Dementia causes over 12% of deaths in several countries around the world. The two most common forms of dementia are vascular dementia (due to insufficient circulation to a part of the brain) and Alzheimer’s disease (AD).
Aging is the most well-established risk factor for AD, which is the most frequently diagnosed form of dementia. Other factors include having a family member with the condition, positive APOE-4 alleles, female gender, heart disease, head injury, Down syndrome, and low education.
Air pollution is now being incriminated in the causation of dementia.
Air Pollution and Aging
Air pollution acts synergistically with aging in promoting dementia. In addition, air pollution causes oxidative stress, which is a known risk factor for AD.
Thus it is logical to conclude that the long-term exposure to air pollution that occurs with aging is likely to result in an increased incidence of dementia in this subgroup.
Air Pollution and Dementia
High levels of air pollution lead to lower performance on human tests of cognitive skills over the course of time, but there is no evidence that these individuals presently have or will develop dementia.
On the other hand, people who live in highly polluted zones such as near very busy roads, for instance, are more likely to develop dementia, though this does not prove that pollution is the cause of the dementia.
However, several studies have determined that air pollution is the primary determinant of risk for dementia by excluding the possible contribution of noise, or stress, which are other factors present in polluted environments.
It is known that air pollution affects humans not only through the air but also because they can be deposited in water and soil, to be ingested or to offer skin contact.
Cooking over woodstoves, especially without proper chimneys or ventilation, is also an important risk factor for dementia, according to a Swedish study, which found a 70% increase in dementia incidence in the people who had the highest exposure to wood burning compared to those without a wood-burning stove.
Air Pollution and the Brain
Several independent animal studies have indicated a higher risk of amyloid plaque development in polluted surroundings. Cognitive changes also occurred in animals exposed to air pollution.
Air pollution contains gases, chemicals and metal particles in very fine form (fine particulate matter, when the size is below 1/40 of a human hair’s width). Some important pollutants include ozone, carbon and nitrogenous compounds. When these enter the lungs at high levels, the lung and heart health of the individual are affected. These particles are also known to enter the brain, though their effects are not completely known.
Air pollution often contains magnetite, and the subsequent course of these particles into the brain and their effects can be easily studied using their magnetic properties.
An interesting study has traced these magnetite particles, confirmed to be from engine emissions and not natural sources, to the interior of amyloid deposits. This is significant because amyloid is found in the brains of individuals with AD.
Much more work is needed to prove that magnetite induces or promotes the formation or growth of amyloid plaques, or that it contributes to brain neuronal damage in any way.
Another possibility could be that such particles are found inside the plaques as a natural waste disposal sink, to be segregated from the remaining brain matter.
Air pollution may also cause upregulation of genes which cause increased oxidative stress and inflammation, as well as cell proliferation and apoptosis. They may also promote DNA damage, and affect the strength and number of cellular microtubules. This may eventually cause oxidative stress leading to neuronal damage.
Air Pollution and Oxidative Stress
Dementia is known to be affected at least in part by oxidative stress. This occurs when there is overproduction of reactive oxygen species (ROS), exceeding the ability of the antioxidant systems of the body. Excess of ROS does occur in heavily polluted areas, which provides a biologically plausible explanation for the observed link between dementia and air pollution.
Oxidative stress affects multiple cell components, and especially mitochondrial DNA for several reasons such as the fact that free radicals are formed in the mitochondria, the DNA in this organelle is not protected by a histone coat completely, and the capacity for self-repair is limited.
Moreover, neurons are extremely differentiated cells and have a high oxygen consumption, which makes them very vulnerable to free radical mitochondrial damage. As a result, the brain is most severely affected by aging as a result of exposure to ROS, and such damage is permanent.
Dementia is known to be marked by oxidative biomarkers. The amyloid plaque could perhaps be a defensive mechanism against oxidative stress and resulting damage, but with time and increased damage, the abnormal proteins themselves become agents of oxidation, initiating a vicious downspin of progressive brain deterioration.
Air pollution is a factor in the development of dementia. The underlying mechanisms have been broadly suggested by current research. Future work could focus on relieving air pollution as a population-level measure to prevent or delay the onset of this insidious and heavy healthcare burden.