Introduction

Population aging and cognition

Numerous advances in the last 100 years, principally in access to new technologies, goods and services, medical care, and expanded availability of medicines, have led to a major increase in the population’s life expectancy worldwide. This has made population aging one of the most important global transformations in human life. However, some nations, especially low and middle-income countries, will experience this increase in life expectancy (and consequently in the elderly population) at an unprecedented pace when compared to the same phenomenon in Europe and North America. These countries will thus have to prepare quickly to deal with a growing burden of highly common diseases at more advanced ages ¹1. National Institute of Health; National Institute of Neurological Disorders and Stroke; National Institute on Aging. The dementias: hope through research. Bethesda: National Institute of Health; 2013..

Currently, an estimated 12% of the global population is over 60 years of age, and projections show that the elderly will represent 21.5% of the world population by the end of this century. It will be the first time in human history that there will be more old people than young people ²2. World Health Organization. Ambient air pollution - a major threat to health and climate. Copenhagen: World Health Organization; 2018.. In this context, cognitive aging and the diseases resulting from this process pose a major challenge for healthy aging. Importantly, normal cognitive aging involves a natural and dynamic process that includes modifications in biological functioning (molecular, cellular, histological, and organic), psychiatric functioning (cognitive and psycho-affective dimensions), and social functioning ³3. Brito FC, Litvoc CJ. Conceitos básicos. In: Brito FC, Litvoc CJ, organizadores. Envelhecimento - prevenção e promoção de saúde. São Paulo: Atheneu; 2004. p. 1-16.^,44. Fechine BRA, Trompieri N. O processo de envelhecimento: as principais alterações que acontecem com o idoso com o passar dos anos. InterSciencePlace - Revista Científica Internacional 2012; 1:106-94.^,55. Günther IA. Envelhecimento, relações sociais e ambiente. In: Falcão DVS, Araújo LF, organizadores. Psicologia do envelhecimento. São Paulo: Alínea; 2011. p. 11-25.. Healthy aging can thus be accompanied by a certain decline in cognitive reserves, but which does not interfere substantially in daily activities. In other cases, when cognitive aging is not successful, there is a sharper decline in cognition, which may be linked to factors extrinsic to the individual, such as sedentary lifestyle, smoking, low schooling, alcohol and drug consumption, and exposure to air pollution.

Air pollutants and human health

In the last 40 years, studies on the impacts of air pollutants, especially in cities, are unanimous in associating greater length of individual exposure to air pollutants with elevated risk of developing inflammatory processes, diseases of the respiratory system, and cardiovascular problems. In most cases, the elderly and children are the most susceptible to developing these diseases. According to recommendations by the World Health Organization (WHO), various countries have adopted more rigorous limits on the concentration of air pollutants. However, even today, 91% of the world population lives in cities where the air quality exceeds the limits suggested by the WHO ²2. World Health Organization. Ambient air pollution - a major threat to health and climate. Copenhagen: World Health Organization; 2018.. As a consequence, in 2016, 8% of all deaths in the world were attributed to exposure to air pollutants, especially in large urban areas, where the main source of emissions is motor vehicle traffic. Collectively, air pollutants consist of a complex and diverse mixture of chemical substances with the capacity to cause harm or discomfort to humans. Such pollutants are thus now the second leading cause of deaths from noncommunicable diseases, after smoking, whereas the latter can be avoided by the individual, unlike exposure to environmental air pollutants. In Brazil, despite the data’s limitations, an estimated 66,000 yearly deaths are attributed to exposure to air pollutants annually ²2. World Health Organization. Ambient air pollution - a major threat to health and climate. Copenhagen: World Health Organization; 2018.. However, this number may be even higher, since projections for the next 15 years only for the State of São Paulo state that if nothing is done, exposure to air pollutants will be the cause of 250 thousand deaths, one million hospitalizations at a cost of 1.5 billion BRL only in the state of São Paulo ⁶6. Vormittag EMPA, Rodrigues CG, Miranda MJ, Cavalcante JA, Costa RR, Camargo CA, et al. Avaliação do impacto da poluição atmosférica no Estado de São Paulo sob a visão da saúde. São Paulo: Instituto Saúde e Sustentabilidade; 2013.^,77. Vormittag EMPA, Saldiva PHN. Qualidade do ar no Estado de São Paulo 2017. Sob a visão da saúde. São Paulo: Instituto Saúde e Sustentabilidade; 2013..

