Does air pollution cause Alzheimer’s disease? – Harvard Health Blog

Have you ever spent the day in a city with such air pollution that when you blow your nose the mucus has a black tint? Have you ever coughed while inhaling diesel fumes from a passing bus and said to yourself, “Well, it’s been a year of my life”? Could this be true – that air pollution leads to premature death? The answer, in fact, is an unqualified yes.

Air pollution causes heart disease, lung disease and premature death

Air pollution has been known for some time to cause lung cancer, chronic obstructive pulmonary disease, emphysema, asthma, heart disease, and stroke. A recent study in China estimated that for people aged 75 and over, there were 1,166 premature deaths per 100,000 people, or more than 1%. But if that doesn’t kill you outright, can air pollution alter your memory and cause dementia in general and Alzheimer’s disease (a cause of dementia) in particular?

Studies from several countries link air pollution to cognitive impairment

Three studies conducted in three different regions of the world suggest that air pollution could lead to cognitive impairment, dementia and Alzheimer’s disease. In the first study, Chinese and American researchers teamed up to analyze data from China. They found that long-term exposure to air pollution is linked to poor performance on verbal and math tests. Additionally, poor performance on verbal tests was more pronounced among the elderly, especially men and the less educated.

In the second study, British researchers studied 130,978 adults aged 50 to 79 in 75 doctor’s offices in Greater London. They found that from 2005 to 2013, 2,181 older people in this sample had been diagnosed with dementia: 39% with Alzheimer’s disease, 29% with vascular dementia, and 32% without a specific dementia diagnosis. Adults living with the highest annual concentration of air pollution had the highest risk of dementia – 1.4 times the risk of those with the lowest annual concentration. They also found that these associations were more consistent for people diagnosed with Alzheimer’s disease.

In the third study, published earlier this year, researchers from the United States, including the University of Southern California and Harvard Medical School, looked at data from 998 women aged 73 to 87 who were tested cognitive and MRI scans. They found that women who had been exposed to higher concentrations of air pollution in the previous three years had two differences compared to those who were exposed to less air pollution. Cognitively, people exposed to greater air pollution showed greater declines in learning a word list. Anatomically, they showed more atrophy (shrinkage) in areas of the brain that typically shrink due to Alzheimer’s disease.

Importantly, in all three studies, the researchers controlled for all other possible factors that they thought could make a difference. For example, in this third study, they controlled for: socio-demographic factors (age, geographic region, race / ethnicity, education, income); lifestyle (smoking, alcohol, physical activity); employment status; clinical features (diabetes, high cholesterol, hypertension, cardiovascular disease, hormone therapy); and cerebrovascular disease measured by MRI.

Air pollution is associate with a greater risk of Alzheimer’s disease, but this may not be the cause of cognitive decline

The first thing to say is that I believe this correlation is real. The fact that three different groups analyzed data from three different continents and came to similar conclusions cannot be due to chance alone. So, I firmly believe the following statement is true: Higher levels of air pollution are associated with an increased risk of cognitive decline, dementia in general, and Alzheimer’s disease in particular.

However, this is not the same as saying that high levels of air pollution cause cognitive decline, dementia and Alzheimer’s disease. Air pollution could cause Alzheimer’s disease, and many researchers have provided possible mechanisms as to how this could happen.

However, it is also possible that air pollution is linked to an as yet unidentified factor that explains the association. For example, it is already fairly well established that certain viral diseases are associated with Alzheimer’s disease. It has also been well established that viral diseases are more likely to be transmitted when people are gathered indoors rather than outdoors. So it may simply be that where the pollution is greatest, people are more likely to congregate indoors, close windows, and swap viruses with each other. The new virus they acquire may be the real cause of the increased risk of Alzheimer’s disease. Now this is all speculation – just an example of how true association is not the same as proof of causation.

What can you do if you want to reduce air pollution to lower your risk of Alzheimer’s disease?

Directly or indirectly, we are all responsible for air pollution in our cities, our country and our planet. We all need to work to do what we can to reduce our carbon footprint. We can work to reuse and recycle materials so factories don’t need to produce as much. We can buy local food that doesn’t need to be trucked across the country and shipped around the world. We can walk and cycle instead of driving our cars (and, once we’re done with COVID, carpool and take public transportation). Finally, we can elect public officials who will advocate for a local, national and international pollution reduction policy. And these are just a few of the things we can do to clean the air.

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Jothi Venkat

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