Does exposure to pollution increase your risk of dementia?
Key Takeaways
Recent studies demonstrated that air pollution can increase dementia risk.
Although more research is needed, air pollution was found to potentially increase cardiac morbidity and inflammation—both of which contribute to cognitive aging.
At-risk patients should be counseled on means to avoid air pollution when levels are high.
The worldwide burden of dementia is on the rise, with 57 million people currently struggling with this condition. Consequently, there’s been heightened interest in identifying modifiable risk factors.
Established data support a significant link between air pollution and cognitive impairment, as well as stroke and heart disease risk. At the same time, decreasing exposure to air pollution has been shown to decrease mortality.
What does the most recent data say?
Over the years, researchers have used different strategies to study the relationship between ambient air pollution and the development of dementia. They have used different criteria to identify dementia cases, and varying methods to peg long-term exposure to ambient pollution and quantify any correlations.
Systematic reviews have either avoided pooling data from such studies or neglected to acknowledge this heterogeneity in study design.
In a 2023 systematic review and meta-analysis published in the BMJ, Harvard researchers aimed to shore up some of the shortcomings from prior research.[] Their approach was to utilize the Risk of Bias In Non-randomized Studies of Exposures (ROBINS-E) tool and incorporate several studies that used active case ascertainment approaches. Active case ascertainment refers to a process where professional abstractors identify cases by assessing multiple data sources, including log books, medical discharge records, and death records.[]
The researchers found that exposure to ambient particulate matter <2.5 µm in diameter (PM2.5) predicted a higher rate of dementia, as did exposure to NO2 and NOx (though to a lesser extent).
The associations were small, according to the authors, but align with data from other studies regarding the health consequences of ambient air pollution. These results also jibed with associations between air pollution and cardiovascular/respiratory mortality. The effect estimates are smaller vs other risk factors for dementia, such as education and smoking, but can still translate to substantial health implications in terms of population health.
Based on studies that employed active case ascertainment, the effect of a 2 μg/m3 higher concentration of PM2.5 represents a hazard ratio of 1.42, with a more conservative estimate of 1.17. As for biases, the authors suggest that they likely favored the null hypothesis.
“Our results strengthen the evidence that air pollutants are risk factors for dementia, further suggesting that efforts to reduce population exposures to these contaminants might help to reduce the personal, financial, and societal burden of dementia,” the authors wrote. “To some degree, this reduction can be done on a personal level and clinicians should communicate the risks of air pollutant exposures to their patients.” The authors also note that these findings show a need for changes in public policy, as they can provide regulatory agencies with a “best estimate for use in burden of disease estimation and regulation setting efforts, as well as inform summaries of risk factors for dementia.”
A 2023 cohort study funded by the NIH took this a step further by not only finding that PM2.5 exposure was associated with an increased risk of dementia, but also that agriculture and wildfire sources were the biggest offenders when it came to the source of the particulate matter.[][] Other sources examined included road and nonroad traffic, burning coal for energy, other energy production, windblown dust, and other industrial sources.
The authors publishing in JAMA Internal Medicine wrote that their evidence supports “reduction as a population-based approach to promote healthy cognitive aging.”
"These findings also indicate that intervening on key emission sources might have value, although more research is needed to confirm these findings."
— Authors, JAMA Internal Medicine
Underlying mechanisms
Experts have proposed various mechanisms explaining the association between air pollution and dementia. First, cardiovascular effects could increase the risk of dementia. Second, particulate matter exposure might mediate systemic inflammation and neuroinflammation, interfere with the blood–brain barrier, and change neurotransmitter levels, thus resulting in neuronal death.
Microglia may be most sensitive, with their toxic activation leading to aberrant synapse elimination in senescence that contributes to dementia.
How to advise your patients
The government website AirNow, which is home to the US Air Quality Index, offers insights into how your patients can reduce their exposure to PM2.5 when levels are high.[] These recommendations are particularly important among high-risk patients with heart or lung disease (eg, asthma and COPD); older adults; and children. Their recommendations include the following:
Staying indoors with filtered air
Keeping indoor activity levels low (eg, reading, watching tv)
Buying air filters for the home
Avoiding at-home products that burn (eg, candles, incense, firewood)
Closing windows
Not smoking
Wearing an N-95 or a P-100 respirator
The AAFP also urges caution for people who are susceptible to the adverse effects of air pollution.[]
“People with heart or lung disease react more severely to polluted air. During times of heavy pollution, their condition may worsen to the point that they must limit their activities or even seek additional medical care. In the past, a number of deaths have been associated with severely polluted conditions,” says the AAFP.
"Today, pollution this bad is rare in the United States."
— The AAFP
If outdoor activity is absolutely necessary for high-risk patients, the early morning hours or the hours after sunset are best to limit ozone exposure—something that is increased by sunshine.
What this means for you
Recent studies are providing more evidence that air pollutants are risk factors for dementia, and that efforts to reduce air pollution may help reduce the personal, financial, and societal burden of dementia. As air pollution also increases the risk of stroke and heart disease, physicians may want to counsel older patients, children, and those with heart/lung disease to limit their exposure to poor air quality. Staying indoors when pollution levels are high is an important step, as is equipping the home with air filters and keeping the windows closed.