In relation to the harmful effects of air pollution on cognitive performance, a cohort study of 2.1 million persons in Ontario, Canada, showed a positive association between fine particulate matter (PM_2.5), even at low concentrations, and incidence of dementia ⁸8. Chen H, Kwong JC, Copes R, Hystad P, van Donkelaar A, Tu K, et al. Exposure to ambient air pollution and the incidence of dementia: a population-based cohort study. Environ Int 2017; 108:271-7.. In Sweden, a study of 1,806 adult volunteers found a positive association between air pollutants and increased incidence of dementia and Alzheimer’s disease ⁹9. Oudin A, Forsberg B, Adolfsson AN, Lind N, Modig L, Nordin M, et al. Traffic-related air pollution and dementia incidence in northern Sweden: a longitudinal study. Environ Health Perspect 2016; 124:306-12.. In Taiwan, two cohort studies, the first with 96,000 and the second with 30,000 volunteers, found that exposure to the pollutants nitrogen dioxide (NO₂), fine particulate matter (PM_2.5), ozone (O₃), and carbon monoxide (CO) was also associated with increased incidence of dementia ¹⁰10. Chang KH, Chang MY, Muo CH, Wu TN, Chen CY, Kao CH. Increased risk of dementia in patients exposed to nitrogen dioxide and carbon monoxide: a population-based retrospective cohort study. PLoS One 2014; 9:e103078.^,1111. Jung CR, Lin YT, Hwang BF. Ozone, particulate matter, and newly diagnosed Alzheimer's disease: a population-based cohort study in Taiwan. J Alzheimers Dis 2015; 44:573-84.. Taken together, these studies suggest that prolonged exposure to air pollutants, especially those produced by motor vehicle traffic in large cities, like particulate matter, show a positive association with cognitive decline and increased incidence of dementia ¹²12. Block ML, Calderón-Garcidueñas L. Air pollution: mechanisms of neuroinflammation and CNS disease. Trends Neurosci 2009; 32:506-16.^,1313. Calderón-Garcidueñas L, Reed W, Maronpot RR, Henríquez-Roldán C, Delgado-Chavez R, Calderón-Garcidueñas A, et al. Brain inflammation and Alzheimer's-like pathology in individuals exposed to severe air pollution. Toxicologic Pathology 2004; 32:650-8.. This association is biologically plausible, because once inside the body, these pollutants can trigger chronic and systemic inflammation, besides directly harming the nervous system ¹⁴14. Calderón-Garcidueñas L, Azzarelli B, Acuna H, Garcia R, Gambling TM, Osnaya N, et al. Air pollution and brain damage. Toxicologic Pathology 2002; 30:373-89.^,1515. Calderón-Garcidueñas L, Mora-Tiscareño A, Ontiveros E, Gómez-Garza G, Barragán-Mejía G, Broadway J, et al. Air pollution, cognitive deficits and brain abnormalities: a pilot study with children and dogs. Brain Cogn 2008; 68:117-27.^,1616. Tuppo E, Arias H. The role of inflammation in Alzheimer's disease. Int J Biochem Cell Biol 2005; 37:289-305.^,1717. Nunomura A, Castellani RJ, Zhu X, Moreira PI, Perry G, Smith MA. Involvement of oxidative stress in Alzheimer disease. J Neuropathol 2006; 65:631-41.^,1818. Genc S, Zadeoglulari Z, Fuss SH, Genc K. The adverse effects of air pollution on the nervous system. J Toxicol 2012; 2012:782462.^,1919. Numan M, Brown J, Michou L. Impact of air pollutants on oxidative stress in common autophagy-mediated aging diseases. Int J Environ Res Public Health 2015; 12:2289-205..

Finally, it is important to recall that before individuals present dementia per se, a smoldering process of cognitive impairment occurs, which is not sufficiently serious in the beginning to lead to functional loss and interfere in quality of life. However, this cognitive impairment is of great importance for early diagnosis of risk factors for dementia at more later ages ¹1. National Institute of Health; National Institute of Neurological Disorders and Stroke; National Institute on Aging. The dementias: hope through research. Bethesda: National Institute of Health; 2013.^,2020. Burlá C, Camarano AA, Kanso S, Fernandes D, Nunes R. Panorama prospectivo das demências no Brasil: um enfoque demográfico. Ciênc Saúde Colet 2013; 18:2949-56..

Timeliness of the study

Brazil and its Latin American neighbors are experiencing an accelerated and irreversible process of population aging. In addition, many countries have cities whose air pollutant emissions fail to comply with the WHO guidelines ²2. World Health Organization. Ambient air pollution - a major threat to health and climate. Copenhagen: World Health Organization; 2018.. Meanwhile, according to forecasts for 2030, 63% of the world population with dementia will be living in low and middle-income countries ¹1. National Institute of Health; National Institute of Neurological Disorders and Stroke; National Institute on Aging. The dementias: hope through research. Bethesda: National Institute of Health; 2013.. Thus, studies that seek to identify and understand possible environmental risk factors and their influence on the pathological cognitive aging process in young and middle-aged adults are highly important for avoiding such outcomes in the future. Research on chronic exposure to air pollutants and their role in neurodegenerative processes in young and elderly populations take on great importance in a scenario of growing life expectancy and high emissions of air pollutants.

Final remarks

The relationship between exposure to air pollutants and neurodegenerative outcomes is still not fully established, but it is already considered a global concern. Many countries, including most of the Latin American nations, still lack data and studies that seek to understand this association in their realities, which indicates a relevant gap. In the coming years, Brazil and other Latin American countries should thus make progress in their understanding of the association between air pollutants and cognitive outcomes, aimed at minimizing the impact of this association in order to provide individuals with healthier aging in urban settings.

Publication Dates

History