A team of researchers has found that patients with Alzheimer's disease have significantly higher levels of DDE, the long-lasting metabolite of the pesticide DDT, in their blood than healthy people.

In a case-control study involving 86 Alzheimer's patients and 79 healthy elderly controls, researchers found that DDE levels were almost four times higher in serum samples from Alzheimer's patients than in controls. Having DDE levels in the highest third of the range in the study increased someone's risk of Alzheimer's by a factor of four. The results were published Jan. 27, 2014, in JAMA Neurology.

"This is one of the first studies identifying a strong environmental risk factor for Alzheimer's disease," says co-author Allan Levey, MD, PhD, director of Emory's Alzheimer's Disease Research Center and Betty Gage Holland chair of neurology at Emory University School of Medicine. "The magnitude of the effect is strikingly large -- it is comparable in size to the most common genetic risk factor for late-onset Alzheimer's."