Gerontology

Gerontology (from the Greek γέρων, geron, "old man" and -λογία, -logia, "study of"; coined by Ilya Ilyich Mechnikov in 1903) is the study of the social, psychological, cognitive, and biological aspects of aging. It is distinguished from geriatrics, which is the branch of medicine that specializes in the treatment of existing disease in older adults. Gerontologists include researchers and practitioners in the fields of biology, nursing, medicine, criminology, dentistry, social work, physical and occupational therapy, psychology, psychiatry, sociology, economics, political science, architecture, geography, pharmacy, public health, housing, and anthropology.[1]

Gerontology encompasses the following:


The multidisciplinary nature of gerontology means that there are a number of subfields, as well as associated fields such as psychology and sociology that overlap with gerontology. Gerontologists view aging in terms of four distinct processes: chronological aging, biological aging, psychological aging, and social aging.[1] Chronological aging is the definition of aging based on a person's years lived from birth.[1] Biological aging refers to the physical changes that reduce the efficiency of organ systems.[1] Psychological aging includes the changes that occur in sensory and perceptual processes, cognitive abilities, adaptive capacity, and personality.[1] Social aging refers to an individual's changing roles and relationships with family, friends, and other informal supports, productive roles and within organizations.[1]

History

In the medieval Islamic world, several physicians wrote on issues related to Gerontology. Avicenna's The Canon of Medicine (1025) offered instruction for the care of the aged, including diet and remedies for problems including constipation.[2] Arabic physician Ibn Al-Jazzar Al-Qayrawani (Algizar, c. 898–980) wrote on the aches and conditions of the elderly (Ammar 1998, p. 4).[3] His scholarly work covers sleep disorders, forgetfulness, how to strengthen memory,[4][5] and causes of mortality[6] Ishaq ibn Hunayn (died 910) also wrote works on the treatments for forgetfulness (U.S. National Library of Medicine, 1994).[7]

While the number of aged humans, and the life expectancy, tended to increase in every century since the 14th, society tended to consider caring for an elderly relative as a family issue. It was not until the coming of the Industrial Revolution that ideas shifted in favor of a societal care-system. Care homes for the aged emerged in the 19th century.

Some early pioneers, such as Michel Eugène Chevreul, who himself lived to be 102, believed that aging itself should be a science to be studied. Élie Metchnikoff coined the term "gerontology" c. 1903[8]

It was not until the 1940s, however, that pioneers like James Birren began organizing gerontology into its own field. Recognizing that there were experts in many fields all dealing with the older population, it became apparent that a group like the Gerontological Society of America (founded in 1945) was needed. Two decades later, James Birren was appointed as the founding director of the first academic research center devoted exclusively to the study of aging, the Ethel Percy Andrus Gerontology Center[9] at the University of Southern California.[10][11] The Baltimore Longitudinal Studies of Aging began in 1958 in order to study physiological changes in healthy middle-aged and older men living in the community by testing them every two years on numerous physiological parameters.[1] In 1967, the University of South Florida[12] and the University of North Texas (formerly North Texas State University) received Older Americans Act training grants from the U.S. Administration on Aging to launch the nation's first degree programs in gerontology, at the master's level. In 1975, the University of Southern California's Leonard Davis School of Gerontology,[13] with Birren as its founding dean, became the country's first school of gerontology within a university and, later, offered the first PhD in Gerontology degree. Since that time, a number of other universities have formed departments or schools of gerontology or aging studies.

More generally, gerontological education has flourished in the United States since 1967 and degrees at all academic levels are now offered by a number of colleges and universities.[14] One of the pioneering gerontologists, Robert Neil Butler, has pushed for care and respect of the elderly. Butler won a Pulitzer Prize for his book titled, Why Survive? Being Old in America, where he discusses how the elderly are overlooked, mistreated, and sometimes even abused. His book argues that we as a society must modify our behavior toward the elderly. Several university-based centers on aging have been founded such as the Duke University Center on Aging, the University of Georgia Institute of Gerontology, the Center of Aging at the University of Chicago,[15] and the Stanford Center on Longevity.[1] Relatively few universities offer a PhD in gerontology. A Certificate in Aging Studies and Master of Science degree are available online and in-class through Virginia Commonwealth University's Department of Gerontology. Currently, PhD programs in gerontology are available at Virginia Commonwealth University (PhD in Health Related Sciences/Gerontology track), Miami University,[16] the University of Kansas,[17] University of Kentucky,[18] University of Maryland Baltimore,[19] University of Massachusetts Boston,[20] and the University of Southern California.[21] The substantial increase in the aging population in post-industrial Western nations has led to this becoming one of the most rapidly growing fields.

