Toxic air pollution increases the risk of Alzheimer’s disease. The closer you live to major roads, the higher the chances that you’ll develop Alzheimer’s disease (AD). Oddly, traffic air pollution makes no difference to your risk for Parkinson’s disease (PD) or multiple sclerosis (MS). What a remarkable statement!! I love research of this kind; this is epidemiology at its best, so let me share some snippets from this project with you.
Start with a large geographic area, like Ontario, a province of Canada. Take two massive samples. For the multiple sclerosis group, try about 4.4 million people and get them before midlife (20-50 years of age). For the neurodegenerative disease cohort (AD & PD), take 2.2 million people but let them be older, between 55-85 years of age. Make sure they are all free of neurological disease and then track them for 11 years, and count the number of new cases (“incident”) of AD, PD and MS.
This is just what Chen et al did, identifying 243,611 incident cases of AD, 31,577 cases of PD, and 9247 cases of MS. Then, they related their data to how near people lived to major roads. They controlled for factors such as diabetes, brain injury, and income. Chen’s paper appeared in the Lancet in 2017.
Air Pollution Disease Gradient
Compared to people living more than 300 m from major roads, the adjusted hazard ratio (HR) for Alzheimer’s was 1.07 for people living less than 50 m from a major road, 1.04 for 50-100 m, 1.02 for 101-200 m, and 1.00 for 201-300 m. So, risk drops 7 – 4 – 2 – 0 as we move away from the traffic. That’s a gradient of specific disease defined by air pollution!
Fine Particulate Pollution
Probing the link between air pollution and dementia continues. In particular, there has been a focus on pollution due to fine particulate matter in the air. Of relevance, we define fine particulate matter as air particles with a diameter of 2.5 micrometers or smaller (PM2.5). That’s about 30 times smaller than the width of a human hair. Furthermore, these tiny particles get suspended in the air and can be inhaled deeply into the lungs, and even enter the bloodstream.
Increased Dementia Risk
Zhang et al published a study in 2023. They considered fine particulate matter (PM2.5) pollution from agriculture, traffic, coal combustion, and wildfires. Specifically, their sample was a large cohort of older adults in the U.S. They report an association between PM2.5 and increased dementia risk (adjusted hazard ratio (HR, of 1.08). The strongest associations are with PM2.5 are from agriculture (HR 1.13) and wildfires (HR 1.05).
Pollution in agriculture come from pesticides and herbicides, which can poison the brain. Additionally, ammonia released during fertilization and manure management is a source of pollution. Wildfires emit highly toxic particulates due to the burning of natural and synthetic materials.
Links to Epigenetics
A 2023 study by Li et al explored how tiny particles in polluted air, particularly from traffic emissions, could affect the brain and push up the risk of AD. They focused on DNA methylation, a process that can turn genes “on” or “off.” Specifically, they wanted to see if air pollution could change how certain brain genes work and promote Alzheimer’s. They found specific chemical changes in the brain’s DNA related to long-term exposure to traffic pollution (PM2.5). These changes trigger swelling and damage in the brain, like we see in AD.
In other words, certain genes affected by pollution play a role in brain inflammation, cell aging, and protecting brain connections. These DNA changes from air pollution help explain how pollution increases the risk of Alzheimer’s over time.
Implications
Now you need to ask yourself if it is time to move house!
