Air pollution and lack of vitamin D caused by too much time spent indoors may raise the risk of dementia, according to experts at the University of Edinburgh.
Around 600,000 people in Britain suffer from dementia, the majority of which is Alzheimer's disease, costing the country more than £26 billion annually.
‘Addressing environmental factors and advising people to consider a daily supplement of vitamin D, particularly if they are unlikely to gain enough through their diet and daily exposure to sunlight can help reduce risk of dementia.’
Scientists believe that around two thirds of the risk of dementia is caused by the lifestyle factors such as smoking, diabetes, obesity, as well as genetics.
The remaining one third of the risk is still unexplained and Edinburgh researchers believe that some environmental factors could be to blame.
The research, published in the journal BMC Geriatrics
, looked back over more than 4,000 studies to draw up a shortlist of the most likely culprits.
They found that a lack of vitamin D - produced by the body through exposure to sunlight - and exposure to air pollution were strongly implicated. The 'sunshine vitamin' helps to clear protein build up which can cause Alzheimer's disease.
Living under power lines.
It found mixed evidence that minerals found in tap water may be linked to the disease, with more than 0.1mg a day of aluminium or selenium in drinking water found to triple the risk of Alzheimer's disease in a large French study.
People exposed to pesticides, through working as farmers for example, could also be at risk.
Air pollution has been found to raise the risk of cardiovascular disease so it may also cause oxidative stress or inflammation in the brain, the researchers speculate. Tiny magnetic particles of materials produced by car engines and brakes have also recently been found in the brains of Alzheimer's sufferers.
Public Health England is so concerned that Britons are not getting enough vitamin D that they recently encouraged everyone to take a supplement in the winter months. The vitamin is important for a healthy immune system and helps clear amyloid plaques from the brain which are a hallmark of Alzheimer's disease.
Dr Tom Russ, of the Alzheimer Scotland Dementia Research Centre at the University of Edinburgh, said "Our ultimate goal is to prevent or delay the onset of dementia. Environmental risk factors are an important new area to consider here, particularly since we might be able to do something about them.
The evidence is particularly strong for air pollution and vitamin D deficiency. But more research is needed to find out whether these factors are actually causing dementia and what can be done to prevent it.
But doctors and scientists believe that a significant proportion of cases could be prevented or delayed by addressing environmental and lifestyle factors linked to the disease.
The team behind the latest research says future studies should focus on the short list of environmental risk factors flagged up in their study.
Prof Tom Dening, Professor of Dementia Research at the University of Nottingham, said "What is difficult is to tell whether the environmental exposures are themselves contributing to dementia or whether they are in fact acting as proxies for some underlying variable. For instance, many unpleasant environmental exposures (traffic fumes, living near power lines, poor water quality) are related to socio-economic deprivation, which itself is related to poor diet, low education, higher stress and worse health. So we really cannot easily tell what is causing what."
Jim Pearson, Director of Policy and Researcher at Alzheimer Scotland,added: "The research study substantially improves our knowledge and understanding of environmental factors which may increase the risk of developing dementia and provides a basis for further, and more focused, research in this area."