New research has found that prolonged exposure to polluted air can lead to physical changes in the brain as well as learning and memory problems and even depression.  These findings could have significant and troubling implications for people who live and work in polluted urban areas.

Previous studies have shown the damaging effects of polluted air on the heart and lungs but researchers at Ohio State’s Department of Neuroscience working with colleagues in the university’s Davis Heart and Lung Research Institute have now shown that polluted air is detrimental to the brain.  The study, conducted using mice, is published in this week’s journal Molecular Psychiatry.

In earlier work with mice, the Ohio research group found that fine air particulate matter causes widespread inflammation in the body which could be linked to high blood pressure, diabetes and obesity – common chronic health problems experienced by industrialised populations worldwide.  The new research takes the work a step further and opens up the possibility of  a link between air pollution and the burgeoning numbers of people experiencing problems with their brains.

In the Ohio study mice were exposed to either filtered air or polluted air for 6 hours a day, 5 days a week for 10 months – that is nearly half the lifespan of the mice.

The polluted air contained fine particulate matter, the kind of pollution created by cars, factories and natural dust.  The fine particulates are tiny, about 2.5 micrometers in diameter, or about 1/30th of the width of a human hair.  These particles can reach deep into areas of the lungs and other organs of the body.

The concentration of particulate matter that the mice were exposed to was equivalent to what people might be exposed to in some polluted towns and cities.

After 10 months exposure to the polluted or filtered air the mice underwent a variety of behavioural tests.

In a learning and memory test, the mice were placed in the middle of a brightly lit arena and given 2 minutes to find an escape hole leading to a dark box where they feel more comfortable.  They were given 5 days of training to locate the escape hole but the mice who breathed the polluted air took longer to learn where the escape hole was located.  The mice exposed to polluted air were also less likely to remember where the escape hole was when tested later.

In another experiment, mice exposed to the polluted air showed more depressive-like behaviours than did the mice that breathed the clean filtered air.  The polluted-air mice also showed signs of higher levels of anxiety-like behaviours.

In order to try to understand how air pollution could lead to the observed changes in learning, memory and mood the researchers went on to examine the hippocampus in the brain, a key area associated with learning, memory and depression.

They found distinct physical differences in the hippocampuses of mice who were exposed to the polluted air compared to those who were not.

The researchers looked specifically at the branching structures called dendrites at the ends of nerve cells ( neurones).  The dendrites have small projections on them called spines, which transmit signals from one nerve cell to another.

Mice exposed to the polluted air had fewer spines in parts of the hippocampus, shorter dendrites and overall reduced cell complexity.  Previous research has shown that these types of changes are linked to decreased learning and memory abilities.

In earlier work researchers from the Ohio group found that chronic exposure to polluted air leads to widespread inflammation in the body which is linked to a range of different health problems in humans.  This new study found evidence of low-grade inflammation in the hippocampus.

In the mice that breathed the polluted air, chemical messengers that cause inflammation, called pro-inflammatory cytokines, were more active in the hippocampus than they were in the mice who breathed the filtered air.  The hippocampus is particularly sensitive to damage caused by inflammation.

This work was conducted in mice and while it is unwise to extrapolate the findings of animal work to humans it is hard to ignore the potential significance these findings may have for humans.  Clearly, we need to better understand the effects of breathing polluted air on the human brain and in particular, the effects of polluted air on learning and memory in children as well as the longer term consequences i.e., the risk in later life of developing cognitive impairment and a raft of other neurological conditions.

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