Exposure to the pesticide DDT – once widely used in the U.S. until banned in the 1970s – seems to be a risk factor for Alzheimer’s disease, according to a new study.

Scientists from Rutgers University, Emory University and the University of Texas Southwestern Medical Center examined blood samples from 86 Alzheimer’s patients and 79 healthy elderly control subjects. The Alzheimer’s patients had significantly higher levels of DDE, a molecule formed by the metabolic breakdown of DDT, according to a paper published in the journal JAMA Neurology on Monday.

“We were very surprised,” Emory University researcher Allan Levey, a co-author on the study, said in a phone interview. “The magnitude of the effect seems pretty strong – as strong as any other risk factor including genetics at this point.”

Having DDE levels in the highest third of the range examined in the study raised a person’s risk for Alzheimer’s fourfold, the scientists found.

However, high DDE levels aren’t the sole factor in developing Alzheimer’s, the authors point out. In their study, some patients with the disease had almost undetectable levels of the chemical in their systems, while some of the healthy controls had very high blood levels of DDE.

“We still think it’s prudent for us to be very cautious and try to replicate this in additional patient populations,” Levey says.

It’s unclear just how DDE might possibly kick-start processes in the body that lead to Alzheimer’s. When the researchers exposed some human neural cells in the lab to DDT, the cells soon had increased levels of a protein that is a precursor of beta-amyloid, one of the prime suspects in Alzheimer’s.

But any attempt to construct a more detailed explanation at this point would be “a lot of hand-waving,” Levey says.

There may be some interaction between genetics and environmental risk factors in the development of Alzheimer’s – perhaps people who carry certain genetic variants associated with Alzheimer’s are more susceptible to the effects of DDT or DDE. If so, it may be possible to identify potential Alzheimer’s cases earlier in life by looking for elevated DDE blood levels in people who carry genetic variants like the ApoE variant associated with Alzheimer’s risk.

Members from this same research team previously found a link between pesticide exposure and Parkinson’s disease. That Parkinson’s research (focused on a different pesticide called beta-HCH) led them to a pool of Alzheimer’s patients that were initially used as control subjects.

While scientists have been delving into environmental risk factors for other neurodegenerative diseases for decades, “the [research] focus on environmental factors in Alzheimer’s is nonexistent, compared to Parkinson’s,” Levey says.

Despite the long-standing ban on DDT, the pesticide and its products still linger. Analyses from the U.S. Centers for Disease Control and Prevention still show that DDE is detectable in the vast majority of U.S. citizens, according to Levey.

“DDT is pretty persistent in the environment,” Levey says. “It’s still in the food chain – animals eat it, it could be part of crops, et cetera. We don’t metabolize it very well, so the older you get, the more you accumulate.”

SOURCE: Richardson et al. “Elevated Serum Levels of p,p’-DDE, the Metabolite of the Pesticide DDT, are Associated with Increased Risk for Alzheimer’s Disease.” JAMA Neurology 27 January 2014.