The connection between air pollution and dementia

You probably know that inhaling air pollution increases your risk of respiratory and cardiovascular conditions like asthma, lung cancer, and heart disease. Now new research indicates a possible connection to brain diseases like dementia, as well.

Though a causal connection is yet to be established, the suspected link between dementia, which includes Alzheimer’s disease, and poor air quality hinges on the size of the pollutant air particles. In general, the smaller the particle, the more harmful it is to inhale.

How these ultrafine particles attack the brain, scientists aren't sure, but a study using lab mice at the University of Southern California found that these tiny particles can penetrate a mouse’s nasal cavities and reach the brain by traveling over neurons. This caused several inflammatory reactions—including the release of “tumor necrosis factor a,” which is found in higher quantities in the brains of humans with Alzheimer’s, as compared to those without. Similarly, a study of demented dogs in Mexico City found that they more often lived in polluted areas of the city and that their brains showed higher amounts of protein amyloid, the “plaque” associated with Alzheimer’s disease.

Other epidemiological studies examined humans with heavy exposure to microscopic airborne particles to assess the link between air quality and brain health. A review of 18 long term studies from multiple countries indicated an association between high exposure to one component of air pollution and a symptom of dementia. Similarly, an imaging study found a correlation between people who live near a major highway, with higher exposure to air pollution, and decreased brain volume. A third study of elderly women found that the total volume of white matter in the brain decreased with exposure to ultrafine particles. As these studies relate to indoor air quality, oil, gas, firewood, and cigarette smoke all produce ultrafine particulates that can be inhaled.

Again, these are not controlled studies in clinical conditions, and some people may be more susceptible to dementia than others based on genetic risk. But the evidence linking air pollution, ultrafine particles, and dementia is certainly compelling.