Exposure to the pesticide DDT, which was banned in the United States since 1972 but still being used in some countries, may increase the risk and severity of Alzheimer's disease in some people, a US study said Monday.

The study, published online in the US journal JAMA Neurology, found patients with Alzheimer's disease have significantly higher levels of DDE, the chemical compound left when DDT breaks down, in their blood than healthy people.

Of the 86 Alzheimer's patients involved in the study, 74 people, whose average age was 74, had DDE blood levels almost four times higher than the 79 people in the control group who did not have Alzheimer's disease, researchers from the Rutgers University, the Emory University and the University of Texas said.

"Our data may help identify those that are at risk for Alzheimer's disease and could potentially lead to earlier diagnosis and an improved outcome," lead author Jason Richardson, assistant professor of the Rutgers University, said in a statement.

The researchers said DDT and DDE could increase the amount of a protein associated with plaques believed to be a hallmark of Alzheimer's disease. These sticky amyloid proteins, which may form in regions of the brain involved in memory, learning and thinking, break off and clump together in the brain and increase as the disease progresses.

This new research is important, Richardson said, because it suggests that DDT and DDE may directly contribute to the process of plaque development.

DDE levels, however, weren't the sole determinant of whether someone gets Alzheimer's. Some Alzheimer's patients in the current study had non-detectable levels of DDE and some healthy control samples had DDE levels that were relatively high.

The researchers said that a gene called ApoE4 may combine with environmental exposures to drive disease development.

"We need to conduct further research to determine whether this occurs and how the chemical compound interacts with the ApoE4 gene, " Richardson said.

In the United States, DDT was used extensively in agriculture and for mosquito control from the 1940s until it was banned in 1972.

Concerns over DDT's effects on wildlife, especially birds, played an important role in the history of the environmental movement. Around the world, DDT's use continued in many countries until more recently.