From the 1950s to the 1970s, the field was mainly social and concerned with issues such as nursing homes and health care. However, research by Leonard Hayflick in the 1960s (showing that a cell line culture will only divide about 50 times) helped lead to a separate branch, biogerontology. It became apparent that simply treating aging was not enough. Developing an understanding of the aging process, and what could be done about it, became an issue.

Biogerontology was also bolstered when research by Cynthia Kenyon and others demonstrated that life extension was possible in lower life forms such as fruit flies, worms, and yeast. Substantial increases in lifespan have been achieved in laboratory mammals such as mice and rats through interventions such as Calorie restriction and mutations in the insulin-like growth factor-1 pathway. So far, however, nothing more than incremental (marginal) increases in life span have been seen in humans.

Aging demographics

The world is forecast to undergo rapid population aging in the next several decades. In 1900, there were 3.1 million people aged 65 years and older living in the United States. However, this population continued to grow throughout the 20th century and reached 31.2, 35, and 40.3 million people in 1990, 2000, and 2010, respectively. Notably, in the United States, the "baby boomer" generation began to turn 65 in 2011. Recently, the population aged 65 years and older has grown at a faster rate than the total population in the United States. The total population increased by 9.7%, from 281.4 million to 308.7 million, between 2000 and 2010. However, the population aged 65 years and older increased by 15.1% during the same period.[22] It has been estimated that 25% of the population in the United States and Canada will be aged 65 years and older by 2025. Moreover, by 2050, it is predicted that, for the first time in United States history, the number of individuals aged 60 years and older will be greater than the number of children aged 0 to 14 years.[23] Those aged 85 years and older (oldest-old) are projected to increase from 5.3 million to 21 million by 2050.[24] Adults aged 85–89 years constituted the greatest segment of the oldest-old in 1990, 2000, and 2010. However, the largest percentage point increase among the oldest-old occurred in the 90- to 94-year-old age group, which increased from 25.0% in 1990 to 26.4% in 2010.[22]

With the rapid growth of the aging population, social work education and training specialized in older adults and practitioners interested in working with older adults are increasingly in demand[25][26] In the last decade, geriatric social work education, practice, and research have received substantial support from foundations such as the John. A Hartford Foundation, Robert Wood Johnson Foundation, and Atlantic Philanthropies.[27]

Of the roughly 150,000 people who die each day across the globe, about two thirds—100,000 per day—die of age-related causes.[28] In industrialized nations, the proportion is much higher, reaching 90%.[28]

Gender differences with age

There has been a considerable disparity between the number of men and women in the older population in the United States. In both 2000 and 2010, women outnumbered men in the older population at every single year of age (e.g., 65 to 100 years and over). The sex ratio, which is a measure used to indicate the balance of males to females in a population, is calculated by taking the number of males divided by the number of females, and multiplying by 100. Therefore, the sex ratio is the number of males per 100 females. In 2010, there were 90.5 males per 100 females in the 65-year-old population. However, this represented an increase from 1990 when there were 82.7 males per 100 females, and from 2000 when the sex ratio was 88.1. Although the gender gap between men and women has narrowed, women continue to have a greater life expectancy and lower mortality rates at older ages relative to men. For example, the Census 2010 reported that there were approximately twice as many women as men living in the United States at 89 years of age (361,309 versus 176,689, respectively).[22] The age-related changes males experience is likely to progress faster than the age-related changes in which females experience. Neural systems associated with behavioral domains identify age effects on behavior and brain parameters, which indicate more pronounced age-related changes in men than in women.[29] Research and studies have found strong evidence of ovarian hormones playing a key role in mediating behavior and brain function.

