The world’s population is aging, and Alzheimer's disease has become the fourth largest health hazard for elderly people worldwide. The 2018 report of the International Alzheimer's Disease Association revealed that an estimated 50 million people worldwide suffer from cognitive impairment. By 2050, this number is projected to increase to 152 million. China, as the country with the largest number of aged people in the world, is at the forefront regarding the health issues associated with aging. In association with the huge and rapidly growing elderly population in China, the number of Alzheimer’s patients has exceeded 10 million. China has the most Alzheimer’s disease patients in the world. The number of patients is increasing by more than 300,000 per year. There is no doubt that Alzheimer's disease will become one of the most severe health threats facing China in the future. Unfortunately, current treatments of late-stage Alzheimer's disease are not effective, so early recognition and prevention of cognitive dysfunction is critical. The factors leading to a decline in the cognitive function of middle-aged and elderly people are complex. Early research mainly focused on individual factors, such as genetic factors, health conditions, and chronic diseases. In recent years, environmental factors have also attracted the attention of researchers. Many studies in developed countries have found that long-term exposure to PM2.5 has a negative impact on the cognitive ability of middle-aged and elderly people. China is the world’s largest developing country and has the world’s second largest economy. The rapid industrialization and urbanization of China has undoubtedly resulted in a deterioration of urban air quality. Following the outbreak of haze weather nationwide in early 2013, the impact of PM2.5 on human health has become a significant concern among the public. PM2.5 may be affecting the cognitive ability of middle-aged and elderly people in China, potentially contributing to the prevalence of Alzheimer's disease. There is a need to consider how to respond more proactively to the combination of climate change and an aging population.
1 Literature review
1.1 Cognitive Impairment Of The Elderly And Measurement Methods
Alzheimer’s disease, commonly known as senile dementia, is a chronic and progressive mental degenerative condition that is common in old age. Its high mortality and disability rates impose a heavy social and economic burden (Bemett DA et al., 2002). Alzheimer's disease is characterized by advanced brain dysfunction caused by chronic or progressive organic damage to the brain, and persistent and comprehensive loss of intelligence manifested in loss of memory, calculation ability, judgment, attention, and abstract thinking ability, as well as in language impairment, emotional and behavior disorders, and loss of independent living and working ability (John A. Hardy et al., 1992). The most commonly used tools for screening the cognitive ability of the elderly include the Mini-Mental State Examination (MMSE) (Folstein MF et al., 1975), which mainly tests orientation, memory, calculation ability, language skills, visuospatial ability, and attention. The Montreal Cognitive Assessment (MCA) (Nasreddine ZS et al., 2005) is mainly applied for the assessment of attention, executive ability, memory, language skills, abstract thinking, computing, orientation and other cognitive abilities in the elderly. The Clock Drawing Test (CDT) (Grober E et al., 1988) tests comprehension, planning, visual memory, the ability to reconstruct images, visuospatial function, executive function, computing power, attention, and abstract thinking. The MMSE is the most widely used of these tools for the measurement of cognitive impairment.
1.2 Relationship between PM2.5 and cognitive impairment in middle-aged and elderly people
Researchers have long been concerned about the impact of PM2.5 on the health of middle-aged and elderly people, but most of the early studies focused on the impact of PM2.5 on respiratory diseases, cardiovascular diseases, and regional elderly mortality (Endawoke Amsalu et al., 2019; Gao Ying et al., 2019; Fengping Hu et al., 2021) In the past 5 years, substantial progress has been made in research on the effects of PM2.5 on mental problems, such as anxiety, depression, powerlessness, and restless or fidgety behavior (Hejun Gu et al., 2020; Aarón Salinas-Rodrígueza et al., 2018), although the evidence is still very limited. Most studies have focused on two aspects of mental illness. The first is the impact of PM2.5 on the risk of neurological diseases. Researchers have found that long-term exposure to PM2.5 significantly increases the risk of Alzheimer’s and Parkinson’s disease (Fu, Pengfei et al., 2019; Wu YC et al., 2015; Gabriele Cipriani et al., 2018). PM2.5 can enter the human body in a variety of ways, exerting pathological effects on the central nervous system. For example, PM2.5 can disrupt the integrity of the blood-brain barrier, making it easy for systemic inflammation to affect the central nervous system (Gabriele Cipriani et al., 2018). The olfactory nerve is another route by which PM2.5 particles enter the brain. Surprisingly, PM2.5 can also enter the gastrointestinal tract, causing a microecological imbalance in the intestines and affecting the central nervous system (Shou, Yikai et al., 2019). The second research focus has been on the impact of PM2.5 on the cognitive ability of elderly people. The results of a national health survey of retirees in the United States showed that PM2.5 exposure reduced memory and increased cognitive errors in the elderly (Jennifer A. Ailshire et al., 2014). The results of a German study on elderly women also showed that the PM2.5 concentration reduced cognitive ability and performance of elderly women (Hejun Gu et al., 2020). In addition, some studies have found that old people with certain characteristics, such as smoking, hypertension, and obesity, are more likely to show cognitive ability deficits in association with PM2.5 (Wellenius G.A. et al., 2012; Power M.C. et al., 2015). Factors affecting the cognitive ability include age, gender, years of education, extent of disability, history of falls, hypertension, and stroke (Aarón Salinas-Rodrígueza et al., 2018; Zhao Jiangang et al., 2015; Zheng Jiaying et al., 2017; Yang Yanrong et al., 2020; Yang Guang et al., 2018). However, some researchers believe that intergenerational interactions within the family is the main factor affecting the mental health of the elderly (Claire Scodellaro et al., 2012; Huang Qingbo et al., 2017).
Most of the research on PM2.5 and cognitive impairment has been conducted in developed countries, mainly foucs on eldly people,with little attention paid to the middle-aged. Research from developing countries is even scarce (Jennifer A. Ailshire et al., 2014).The formation of cognitive impairment is a gradual process. To prevent the cognitive impairment of elderly people should start from paying attention to the cognitive ability of the middle-aged. Moreover, as the largest developing country in the world, China faces a difficult task in reforming its social security system and environmental protection policies due to its rapid economic growth and rapidly aging population. China is likely to face great challenges in association with cognitive impairment among the elderly, with air pollution control still being a critical issue (Fengping Hu et al., 2021). It is therefore of practical significance to explore the impact of PM2.5 on the cognitive ability of middle-aged and elderly people in China.