The causes of dementia are complex and often not well-understood. Only a small percentage of dementia cases can be explained by genetic mutations. In addition, 7 potentially modifiable risk factors have been identified: diabetes, midlife hypertension and obesity, smoking, depression, cognitive inactivity, and low educational attainment.
However, about one third of dementia remains unexplained. Research assessing geographical variation in dementia rates indicates that environmental risk factors may be of importance. In a 2016 systematic literature review Killin and colleagues analysed the existing literature to determine the role potential environmental factors may play. They identified a number of factors, of which the two most important ones appeared to be Vitamin D and air pollution.
Killin and colleagues found moderate evidence for air pollution exposures being related to dementia risk. In particular, they looked at three factors that contribute to air quality: nitrogen oxides, particulate matter and ozone.
Nitrogen oxide is one of the gases most relevant for air pollution, as it contributes to the formation of smog and acid rain, as well as ozone in the lowest layer of the earth’s atmosphere. Ozone (O3) is a colourless, unstable toxic gas formed from oxygen. Particulate matter refers to all the solid and liquid particles suspended in the air, many of which are hazardous. Some of these include for example dust, smoke, and pollen.
Studies have shown that poor air quality is bad for your brain. Although this can be affected by socio-economic status, even studies who take this into account show a clear association between air pollution and dementia. Similarly, it appears that there is a link between air pollution and heart disease. Some research has even shown that particles of air pollution can make it into the brain, which could potentially cause inflammation.
Air pollution is associated with reduced cerebral blood flow, and seems to be toxic to neurons. In children living in areas with high levels of air pollution, brain changes, including cognitive changes and biomarkers can be identified (see study here) .
In two cohort studies in Sweden and Taiwan, higher levels of nitrogen oxides were associated with an increased dementia risk. In a cross-sectional study of almost 6000 people in five Chinese provinces, increased exposure to environmental tobacco smoke was associated with an increased risk of severe dementia. A prospective Taiwanese study that followed almost 1 million people over a period of 10 years found an association between baseline ozone and incident Alzheimer’s Disease. Similarly, another Taiwanese study of 871 people examined particulate matter and ozone concentration, finding increased Alzheimer’s and vascular dementia risk with the highest particulate matter or ozone concentrations.
Overall, the evidence does seem to show that air pollution can increase one’s risk of developing dementia. However, this is only one of many environmental risk factors that has been identified, and no studies to date have shown that there is a causal relationship, i.e. that dementia is caused by air pollution. Further research is required to identify the significance of the relationship between air pollution and brain health.