Geographic distribution of older adults

The number and percentage of older adults living in the United States vary across the four different regions (Northeast, Midwest, West, and South) defined by the United States census. In 2010, the South contained the greatest number of people aged 65 years and older and 85 years and older. However, proportionately, the Northeast contains the largest percentage of adults aged 65 years and older (14.1%), followed by the Midwest (13.5%), the South (13.0%), and the West (11.9%). Relative to the Census 2000, all geographic regions demonstrated positive growth in the population of adults aged 65 years and older and 85 years and older. The most rapid growth in the population of adults aged 65 years and older was evident in the West (23.5%), which showed an increase from 6.9 million in 2000 to 8.5 million in 2010. Likewise, in the population aged 85 years and older, the West (42.8%) also showed the fastest growth and increased from 806,000 in 2000 to 1.2 million in 2010. It is worth highlighting that Rhode Island was the only state that experienced a reduction in the number of people aged 65 years and older, and declined from 152,402 in 2000 to 151,881 in 2010. Conversely, all states exhibited an increase in the population of adults aged 85 years and older from 2000 to 2010.[22]

Biogerontology

Biogerontology is the sub-field of gerontology concerned with the biological aging process, its evolutionary origins, and potential means to intervene in the process. It involves interdisciplinary research on biological aging's causes, effects, and mechanisms. Conservative biogerontologists such as Leonard Hayflick have predicted that the human life expectancy will peak at about 92 years old,[30] while others such as James Vaupel have predicted that in industrialized countries life expectancies will reach 100 for children born after the year 2000.[31] and some surveyed biogerontologists have predicted life expectancies of two or more centuries.[32] with Aubrey de Grey offering the "tentative timeframe" that with adequate funding of research to develop interventions in aging such as Strategies for Engineered Negligible Senescence, "we have a 50/50 chance of developing technology within about 25 to 30 years from now that will, under reasonable assumptions about the rate of subsequent improvements in that technology, allow us to stop people from dying of aging at any age," leading to life expectancies of 1,000 years.[33]

Biomedical gerontology, also known as experimental gerontology and life extension, is a sub-discipline of biogerontology that endeavors to slow, prevent, and even reverse aging in both humans and animals. By preventing senescent changes in animals, these therapies would prevent the onset of age-related disease and frailty, extending both average and maximum lifespan. Most "life extensionists" believe the human life span can be increased within the next century, if not sooner. Optimists such as Aubrey de Grey estimate that the first person to live to a thousand years may have already been born. Biogerontologists vary in the degree to which they focus on the study of the aging process as a means of mitigating the diseases of aging or extending lifespan, although most agree that extension of lifespan will necessarily flow from reductions in age-related disease and frailty, although some argue that maximum life span cannot be altered or that it is undesirable to try.

In contrast with biogerontology, geriatrics studies the treatment of disease in aging people. Both fields are considered by many scientists to be the most important medical frontiers in aging research.

Biological theories of aging

Theories of aging are numerous and no one theory has been accepted. There is a wide spectrum of the types of theories for the causes of aging with programmed theories on one extreme and error theories on the other. Regardless of the theory, a commonality is that as humans age, functions of the body decline.[23]

Wear and tear

Wear and tear theories of aging suggest that as an individual ages, body parts such as cells and organs wear out from continued use. Wearing of the body can be attributable to internal or external causes that eventually lead to an accumulation of insults which surpasses the capacity for repair. Due to these internal and external insults, cells lose their ability to regenerate, which ultimately leads to mechanical and chemical exhaustion. Some insults include chemicals in the air, food, or smoke. Other insults may be things such as viruses, trauma, free radicals, cross-linking, and high body temperature.[23]

Genetic

Genetic theories of aging propose that aging is programmed within each individual's genes. According to this theory, genes dictate cellular longevity. Programmed cell death, or apoptosis, is determined by a "biological clock" via genetic information in the nucleus of the cell. Genes responsible for apoptosis provide an explanation for cell death, but are less applicable to death of an entire organism. An increase in cellular apoptosis may correlate to aging, but is not a 'cause of death'. Environmental factors and genetic mutations can influence gene expression and accelerate aging. More recently epigenetics have been explored as a contributing factor. The epigenetic clock, which objectively measures the biological age of cells and tissues, may become useful for testing different biological aging theories.[34]

General imbalance

General imbalance theories of aging suggest that body systems, such as the endocrine, nervous, and immune systems, gradually decline and ultimately fail to function. The rate of failure varies system by system.[23]

