Environmental factors such as air pollution and a lack of vitamin D may raise the risk of developing dementia but more research is required, according to experts.

Pesticides and minerals found in drinking water are also on a new shortlist of possible causes of the mental illness.

Dr Rosa Sancho, head of research at Alzheimer's Research UK, said: "Dementia is one of the most feared conditions and public interest in what might influence dementia risk is understandably high.

"This new analysis of existing studies has identified a shortlist of environmental factors that could be associated with dementia risk, but strong evidence behind these factors is still lacking".

Dr Sancho continued: "Many of the factors identified in this study have only been observed in a single study, or multiple studies have presented a mixed picture, making it hard to draw firm conclusions.

"It is important to remember that association does not necessarily indicate causality, and that diseases like Alzheimer's have a complex mix of risk factors including age and genetics.

"Alzheimer's Research UK has been working with experts to identify key challenges in risk reduction research that could be tackled to boost the number of studies in this important area.

"We are working with researchers across the globe to develop new ways to support the most promising research into dementia risk reduction.

"This study indicated an association between vitamin D deficiency and increased dementia risk.

"While the reasons for this relationship are unclear, current government advice recommends that people consider a daily supplement of vitamin D, particularly if they are unlikely to gain enough through their diet and daily exposure to sunlight.

"Anyone concerned that they may be deficient in vitamin D should speak to their GP."

The study by the Alzheimer Scotland Dementia Research Centre at the University of Edinburgh is published in the journal BMC Geriatrics.