Exposure to the pesticide dichlorodiphenyltrichloroethane (DDT) has long been linked to a number of health conditions. Now, a new study has found evidence that patients with Alzheimer's disease had almost four times as much DDT in their body than healthy people, linking exposure to the once widely used pesticide to an increased risk of developing Alzheimer's disease.

The study, published in JAMA Neurology Jan. 27, stemmed out of an earlier research by its lead author Jason Richardson, an associate professor at Robert Wood Johnson Medical School in New Jersey, and his colleagues, where they found that people with Alzheimer's disease have higher than normal levels of dichlorodiphenyldichloroethylene (DDE), a broken down form of DDT, in their blood.

In the study "Elevated Serum Pesticide Levels and Risk for Alzheimer Disease," the researchers analyzed and compared the blood samples of 86 people with Alzheimer's and 79 healthy subjects. The researchers found that DDE levels were 3.8 times higher in those with Alzheimer's than those without. Researchers also found that DDE increases the amount of a protein associated with brain plaques seen in Alzheimer's patients.

"This is one of the first studies identifying a strong environmental risk factor for Alzheimer's disease," said study co-author Allan Levey, director of the Alzheimer's Disease Research Center at Emory University School of Medicine in Atlanta. "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."

The researchers noted that exposure to DDT does not mean one will develop Alzheimer's, since some of the Alzheimer's patients involved in the study had no trace of the chemical in their blood. There was even evidence of high exposure in some of the healthy participants. Researchers believe that the presence of a mutant gene strongly linked to Alzheimer's known as ApoE may make people more susceptible to the effects of DDE.

DDT was banned in the U.S in the 1970's, but the people may still be exposed to it because of imported food or environmental contamination. Traces of it still appear in human blood samples due to the chemical's long half-life. DDE can remain in the human body for up to 10 years.

"We are still being exposed to these chemicals in the United States, both because we get food products from other countries and because DDE persists in the environment for a long time," Richardson explained.