Accumulation

Accumulation theories of aging suggest that aging is bodily decline that results from an accumulation of elements. Elements can be foreign and introduced to the body from the environment. Other elements can be the natural result of cell metabolism.[23] An example of an accumulation theory is the Free Radical Theory of Aging. According to this theory, byproducts of regular cell metabolism called free radicals interact with cellular components such as the cell membrane and DNA and cause irreversible damage.[35] A more recent and comprehensive accumulation theory by Dr. Aubrey de Grey posits that aging is the consequence of the accumulation of 7 types of 'damage' at the molecular, cellular and intracellular levels.[36]

The free radical theory of aging

The idea that free radicals are toxic agents was first proposed by Rebeca Gerschman and colleagues.[37] In 1956, Denham Harman proposed the free-radical theory of aging and even demonstrated that free radical reactions contribute to the degradation of biological systems.[38] Oxidative damage of many types accumulate with age, such as oxidative stress that oxygen-free radicals,[39] because the free radical theory of aging argues that aging results from the damage generated by reactive oxygen species (ROS).[40] ROS are small, highly reactive, oxygen-containing molecules that can damage a complex of cellular components such as fat, proteins, or from DNA, they are naturally generated in small amounts during the body's metabolic reactions. These conditions become more common as we age, including diseases related to aging, such as dementia, cancer and heart disease. The Free Radical Theory of Aging is not accepted by everyone because we cannot determine if the free-radicals or the aging came first and if it is why the human body ages. Reasonably, the damage done by free radical may only begin after the aging process does. Future research must be conducted to see if one of the solutions to stabilize the free-radicals may postpone aging.

The DNA Damage Theory of Aging

DNA damage has been one of the many causes in diseases related to aging. The stability of the genome is defined by the cells machinery of repair, damage tolerance, and checkpoint pathways that counteracts DNA damage. One hypothesis proposed by Gioacchino Failla in 1958[41] is that damage accumulation to the DNA causes aging. The hypothesis was developed soon by physicist Leó Szilárd.[42] This theory has changed over the years as new research has discovered new types of DNA damage and mutations, and several theories of aging argue that DNA damage with or without mutations causes aging.[43] If DNA damage and other stressors can be conserved, the anticancer and survival responses from cells may boost defenses that maintain the integrity of the cell. This may improve health and extend the lifespan. The importance of DNA damage and the genome maintenance in relation to aging can be investigated further more for treatment.

Social gerontology

Social gerontology is a multi-disciplinary sub-field that specializes in studying or working with older adults.

Social gerontologists may have degrees or training in social work, nursing, psychology, sociology, demography, or other social science disciplines. Social gerontologists are responsible for educating, researching, and advancing the broader causes of older people.

Because issues of life span and life extension need numbers to quantify them, there is an overlap with demography. Those who study the demography of the human life span differ from those who study the social demographics of aging.

Social work with older adults

Social work with older adults, known as geriatric social work practice, is considered to be both a macro and micro practice with individuals over the age of 60 or 65, their families and communities, aging related policy, and aging research. Geriatric social workers typically provide counseling, direct services, care coordination, community planning, and advocacy in an array of agencies and organizations including private practice, in home, neighborhoods, hospitals, senior congregate living, hospice/end of life care, senior centers, oncology centers and residential long term care facilities such as nursing facilities. At the macro level, geriatric social workers work within state departments of health, adult protective services, and at universities and colleges, as well as Administration on Aging offices on a federal level in the United States.

Social theories of aging

According to Dannefer, aging is an interactive process where the individual is affected by the environment while also influencing the environment in which he/she ages. Several theories of aging are developed to observed the aging process of older adults in society as well as how these processes are interpreted by men and women as they age.[44]

Activity theory

Activity theory was developed and elaborated by Cavan, Havighurst, and Albrecht. According to this theory, older adults' self-concept depends on social interactions. In order for older adults to maintain morale in old age, substitutions must be made for lost roles. Examples of lost roles include retirement from a job or loss of a spouse.[44]

Activity is preferable to inactivity because it facilitates well-being on multiple levels. Because of improved general health and prosperity in the older population, remaining active is more feasible now than when this theory was first proposed by Havighurst nearly six decades ago. The activity theory is applicable for a stable, post-industrial society, which offers its older members many opportunities for meaningful participation. Weakness: Some aging persons cannot maintain a middle-aged lifestyle, due to functional limitations, lack of income, or lack of a desire to do so. Many older adults lack the resources to maintain active roles in society. On the flip side, some elders may insist on continuing activities in late life that pose a danger to themselves and others, such as driving at night with low visual acuity or doing maintenance work to the house while climbing with severely arthritic knees. In doing so, they are denying their limitations and engaging in unsafe behaviors.[45]

