Particulate Matter: Dementia Is In The Air

5. June 2015
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Anybody who lives on a busy street has to have concerns about his or her brain functions. Even a small increase in the concentration of particulate matter has a negative effect: silent brain infarcts are accumulating, which among other things increases the risk of dementia.

Life in the big city offers numerous advantages: diversified cultural lifestyle options, a good choice of schools, universities, workplaces, training opportunities and an almost inexhaustible range of sports and leisure activities. However, city air can also damage brain functions, as a recent study [Paywall] in the journal Stroke has demonstrated. Scientists at the Beth Israel Deaconess Medical Center in Boston have investigated the effects of particulate pollution in big cities on the human brain.

Brain scans show the relationship

It has long been known that air pollution is able to increase stroke risk. Researchers have now shown that long-term exposure to PM (particulate matter) can also damage human brain structures and in older adults can lead to impaired cognitive functions. With this in mind they analysed the MRI recordings of the brains of more than 900 over-60s and determined the volume of individual brain areas. This included the entire cerebral brain volume, which is considered a marker for age-related brain atrophy, and the volume of the hippocampus, which reflects the changes in the area of the brain which controls memories. Furthermore, the researchers recorded the volume of hyperintense activity in the white matter, which can be used as a measure for pathological changes and ageing processes. They searched for evidence of blood clots and small cerebral infarctions. Subjects suffering from dementia and those who had suffered a stroke were among those excluded.

Particulate matter makes the brain shrink

The data derived from the brain scans was taken by the researchers placed against the data relating to the subjects’ residential locations. The level of 2.5 micron (PM 2.5) particulate matter carried by the study participants in their bodies varied depending on how close they lived to major roads.

Info on particulate matter

Particulate matter refers to tiny particles in the air that are imperceptible to the naked eye. Only during certain weather conditions can one see dust in the form of a haze. The particles in the air do not immediately fall to the ground, but remain for a certain time in the atmosphere. Depending on the particle size the dust particulate matter is sub-categorised into so-called fractions: the label ‘less than PM10’ is used in reference to all dust particles whose aerodynamic diameter is under ten micrometers (that’s 10 millionths of a metre). One subset of this PM10 fraction denotes the finer particles whose aerodynamic diameter amounts to less than 2.5 microns. This is labelled “fine fraction”, or 2.5 (in contrast to the size range 2.5 to 10 microns, ie. “coarse fraction”). The smallest of them, with an aerodynamic diameter of less than 0.1 micrometer (ie 100 billionths of a meter) are the ultrafine particles, the German Federal Environment Agency explains.

Faster shrinking process

In actuality, fine dust seems to affect the human brain massively: subjects with higher particulate matter loads have on average a lower brain volume, their brains were subjected to faster shrinkage than the individuals physically carrying less particulate matter. In addition they suffered more silent brain infarcts. These involve the occurrence of small strokes which are not perceived by those affected. “These are startling results,” says study leader Wilker. “Silent infarcts are known to increase the risk of major strokes, but also of dementia, coordination problems and depression”.

Linear relationship recognised

As the scientists report, there even existed a linear relationship between stress and consequences: for every increase in particulate air pollution of two micrograms per cubic meter of air, the subjects had a 46 percent higher risk of silent strokes. In addition, the increase in particulate air pollution of two micrograms per cubic meter of air caused the brain volume of the subjects to shrink by an extent measurably equivalent to being one year older. The higher the pollution load, the “older” was the brain of the participant. The study suggests that there is already today sufficient particulate air pollution in most cities for these impacts to occur.

Fine particulate matter may arise from complex chemical reactions in the atmosphere or be caused by human activity. Human-made factors include motor vehicles (cars, trucks), thermal power stations, waste incinerators, furnaces and heaters in homes and raising livestock as well as certain industrial processes. In urban areas, road traffic in particular is a key source of particulate matter. This particulate matter makes its way into the air not only from engines (foremost from diesel engines), but also from brake and tire wear, as well as through the resuspension of dust on the road surface.

How does particulate matter work?

Firstly, harmful substances such as heavy metals or carcinogenic polycyclic aromatic hydrocarbons can accumulate on the surface of fine dust. What’s more the dust particles themselves represent a health risk: the smaller the dust particles, the greater the risk of becoming ill. Small particles in particular penetrate deeper into the airways than larger ones. This way they make it into areas where they are not expelled again with exhalation. Ultrafine particles may also penetrate the bloodstream via the alveoli and spread via the blood through the body. Precisely which processes in the brain are altered by particulate matter is something yet to be elucidated by further studies.

Fine dust is always harmful

As the WHO has established, there is no fine dust concentration level below which no harmful effect is to be expected. It’s not only briefly elevated levels which lead to adverse health effects; especially over the longer term the presence of lower concentrations have health-damaging effects. Fine particulate pollution should therefore always be as low as possible.

Original publication:

Long-Term Exposure to Fine Particulate Matter, Residential Proximity to Major Roads and Measures of Brain Structure [Paywall]
Elissa H. Wilker et al.; Stroke, doi: 10.1161/STROKEAHA.114.008348; 2015

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