Dementia is known to be associated with genetics and lifestyle factors such as high blood pressure, smoking and diabetes, yet about a third of a person’s risk of developing the condition remains unknown.
Experts from Edinburgh University now think that everyday factors such as traffic fumes and lack of sunshine could be the missing link, after conducting the first major review into research on risks caused by our surroundings.
However, the team insisted that there is not enough evidence to prove these factors directly cause the neurological condition, which affects around 90,000 Scots.
It comes as part of a growing understanding that a significant number of dementia cases could be prevented or at least delayed by cutting risk factors such as obesity and smoking.
Lead author Dr Tom Russ, of the university’s Alzheimer Scotland Dementia Research Centre, 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.
“We found that the evidence is particularly strong for air pollution and vitamin D deficiency. But we really need more research to find out whether these factors are actually causing dementia and how, and if so, what we can do to prevent this.”
Researchers examined dozens of studies from all over the world, including analysis of more than 19,000 women living in different parts of the United States, which suggested smog and other air pollutants may contribute to memory loss and dementia.
Another study of British people over 65 found those with very low vitamin D levels were more than twice as likely to develop the disease.
The Edinburgh research, published today in the journal BMC Geriatrics, also found that repeated exposure to certain pesticides and high levels of minerals in drinking water may also be linked to the disease.
Dr Russ said: “This is the first step towards trying to find out what the impact of environment factors is.
“For example, air pollution has been robustly linked to cardiovascular disease. It may well be that there is a vascuar link between air pollution and dementia.”
Dementia is a major public health concern, which costs the UK an estimated £26 billion each year. The condition is expected to affect more than 131 million people worldwide by 2050.
Dementia experts stressed that environmental factors represent only “a vanishingly tiny increased risk” of developing the disease.
Prof Robert Howard, professor of old age psychiatry and psychopathology at UCL, said: “There is robust evidence that head trauma and poor cardiovascular health increase dementia risk.
“But most of the environmental factors identified in this review probably represent no realistic increase or only a vanishingly tiny increased risk for dementia.
“If you want to avoid dementia, look after your heart and try to avoid getting knocked unconscious.”
His concerns were echoed by other scientists. Professor Tom Dening, a dementia expert at Nottingham University, 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.”