Disengagement theory

Disengagement theory was developed by Cumming and Henry. According to this theory, older adults and society engage in a mutual separation from each other. An example of mutual separation is retirement from the workforce. A key assumption of this theory is that older adults lose "ego-energy" and become increasingly self-absorbed. Additionally, disengagement leads to higher morale maintenance than if older adults try to maintain social involvement. This theory is heavily criticized for having an escape clause - namely, that older adults who remain engaged in society are unsuccessful adjusters to old age.[44]

Gradual withdrawal from society and relationships preserves social equilibrium and promotes self-reflection for elders who are freed from societal roles. It furnishes an orderly means for the transfer of knowledge, capital, and power from the older generation to the young. It makes it possible for society to continue functioning after valuable older members die. Weakness: There is no base of evidence or research to support this theory. Additionally, many older people desire to remain occupied and involved with society. Imposed withdrawal from society may be harmful to elders and society alike. This theory has been largely discounted by gerontologists.[45]

Continuity theory

Continuity theory is an elusive concept. On the one hand, to exhibit continuity can mean to remain the same, to be uniform, homogeneous, unchanging, even humdrum. This static view of continuity is not very applicable to human aging. On the other hand, a dynamic view of continuity starts with the idea of a basic structure which persists over time, but it allows for a variety of changes to occur within the context provided by the basic structure. The basic structure is coherent: It has an orderly or logical relation of parts that is recognizably unique and that allows us to differentiate that structure from others. With the introduction of the concept of time, ideas such as direction, sequence, character development, and story line enter into the concept of continuity as it is applied to the evolution of a human being. In this paper, a dynamic concept of continuity is developed and applied to the issue of adaptation to normal aging.[46]

A central premise of continuity theory is that, in making adaptive choices, middle-aged and older adults attempt to preserve and maintain existing internal and external structures and that they prefer to accomplish this objective by using continuity (i.e., applying familiar strategies in familiar arenas of life). In middle and later life, adults are drawn by the weight of past experience to use continuity as a primary adaptive strategy for dealing with changes associated with normal aging. To the extent that change builds upon, and has links to, the person's past, change is a part of continuity. As a result of both their own perceptions and pressures from the social environment, individuals who are adapting to normal aging are both predisposed and motivated toward inner psychological continuity as well as outward continuity of social behavior and circumstances.[47]

Continuity theory views both internal and external continuity as robust adaptive strategies that are supported by both individual preference and social sanctions. Continuity theory consists of general adaptive principles that people who are normally aging could be expected to follow, explanations of how these principles work, and a specification of general areas of life in which these principles could be expected to apply. Accordingly, continuity theory has enormous potential as a general theory of adaptation to individual aging.[48]

Age stratification theory

According to this theory, older adults born during different time periods form cohorts that define "age strata". There are two differences among strata: chronological age and historical experience. This theory makes two arguments. 1. Age is a mechanism for regulating behavior and as a result determines access to positions of power. 2. Birth cohorts play an influential role in the process of social change.[44]

Life course theory

According to this theory, which stems from the Life Course Perspective (Bengston and Allen, 1993),[49] aging occurs from birth to death. Aging involves social, psychological, and biological processes. Additionally, aging experiences are shaped by cohort and period effects.[44]

Also reflecting the life course focus, consider the implications for how societies might function when age-based norms vanish—a consequence of the deinstitutionalization of the life course— and suggest that these implications pose new challenges for theorizing aging and the life course in postindustrial societies. Dramatic reductions in mortality, morbidity, and fertility over the past several decades have so shaken up the organization of the life course and the nature of educational, work, family, and leisure experiences that it is now possible for individuals to become old in new ways. The configurations and content of other life stages are being altered as well, especially for women. In consequence, theories of age and aging will need to be reconceptualized.[50]

