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.
Exposure to the synthetic pesticide DDT - widely known to be harmful in the environment - may also increase the risk and severity of Alzheimer's disease, a team of US scientists has found.
The new study published in JAMA Neurology showed patients with Alzheimer's had four times the level of DDE, the chemical compound left when DDT breaks down, lingering in their body compared to healthy people.
Out of the 86 patients involved 74 had DDE blood levels almost four times higher than the 79 in the control group who did not have Alzheimer's disease.
DDT may contribute to brain cell death
The researchers believe the chemical increases the risk of Alzheimer's and may be involved in the development of amyloid plaques in the brain, which contribute to the death of brain cells.
Professor Allan Levey, director of the Alzheimer's Disease Research Centre at Emory, said: "This is one of the first studies identifying a strong environmental risk factor for Alzheimer's disease.
"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."
Banned since the 70s but still used
DDT has been banned in many countries since 1972, when its impact on human health and wider environmental concern was first questioned.
The chemical is still used to control malaria in some areas - its original purpose when introduced during World War Two. The World Health Organization still recommends DDT for keeping malaria in check. It is also used to fight typhus disease, which is transmitted by lice and fleas.
DDT is still used as an agricultural pesticide in some countries. It has not been used in the UK since the 1980s, and is now banned across the EU, the USA and most industrialised countries.
However DDT may be present in some imported crops. It is also highly persistent in soils and the wider environment.
The toxic chemical lingers for decades
Dr Jason Richardson, one of the scientists conducting the research said:"We are still being exposed to these chemicals ... We get food products from other countries and DDE persists in the environment for a long time."
Levels of DDT and DDE have decreased significantly over the last three decades, yet the toxic pesticide is still found in 75 to 80% of the blood samples collected from the Centers for Disease Control and Prevention for a national health and nutrition survey in the US.
This occurs, scientists say, because "the chemical can take decades to break down in the environment". In addition, people may be exposed to the pesticide by consuming imported fruits, vegetables and grains from places where DDT is still being used and eating fish from contaminated waterways.
More research needed
Alzheimer's Research UK commented that more evidence was needed to prove the role of DDT in dementia.
The study was conducted by Rutgers, Emory University and University of Texas Southwestern Medical School.
Sophie Morlin-Yron is a freelance journalist.
More information: http://whqlibdoc.who.int/hq/2011/WHO_HTM_GMP_2011_eng.pdf?ua=1