Air pollution increases dementia risk

Metastudy shows links between particulate matter pollution and dementia

Air pollution has long been suspected of increasing the risk of dementia. Particulate matter pollution in particular is said to lead to a significantly increased risk of developing dementia. This also applies to air pollution levels that are far below the limit values applicable in the EU.

A meta-analysis of 16 studies now published in the “British Medical Journal” (“BMJ”) confirms this connection: According to the study, particle concentrations significantly below the limit values applicable in the EU also increase the risk of disease. The study underlines the urgency of reducing air pollution worldwide, also through political measures, according to a commentary in the journal.

More than 57 million people worldwide live with dementia and the number is rising, writes the team led by Marc Weisskopf of Harvard University in Boston. It has long been suspected that air pollution increases the risk of the disease – especially exposure to small fine dust particles with a diameter of up to 2.5 micrometres (thousandths of a millimetre; PM2.5).

To test this association, the team analysed 16 studies, mostly from North America and Europe. Nine of them used a more modern methodology that did not just cross-check information from data sets, but actively examined the participants. 14 studies focused on PM2.5 particulate matter, and several also included various nitrogen oxides and ozone (O3).

Higher air pollution, higher dementia risk – by up to 42 percent

The studies evaluated showed a higher risk of disease with increasing particulate matter pollution. On average, this risk increased by 4 percent with an increase in the average annual PM2.5 concentration of 2 micrograms per cubic metre. In those studies in which the participants were actively examined, the risk even increased on average by 42 percent per 2 micrograms per cubic metre. The study also found a connection to the risk of dementia – on a smaller data basis – for nitrogen oxides, but not for higher ozone levels.

The authors admit that their study cannot prove that particulate matter actually contributes to dementia. They emphasise, however, that the results strengthen the suspicion that particulate matter is already a cause for concern below the current limit values. The PM2.5 limit in the USA is an annual average of 12 micrograms per cubic metre, in Great Britain 20 micrograms per cubic metre and in the EU 25 micrograms per cubic metre.

Particulate matter is produced, among other things, in traffic by combustion engines, but also by tyre abrasion. Other sources include industry, power plants and wood stoves. It is true that the effect of air pollution on the risk of dementia is lower than that of smoking, for example – because cigarette smoke also contains fine dust. However, given the sheer number of people exposed to high levels of pollution, the consequences at the population level are substantial, the group writes.

Many biological connections conceivable

Several biological correlations could explain the effect of particulate matter on dementia, the group adds. For example, particulate matter is known to damage the cardiovascular system, and cardiovascular problems are considered risk factors for dementia. In addition, fine dust can damage the blood-brain barrier and promote inflammatory reactions in the brain, including the death of nerve cells. However, the team emphasises that it is difficult to prove that such mechanisms are involved in dementia in humans.

In a BMJ commentary, Andrew Sommerlad and Kathy Liu of University College London point to the drastic differences in particulate matter pollution in different cities: the annual mean value for PM2.5 in Toronto, for example, is less than 10 microgrammes per cubic metre, while in some cities, such as Delhi, it is more than 100 microgrammes. There are hardly any studies from those regions in Africa and Asia where particulate matter pollution is particularly high.

Many countries without air quality regulations

“Air pollution is also associated with other health effects and mortality,” write Sommerlad and Liu. They point to an estimate that more than 6.5 million deaths a year are caused by poor air quality. About 40 per cent of all countries have no regulations at all on air quality, the doctors complain, pointing to the World Health Organisation’s (WHO) goal of targeting particulate matter concentrations below 5 micrograms per cubic metre.

“Any positive effect on dementia and general health would be accompanied by an important effect on climate change and biodiversity,” the commentary continues.