Air pollution “might be a risk factor for dementia”, researchers have said.

Exposure to fine particulate matter (PM2.5) has been linked to an increased risk in a new study published in The BMJ.

Experts from the Harvard TH Chan School of Public Health in the US set out to examine the role ambient air pollution plays in dementia risk.

They specifically looked at 14 previous studies examining the link between dementia and PM2.5 exposure.

Their analysis suggested that for every two micrograms per cubic metre increase in average annual PM2.5 concentration, the overall risk of dementia rose by 4%.

More limited data suggested that exposure to nitrogen dioxide and nitrogen oxide might also be a risk factor.

The study did not find an association between ozone and dementia.

In a linked editorial, experts from University College London warned that air pollution has the “potential to substantially affect dementia risk”.

“Current estimates suggest that PM2.5 concentrations in major cities vary considerably from below 10 micrograms per cubic metre in some cities (eg, Toronto, Canada) to more than 100 micrograms per cubic metre in others (eg, Delhi, India), therefore, air pollution has the potential to substantially affect dementia risk globally,” they wrote.

They highlight that air pollution is also linked to an increased risk of other health conditions, with an estimated 6.5 million deaths attributable to air pollution each year.

“Although individuals can take steps to reduce their own personal exposure, for example, by remaining indoors on high air pollution days, this solution is impractical in the long term, so for many people, the risk is inescapable,” they added.

“(The) findings therefore add urgency to the need for effective policy measures to reduce air pollution globally.”

Commenting on the study, Dr Susan Mitchell, head of policy at Alzheimer’s Research UK, said: “Poor air quality is a significant public health issue, and this new review helps to cement the relationship between certain types of air pollution and dementia risk.

“But as individuals there’s little we can do about the air we breathe. So it’s vital the Government leads from the front in reducing air pollution and the resulting harm to our brain health.

“But what we have seen so far has fallen disappointingly short. In December, the Government missed a clear opportunity for decisive action on air pollution by setting an unambitious and inadequate target of 10 micrograms per cubic metre by 2040 – far less stringent than what Alzheimer’s Research UK and the World Health Organisation recommended.

“The Government should now ensure brain health is central to its Major Conditions Strategy and its wider ambitions for prevention, and invest in population-level interventions that have significant impact on air pollution.

“What remains to be uncovered is the ‘how’ – there are several biological explanations that could be behind the link between air pollution and dementia, and we echo the authors call for more research to better our understanding in this area.”