Cumulative advantage/disadvantage theory

According to this theory, which was developed beginning in the 1960s by Derek Price and Robert Merton and elaborated on by several researchers such as Dale Dannefer,[51] inequalities have a tendency to become more pronounced throughout the aging process. A paradigm of this theory can be expressed in the adage "the rich get richer and the poor get poorer". Advantages and disadvantages in early life stages have a profound effect throughout the life span. However, advantages and disadvantages in middle adulthood have a direct influence on economic and health status in later life.[44]

Constructionist theories

A spectrum of theorizing in gerontology has emerged that is "anti-theoretical" in the sense that its common point of departure is to set aside or "bracket" taken-for-granted realities such as activity, disengagement, the life course, continuity, cumulative advantage, and the like, in order to foreground their construction, use, institutional articulations, and experiential consequences in everyday life. A decided interest in subjectivity, agency, discourses, and their ordinary practices leads the way. The goal is to document how members of speech communities theorize realities in their own terms and how this relates to communicative context. Meaning-making, performativity, categorization, agency, narrativity, discourse, practice, and structuration are leading concepts. Constructionist approaches range from the ideas and conceptual proclivities of European cultural gerontologists, to the growing international network of narrative gerontologists, and longstanding programs of research on selves, social interaction, and meaning-making. Important contributors are Sara Arber, Bill Bytheway, Jaber Gubrium, Haim Hazan, Stephen Katz, Sharon Kaufman, Gary Kenyon, Marc Luborsky, Sara Matthews, William Randall, Robert Rubinstein, Andrea Sankar, Christine Swane, and Julia Twigg, among many others. Cultural gerontology emerged as a desire to depart from the dominant paradigm in favor of a social welfare and public policy framework that tend to depict ageing through the lens of frailty and burden[52]. Strongly influenced by Nordic and UK developments, this school of thoughts places the lived experiences of elder people at the heart of the analysis, by grasping the individuality of older lives through autobiography or narrative.[53].

Environmental gerontology

The environmental gerontology [54][55][56] is a knowledge area which aims Gerontology understand, analyze, modify and optimize the relationship between the aging person and their physical and social environment, from interdisciplinary perspectives and approaches, covering different disciplines like psychogerontology, geography,[57] urban planning, architecture and urban design, social policy, social work, sociology and other sciences at the end . In the 1930s (USA, Canada, UK and Germany), begin to emerge the first studies on Behavioral and Social Gerontology, and associated with deterministic explanations of the relationship between aging and environment from genetic parameters and biological. In the 1970s and 1980s, theories of different researchers, as Kurt Lewin ( living space model as a function of the person and the environment ) and, mainly, M. Powell Lawton (1923-2001) ( ecological adaptation model ), based on the influence of the interactions between the older person and their environment (adaptation - ambient pressure), confirm the importance of the physical and social environment ( objective and subjective ) in the understanding of the aging population and the possibility of improving the quality of life in old age. Environmental Gerontology[58] strives to understand the socio-spatial implications of aging and its complex relationship with the environment, from analysis at different scales : micro scales ( home and family) and macro scales (neighborhood, city, region ), to enable social and environmental policies that enable successful aging . Among the main contributions of the discipline are the contributions to aging at home (aging in place), and that older people prefer to age in their immediate environment ( housing ), where aspects such as spatial experience and place attachment are important for understanding the process.[59]

Researchers Examples of Environmental Gerontology, interdisciplinarity discipline, leading researchers emphasize, as the psychologist and gerontologist M. Powell Lawton (1923-2001) and the architect and gerontologist Paul Gordon Windley PhD. (1941-2007) The latter was one of the pioneers of this field, which showed that a good architectural design can encourage independence and competence of the elderly with their environment. Windley was interested to know the implications of the built environment on aging, highlighting the importance of encouraging a design built integrator . Between late 1970 and 1980 PG . Windley led an interdisciplinary team of architects and psychologists to examine how physical and social characteristics of small rural Kansas towns affected the quality of life of older people . Also noteworthy is the contribution of Graham D. Rowles, PhD . (1946- ), one of the social geographers recognized worldwide for his contributions to the geography of aging and environmental gerontology. Graham D. Rowles has been interested in the changing relationship between the elderly and their environment, first in rural ( Appalachian Region ) and later in urban areas, with special regard to the variety of residential and institutional environments . It is expected that in coming years the aging population worldwide incrementantará interest in environmental gerontology studies, due to social, economic, cultural and environmental factors associated with this phenomenon in cities, mainly in countries development.

See also

Notes

References

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