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Have you ever spent the day in a city with such bad air pollution that when you blew your nose the mucus had a black tinge? Have you ever coughed as you breathed in diesel fumes from a passing bus and thought to yourself, “Well, that’s a year gone from my life”?

Could it actually be true — that air pollution leads to an early death? The answer, in fact, is an unqualified yes.

Air pollution causes heart and lung disease; early death

It has been known for some time that air pollution causes lung cancer, chronic obstructive pulmonary disease, emphysema, asthma, heart disease and stroke. One recent study in China estimated that for those ages 75 and older, there are 1,166 early deaths for every 100,000 people — that’s more than 1%.

But if it doesn’t kill you outright, can air pollution impair your memory and cause dementia in general and Alzheimer’s disease (one cause of dementia) in particular?

Studies link air pollution to cognitive impairment

Three studies from three different parts of the world suggest that air pollution might cause cognitive impairment, dementia and Alzheimer’s disease.

In the first study, researchers from China and the U.S. teamed up to analyze data from China. They found that long-term exposure to air pollution is related to poor performance on both verbal and math tests. Moreover, the poor performance on the verbal tests was more pronounced for older individuals, especially for men and those less educated.

In the second study, researchers in England studied 130,978 adults ages 50 to 79 from 75 medical practices in greater London. They found that from 2005 to 2013, 2,181 older adults from this sample were 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 those given an Alzheimer’s disease diagnosis.

In the third study, published earlier this year, researchers from the United States, including the University of Southern California and Harvard Medical School, studied data from 998 women ages 73 to 87 who had both cognitive tests and MRI scans.

They found that those women who were exposed to higher concentrations of air pollution in the preceding three years showed two differences compared to those who were exposed to less air pollution. Cognitively, those exposed to more air pollution showed greater declines in learning a list of words. Anatomically, they showed more atrophy (shrinkage) in those areas of the brain that typically shrink due to Alzheimer’s disease.

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

Nonetheless, air pollution may not be the cause of the 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.

Thus, I firmly believe that the following statement is true: Higher levels of air pollution are associated with a greater risk of cognitive decline, dementia in general, and Alzheimer’s disease in particular.

However, that is not the same thing 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 provided possible mechanisms as to how that might happen. However, it is also possible that air pollution could be linked to some as-of-yet unidentified factor that explains the association.

For example, it has already been fairly well established that some viral illnesses are associated with Alzheimer’s disease. It has also been well established that viral illnesses are more likely to be transmitted when people are gathered together indoors versus outdoors.

So, it may simply be that where there is greater pollution, people are more likely to gather together inside, shut the windows, and trade viruses with each other. The new virus they acquire may be the real cause of the increased risk of Alzheimer’s disease.

Now, that’s just speculation — just an example of how a real association is not the same thing as evidence 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 the air pollution in our cities, our country and our planet. We should each do what we can to reduce our carbon footprint.

We can work to reuse and recycle materials so that factories don’t need to produce as much. We can buy local foods that don’t need to be trucked across the country and shipped around the world. We can walk and bike instead of driving our cars (and, once we’re done with COVID-19, carpool and take public transportation).

Lastly, we can elect public officials who will advocate for local, national and international policy to reduce pollution. Those are just some of the things that we can do to clear the air.

Andrew E. Budson, M.D., is a contributor to Harvard Health Publications.

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