low levels of exposure. Another possible explanation is that blood TCDD levels reflect some other, unknown risk factor.

The interaction between cancer risk, TCDD levels, and time served in Southeast Asia suggests that a combination of factors are involved. More research will be needed, including longer follow-up of Vietnam-era veterans and more detailed information on their individual tours of duty in Southeast Asia.

ACOEM, an international society of 6,000 occupational physicians and other health care professionals, provides leadership to promote optimal health and safety of workers, workplaces, and environments.

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Journal of Occupational and Environmental Medicine (Apr-2005)