Agent Orange, the controversial herbicide used to clear vegetation in the Vietnam War, has been linked to cases of deadly prostate cancer.
Exposure to the chemical more than doubled the chances of American war veterans who fought in the conflict having the most aggressive and lethal forms of the disease.
Contact with Agent Orange increased the overall risk of prostate cancer by 52% and of high-grade tumours by 75%.
The findings highlight a more widespread potential environmental threat from dioxin pollutants. Agent Orange was often contaminated with dioxins, highly toxic chemicals known to trigger cancer.
Dioxins and dioxin-like compounds can be generated by volcanic eruptions and forest fires but are mainly produced by industrial processes.
In high doses they can cause reproductive and developmental problems, damage to the immune and hormone systems, and cancer.
The chemicals accumulate in fatty body tissue and more than 90% of human exposure is through eating contaminated food, chiefly meat, fish and dairy products.
Uncontrolled waste incineration is one of the major environmental sources of dioxins.
Mass production of Agent Orange resulted in widespread contamination of the herbicide with dioxins and led to millions of Vietnamese people, as well as soldiers, being exposed to the harmful chemicals. Production of Agent Orange was halted in the 1970s.
The new research was based on a study of 2,720 US war veterans who were referred by their doctors to have biopsies for suspected prostate cancer.
Cancer was diagnosed in 896, or almost a third, of the veterans, 17% of whom had high-grade disease.
Agent Orange exposure was not linked to low-grade prostate cancer, only aggressive cases.
The study is published in the latest online edition of the journal Cancer.
Lead researcher Dr Mark Garzatto, from the Portland Veterans Administration Medical Centre in the US, said: "It should raise awareness about potential harms of chemical contaminants in biological agents used in warfare and the risks associated with waste-handling and other chemical processes that generate dioxin or dioxin-related compounds."
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