Air pollution is well known to be among the world's leading causes of death, but there is less awareness of its impact on people's capacity to think. Although many studies have found pollution can harm cognitive functioning, particularly in older populations, almost all have looked at the impact of long-term pollution exposure. New research shows the effect can be seen over a surprisingly short period, and at levels of pollution previously regarded as almost harmless.
The link between particulates in the air and lung disease isn't hard to see, but the connection to our thinking processes is less obvious. Nevertheless, past studies have shown exposure to air pollution over periods of many years is associated with dementia, and can even cause the brain to shrink.
Dr Xu Gao of Peking University wanted to see if more immediate effects could be detected. As part of the Veterans Affairs Normative Aging Study members of Gao's team visited 954 white men with an average age of 70 from the greater Boston area several times over the course of a month (28 days). The men were given two tests of their capacity to remember and reason: global cognitive function and Mini-Mental State Examination.
In Nature Aging, Gao and co-authors compared the men's scores with concentrations of PM2.5 (particles with diameters 2.5μg or less) at the time and over the preceding weeks.
Even over such short periods, the pollution levels had a significant effect on the men's scores on both tests. Even more concerning, the damage appears to kick in below the 10 μg m−3 level widely considered to be safe.
For comparison, India's average particulate concentration before the pandemic was 65 μg m−3, and much more in some cities, while most of the world is getting worse on this score. Even many wealthy countries, where pollution has been declining for decades as a result of clean air acts have average levels around or above the point where the study found the damage starts. Although the study was carried out on older white men, if there is nothing sex- or race-specific about the way particulates affect the brain, this is a problem that could affect most of us, not just those who live in pollution hot spots.
Guo's findings are not all gloom and doom, however. Besides the potential for falling rates of pollution in some regions to restore humanity's thinking power, he also found a remarkably simple way to reduce the damage. Men who were taking nonsteroidal anti-inflammatory drugs (usually aspirin) appeared to experience less harm from particulate exposure. The authors consider this evidence that PM2.5s do at least part of their damage by causing neuroinflammation.
Air pollution's harm to our thinking capacity isn't always small enough to be hard to detect. Previous research found older people exposed to high PM2.5 concentrations made 50 percent more mistakes on working memory tests than those used to breathing cleaner air, even after controlling for demographic factors.