Air pollution linked to dementia at lower levels than guidelines, study finds

A new analysis of studies of air pollution, published in the British Medical Journal, has linked tiny air pollutants to a heightened risk of dementia – at lower levels than previously thought.

While a number of studies have linked tiny particulate matter known as PM 2.5 with a range of illnesses including heart attacks, strokes and dementia, the latest study says damage to humans can occur at levels lower than US, EU and UK guidelines.

In Ireland, the largest concentrations of PM 2.5 come from solid fuel burning, rather than pollution from road traffic in urban areas.

The new analysis and systematic review of a number of ambient air pollution studies was undertaken by US researchers and funded by the Harvard Chan National Institute of Environmental Health Sciences Centre for Environmental Health.

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It comes after last month’s publication of a report from the Irish Heart Foundation which revealed that about 2,600 premature deaths a year can be attributed to air pollution – 1,700 in the Republic and 900 in Northern Ireland

Environmental and health experts in Ireland have long supported scientific findings that air pollution is “likely” to increase the risk of developing dementia and accelerated cognitive decline.

But the new research suggests PM2.5s could be damaging at lower levels than previously thought.

More than 57 million people worldwide are living with dementia and the global burden continues to increase. But interventions to delay or prevent the onset of dementia are scarce.

The team of US researchers set out to investigate the role of air pollutants in dementia risk, accounting for study differences that could influence findings.

Using scientific databases, they identified 51 studies reporting associations between air pollutants averaged over a year or more and dementia cases in adults.

After assessing study quality and risk of bias, they were able to include 16 studies in their main quantitative analysis, mostly from North America and Europe.

The results show that higher exposure to fine particulate pollution was associated with an increased risk of dementia.

In 14 studies that specifically examined the potential effects of PM2.5 on dementia, they found that for every 2 micrograms per cubic metre (µg/m3) increase in average annual PM2.5 concentration, the overall risk of dementia rose by 4 per cent.

Studies that actively assessed participants reported a stronger association between dementia risk and air pollution than studies using passive surveillance methods, such as electronic health records.

Among studies with active assessment, results suggested a 42 per cent greater risk of dementia for every 2 µg/m3 increase in average annual PM2.5 concentration. The most conservative estimate was a 17 per cent greater risk.

The results also suggest an elevated but small increase in dementia risk with exposure to nitrogen dioxide, 2 per cent for every 10 μg/m3 increase. An elevated risk for nitrogen oxide was found to be 5 per cent for every 10 μg/m3 increase, but this was based on more limited data.

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

The researchers said the findings suggest consistent evidence of an association between ambient air pollution and clinical dementia, particularly for PM2.5, even below the current US Environmental Protection Agency annual standard of 12 μg/m3, and well below the limits of the UK (20 μg/m3) and the European Union (25 μg/m3).

According to the researchers, these findings support the public health importance of limiting exposure to PM2.5 and other air pollutants and provide regulatory agencies and others with a best estimate of effect for use in policy deliberations.