Exposure to air pollution may increase the risk of developing dementia, according to a review of prior research.
The new Harvard study is the latest look at a host of health issues — from dementia to heart disease and stroke — linked to pollutants such as fine particulate matter (PM2.5), as well as nitrogen oxide and nitrogen dioxide.
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The findings support the need to strengthen air quality policies in the United States, according to the authors.
"There really hadn't been, to my mind, a good synthesis of all of the data or all of literature that was out there and, in particular, literature that included some of the more recent studies that involved a slightly different approach to doing this study," said study co-author Marc Weisskopf. He is a professor of environmental epidemiology and physiology at the Harvard T.H. Chan School of Public Health, in Boston.
The researchers scanned more than 2,000 studies in their review, finding 51 that looked at an association between ambient air pollution and clinical dementia. Those studies were published within the past 10 years.
Sixteen studies met their criteria after they assessed for bias using something called the Risk of Bias in Non-Randomized Studies of Exposure (ROBINS-E). Weisskopf explained that with issues of environmental exposures, the biases can be subtle and don't lend themselves to easy metrics. ROBINS-E allows for a deeper assessment of the possibility of bias.
The meta-analysis found consistent evidence of an association between PM2.5 and dementia.
That was true even when the exposure was less than the current limit set by the U.S. Environmental Protection Agency (EPA) — 12 micrograms per cubic meter of air (μg/m3) — a level the EPA has announced it may lower.
The investigators found a 17% increase in risk for developing dementia for every 2 μg/m3 increase in average annual exposure to PM2.5.
Associations also existed between nitrogen dioxide and nitrogen oxide and dementia. For nitrogen oxide, there was a 5% increase in risk for every 10 μg/m3 in annual exposure. For nitrogen dioxide, it was a 2% increase in risk for every 10 μg/m3 increase in annual exposure.
Many theories exist for why dirty air might increase dementia risk. It may simply be because pollution affects cardiovascular health and that in turn affects brain health.
Another theory involves the impact of inflammation. The pollution may directly act on nerve cells or other cells of the brain, Weisskopf said, interfering with general brain function.
Although smoking is considered a bigger risk factor for dementia than air pollution, this could still have massive implications at a population level because everyone has to breathe, Weisskopf noted.
"It's unclear what the mechanisms are that lead to this connection, but it's hypothesized that the very small particles of pollutants enter our bodies and penetrate our circulatory system, which helps fuel the brain," said Rebecca Edelmayer, Alzheimer's Association senior director of scientific engagement. Edelmayer was not involved in the study.
She noted that the results should be interpreted with caution because of limitations when conducting a "meta-analysis" of observational studies.
"These data illustrate that there are many factors across the life course that can contribute to our risk of dementia, and this includes the environment. Some of these factors we can control, such as our diet and frequency of physical activity, but some are more difficult to control on an individual level, such as where we live and the quality of the air that surrounds us," Edelmayer said.
She called for action by federal and local governments, as well as businesses, to reduce air pollutants.
Weisskopf said that agencies like the EPA could use these results because they support the public health importance of strengthening limits on pollution.
One big uncertainty is when exposure to pollution matters most in terms of its association with dementia, Weisskopf noted.
"Most of these studies were looking in the year or a few years before the dementia onset, but it could be earlier in life, it could be across all of life. And so until we understand that better, these numbers are going to fluctuate a bit," he explained.
Americans could potentially reduce some of the pollution they take in by the choices they make, said Dr. Afif El-Hasan, a pediatrician with Kaiser Permanente in Southern California and a volunteer medical spokesperson with the American Lung Association. El-Hasan was not involved in this study.
"Obviously, we have to go out. As a pediatrician, I want people to exercise," he said.
El-Hasan suggested being aware of pollution levels, noting that many cities and states have websites offering information about that day's air quality or even variations at different times of day.
"If possible, and we know it's not always possible, take a look and schedule outdoor activities at times when there's less pollution, especially so if you have lung disease or other issues," he stressed.
Use a filter that cleans air in the house, he said. Don't smoke.
If the world is an unhealthy place, that may not only worsen medical issues, but will also have an economic impact, El-Hasan noted.
"I just hope that the study reminds everyone that we're very much affected by the world around us. It's not independent. It is long-term effects, and it can be devastating effects to our whole body," he added.
The study findings were published April 5 in BMJ.