Dementia can begin developing in a person at any age.
Used to describe a group of conditions characterized by brain function impairment and memory loss, dementia is neurodegenerative. This means that people with the disease experience symptoms that gradually get worse over months, years or even decades due to continual brain nerve cell damage. Currently, recommendations for preventative measures against dementia include getting enough exercise, maintaining a nutritious diet, and partaking in activities that involve mental stimulation. Scientists have identified several risk factors for different forms of dementia, including old age and genetic predispositions. However, concrete causes of dementia have been harder to uncover. In recent years, it has been the interest of researchers in several studies and trials to investigate the impact environmental factors can have on a person’s risk of developing dementia. In a recent observational study, researchers examined the impact of air pollution exposure on dementia incidence in older adults in the United States, hoping to discover the extent of the relationship (Shi et al. 2021). This study is the first nationwide, population-based cohort study that focuses on the health effects of PM2.5, NO2 and O3 on dementia and Alzheimer’s disease (Shi et al. 2021).
Fine particulate matter, or PM2.5, along with Nitrogen Dioxide (NO2) and Ozone (O3) are some of the main pollutants that pose health concerns for humans if they are inhaled at high quantities over a long period of time. This is because ultrafine particles such as these are so small that, when breathed in, can move up the olfactory nerve into the brain directly, penetrate into the blood, and reach the blood-brain barrier (Oberdörster et al. 2004). When these particles reach the brain, they directly irritate the delicate tissue there (Pelley 2020). Furthermore, the brains of people with Alzheimer’s and related dementias (ADRD) typically have a high amount of iron, which is something that builds up with age. Excess amounts of iron in the brain induces inflammation and stress in cells and kills them, causing neurodegeneration and the common symptoms of dementia (Pelley 2020). Iron is also a frequent trace contaminant in air arising from combustion and vehicle emissions. This has led scientists to believe that air pollution exposure over a long period of time increases the buildup of iron in the brain and therefore is a contributing factor for the development of ADRD (Cory-Slechta et al. 2019). Overall, it is known that air pollution exposure clogs the brain with plaques, kills neurons, and warps the architecture of developing tissue (Pelly 2020).
Taking the previous knowledge from these studies into consideration, a study by Shi et al. aimed to find more concrete evidence for a relationship between dementia and air pollution exposure based on nationwide Medicare data and daily surface-level concentrations of air pollution across the US. This study was conducted by researchers at Emory University in Atlanta, Georgia with help from experts at the Georgia Institute of Technology, the University of Washington, Harvard University and the Fundação Getúlio Vargas in Brazil. Conducted in 2019, the study focused on historical air pollution and Medicare data from 2000-2018. It was published on November 19, 2021, in the journal of Nature Communications.
The “subjects” in this observational study consisted of all the Americans that were enrolled in the Medicare system from 2000-2018 who also had claims of an ADRD diagnosis. The researchers only included data from the subjects who had hospital and medical insurance and a five-year “clean” period after enrolling in Medicare in which they were not diagnosed with ADRD. This period was required to ensure that cases were diagnosed within 2000-2018, therefore making the incidence rate more accurate. Diagnosis of ADRD was determined by administrative records from inpatient and outpatient claims, doctor visits, nursing facilities and home-health care claims (Shi et al. 2021). This is an improvement upon previous studies that only measured incidence off hospitalizations, most likely missing many cases. In the study, there were 12.2 million subjects with dementia and 12.4 million subjects with Alzheimer’s. Subjects were 65 to 74 years old. To determine the amount of air pollution subjects were exposed to during the study period, exposure was assigned based on resident ZIP code. The levels of PM2.5, NO2 and O3 were measured in each ZIP code using a combination of historical ground observations, satellite data, computer models that simulated atmospheric composition, land use and meteorological data (Shi et al. 2021). This study improved upon other studies that only observed pollution levels in cities by also including rural areas.
The results of the study indicate that there is a positive association between exposure to particulate matter from air pollution and ADRD incidence. The study examined the risk of dementia associated with each of the particulates (PM2.5, NO2 and O3). It was found that if the average levels of PM2.5 were high in the five years leading to diagnosis, there was an increased risk of dementia. The same relationship was found with NO2, although it was a slightly smaller correlation. However, it was found that as average levels of O3 increased, dementia rates were not significantly affected. For both PM2.5 and NO2, a larger effect was found on Alzheimer’s incidence than dementia. This may have to do with the fact that dementia includes a wide range of diseases, all of which may not be affected by air pollution. The study also found that PM2.5 and NO2 have larger effects on dementia and Alzheimer’s incidence in urban areas. This makes sense as urban areas tend to have more emissions from vehicles.
Even though the study found an association between air pollution exposure and dementia, the researchers admit that they cannot determine the association to be causal because their study was observational. The researchers also admit that because administrative records were used to identify diagnoses of dementia, there is a risk that some subjects were not diagnosed correctly. For example, a subject could have been diagnosed with a different form of neurological disease when they actually had Alzheimer’s. Another limitation of the study is that air pollution exposure was assigned to subjects by ZIP code, which does not consider subjects’ lifestyles. For example, one individual could live closer to a highway than another person in the same ZIP code, which could lead to them having more exposure. However, these limitations are not significant enough to alter the credibility or ethicality of the study and its results.
For the conclusion of the study to be agreed upon holistically, further studies on the subject must be done. While this observational study is easily replicable, it would be beneficial to conduct experimental studies that run tests on actual participants and have control groups to better determine causality. It would also be beneficial to conduct observational and experimental studies on the topic in other countries to determine if different types of air pollution present across the world have different impacts on health, and if the results of this study hold up globally.
Whether or not a conclusion about the strength of the association between air pollution exposure and dementia can yet be determined from this study, it is still an important public health subject. Over 90% of the world’s population is chronically exposed to fine particulate air pollution at levels above the World Health Organization’s guideline (Pelley 2020). Yet, humans continue to use nonrenewable sources of energy that only make the problem worse. To protect yourself from air pollution, it is suggested that you use masks in heavily polluted areas and avoid walking along streets and highways. Yet, why have we let air pollution get so bad that we cannot feel safe breathing the air around us? It is of interest to humans around the globe to reduce harmful emissions in order to save our health and the health of our planet.
Bibliography
Cory-Slechta D, Sobolewski M, Marvin E, Conrad K, Merill A, Anderson T, Jackson B, Oberdörster G. 2019. The impact of inhaled ambient ultrafine particulate matter on developing brain: Potential importance of elemental contaminants. Toxicologic Pathology. 47(8):976-92. https://doi.org/10.1177/0192623319878400
Oberdörster G, Sharp Z, Atudorei V, Elder A, Gelein R, Kreyling W, Cox C. 2004. Translocation of inhaled ultrafine particles to the brain. Inhalation Toxicology. 16(6-7):437-45. Doi:10.1080/08958370490439597
Pelley J. 2020. How air pollution messes with our minds. Chemical and Engineering News. [Accessed 2022 Sept 16]. https://cen.acs.org/biological-chemistry/neuroscience/air-pollution-messes-minds/98/i21
Shi L, Steenland K, Li H, Lui P, Zhang Y, Lyles R, Requia W, Llango S, Chang H, Wingo T et al. 2021. A national cohort study (2000-2018) of long-term air pollution exposure and incident dementia in older adults in the United States. Nature Communications. (12:1-9). doi:https://doi.org/10.1038/s41467-021-27049-2
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