People with Alzheimer's disease may have higher levels of a chemical left behind by the pesticide DDT than healthy elderly people, suggested a US study out Monday.

The pesticide, DDT, was phased out in the United States in 1972, but is still used elsewhere in the world and global health authorities consider it an important tool against malaria.

Researchers found DDE, the long-lasting metabolite of DDT, was nearly four times higher in Alzheimer's patients than in peers without the disease.

Having high DDE levels was also found to increase someone's risk of Alzheimer's fourfold, according to the study which compared 86 Alzheimer's patients to 79 people of advanced age.

The patients in the study came from the US states of Texas and Georgia, and their average age was 74, according to the study published in the Journal of the American Medical Association (JAMA) Neurology.

Outside experts cautioned that its sample size was small and should be followed with more research.

"The findings should be a stimulus to further research using more rigorous epidemiological methods, but of themselves, they do not provide strong evidence of a hazard," said David Coggon, professor of occupational and environmental medicine at the University of Southampton in Britain.

The differences in DDE levels were seen in the Texas sample, but not in Georgia, noted an accompanying editorial in JAMA Neurology by doctors at the University of Pennsylvania and the University of Virginia.

The editorial writers, Steven DeKosky and Sam Gandy, noted that the research should be considered "preliminary until there is independent confirmation in other populations."

Little is known about what causes Alzheimer's disease, which afflicts five million people in the United States.

The World Health Organization says some 35 million people around the world are living with dementia.

"This is one of the first studies identifying a strong environmental risk factor for Alzheimer's disease," said a statement by study co-author Allan Levey, chair of neurology at Emory University School of Medicine.

"The potentially huge public health impact of identifying an avoidable cause of Alzheimer's disease warrants more study -- urgently."