Higher Dementia Incidence Linked to Air Pollution

by Bianca Garilli, ND

Dementia has become one of the major causes of disability and dependency in older people and is considered a worldwide public health priority.¹ According to the World Health Organization (WHO), 50 million people globally suffer from this cognitive decline syndrome, with nearly 10 million new cases occurring annually.¹ Although common in today’s society, dementia is not a guarantee, nor a normal part of the aging process. It typically affects older adults leading to symptoms associated with “memory, thinking, orientation, comprehension, calculation, learning capacity, language, and judgement,” leading to potential physical, psychological, social and economic impacts on the individual, their family, and society as a whole.¹

Dementia includes a variety of brain disorders each with a slightly different pathophysiology and presentation. Alzheimer’s disease (AD) is the most common form of dementia accounting for 60-80% of all cases. Vascular dementia (VD) is the 2^nd most common form and accounts for 10% of cases, while the remainder of dementia diagnoses make up all other cognitive decline conditions including dementia with Lewy bodies, mixed dementia, frontotemporal dementia, Creutzfeldt-Jakob Disease, Huntington’s disease, and normal pressure hydrocephalus.² 

In the US, AD is the 6^th leading cause of death across all age groups.³ In England and Wales, dementia accounts for 12% of all registered deaths and is the leading cause of death in these two countries⁴ whereas worldwide, according to WHO data, dementia is the 7^th leading cause of death.⁵

With these alarming numbers, researchers are turning their attention to elucidate potential causative factors. A systematic review found that approximately 1/3 of all AD cases may be associated with modifiable lifestyle factors and, “might be reduced through improved access to education and use of effective methods targeted at reducing the prevalence of vascular risk factors (i.e., physical inactivity, smoking, midlife hypertension, midlife obesity, and diabetes) and depression”.⁶

Environmental factors are also taking center stage in understanding the root causes of dementia. Air pollution has been linked to dementia-related outcomes potentially due to the exposure to nitrogen dioxide (NO₂) and high density of fine particles frequently found near major roads.^7-9 In fact, it is estimated that up to 6.1% of dementia cases may be attributed to elevated air pollution levels.^7-9 Traffic noise and noise pollution may also contribute to a significant number of cases.¹⁰

To learn more about the incidence of dementia related to residential levels of air and noise pollution in London, a retrospective cohort study using primary care data from 75 Greater London practices was conducted. Participants included 130,978 adults aged 50-79 years old with no history of dementia nor use of a residential care home.⁴

During the follow-up period of 8 years from 2005-2013 (average: 6.9 years) it was found that a total of 2181 patients (1.7%, incidence rate of 2.4 per 1000 per year) were diagnosed with dementia.⁴ Categorically, 39% were diagnosed with AD, 29% with VD, and 34% with a non-specific Read code (the standard clinical terminology system GPs use in the UK); 2.2% were diagnosed with both AD and VD.⁴ Similar to earlier reports, this study found new dementia diagnoses positively associated with levels of NO₂ and particulate matter with a median aerodynamic diameter < 2.5µM (PM_2.5).⁴ The associations between dementia and these elevated pollutant levels were most positively associated with the diagnoses of AD and non-specific diagnoses, such that individuals in the top 5^th  exposure category for NO₂ had the highest risk of AD diagnosis when compared to those in the bottom 5^th of exposure levels.⁴ It was further seen that all pollutant levels, except for ozone (O₃), declined with increasing distance from major roads, while nighttime noise levels were higher the closer one lived to major roads.⁴

According to the study’s analysis, the associations could not be explained by confounding and were found to be consistent within subgroups.⁴ There were, however, study limitations. One weakness is the lack of information surrounding the participants’ cumulative lifetime exposures to dementia-associated risks including, not just the immediate London air pollution, but pollution exposure from other locations that these individuals may have lived or been exposed to over their lifetime, as well as other modifiable lifestyle related factors unaccounted for in this study.⁴

Why is this Clinically Relevant?

• Air pollution exposure is a known risk factor for dementia
• Causes of dementia are multi-factorial, although a majority of the risk factors are associated with modifiable lifestyle changes
• Dementia risk factors are cumulative over a lifetime thus reinforcing the importance of a healthy lifestyle and reduction in toxin exposure, including air pollution at all stages of life
• Healthcare providers should screen for lifetime air pollution exposure when assessing for dementia related risks
• Living further away from major roads and highways reduces exposure to air pollutants, particularly NO₂ and PM₅, which are associated with increased risk for dementia

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Citations

1. WHO. Dementia. http://www.who.int/news-room/fact-sheets/detail/dementia. Accessed October 8, 2018.
2. WebMD. Alzheimer’s and dementia: what is the difference? https://www.webmd.com/alzheimers/guide/alzheimers-and-dementia-whats-the-difference#2. Accessed October 8, 2018.
3. Kochanek KD, Murphy SL, Xu JQ, Tejada-Vera B. Deaths: final data for 2014. Natl Vital Stat Rep. 2016;65(4):1-122.
4. Carey IM, Anderson HR, Atkinson RW, et al. Are noise and air pollution related to the incidence of dementia? A cohort study in London, England. BMJ Open. 2018;8:e022404.
5. WHO. Towards a dementia plan: a WHO guide. http://www.who.int/mental_health/neurology/dementia/en/. Accessed October 8, 2018.
6. Norton S, Matthews FE, Barnes DE, Yaffe K, Brayne C. Potential for primary prevention of AD: an analysis of population-based data. Lancet Neurol. 2014;13:788–794.
7. Power MC, Adar SD, Yanosky JD, Weuve J. Exposure to air pollution as a potential contributor to cognitive function, cognitive decline, brain imaging, and dementia: a systematic review of epidemiologic research. Neurotoxicology. 2016;56:235–253.
8. Chen H, Kwong JC, Copes R, et al. Living near major roads and the incidence of dementia, Parkinson’s disease, and multiple sclerosis: a population-based cohort study. Lancet. 2017;389:718–726.
9. Chen H, Kwong JC, Copes R, et al. Exposure to ambient air pollution and the incidence of dementia: a population-based cohort study. Environ Int. 2017;108:271-277.
10. Tzivian L, Jokisch M, Winkler A, et al. Associations of long-term exposure to air pollution and road traffic noise with cognitive function- an analysis of effect measure modification. Environ Int. 2017;103:30–38.