Air Pollution: A Pressing Threat to Functioning in the Older Adults Free

Populations are aging worldwide with a more rapid increase in low- and middle-income countries (LMICs). It is estimated that 21% of the global population will be 60 years or older, and 80% of this older adults group will be living in LMICs by 2050 (1). Age-related physiological decline can lead to dysfunction or even disability (or frailty) in the older adults, greatly jeopardizing their quality of life and resulting in tremendous burden on their families and the society. Identifying the determinants of age-related disability is imperative for the development of effective strategies to promote healthy aging. In recent years, growing interests have been shifted to environmental exposures such as air pollution.

Air pollution is the contamination of the indoor or outdoor environment by any chemical, physical, or biological agents. Air pollution originates from various sources of both natural and anthropogenic emissions, with the combustion of fossil fuels and biomass being the greatest contributor. Nowadays, air pollution has been recognized by the World Health Organization (WHO) as the single biggest environmental threat to health, which is responsible for around 7 million deaths annually (2). Similar estimates are shown in some other global assessments including the Global Burden of Disease Study, with greater disease burden seen in LMICs (3).

Oxidative stress and inflammation induced by air pollution have been recognized as the major biological mechanisms underlying its adverse health effects (4). Considering the critical role that oxidative stress plays in aging by impeding physiological processes and directly damaging tissue components (5), it is biologically plausible that air pollution can accelerate the aging process and related functional decline. In addition, air pollution has been well associated with increased risks of various chronic diseases that are increasingly prevalent among older adults, including hypertension, heart disease, stroke, chronic obstructive pulmonary disease, lung cancer, and diabetes (6,7), all of which can contribute to functional decline and disability.

In recent years, an increasing number of epidemiological studies have been conducted to examine the health effects of air pollution exposure on dysfunction or disability in the older adults. Dementia is one of the most frequently investigated conditions of mental or cognitive decline due to its incurable nature and the devastating impact on patients’ daily lives. A recently published study by Chen et al. approached this issue using the UK Biobank study, a well-established prospective cohort, providing important evidence on air pollution and dementia (8). More than 0.45 million participants were included and followed up for a median of 11.7 years. Exposure to various ambient air pollutants including particulate matter of different sizes (PM_2.5, PM_2.5–10, and PM₁₀) and gaseous air pollutants (NO₂ and NO_x) was estimated using Land Use Regression models, and a weighted air pollution score was constructed to reflect the overall exposure. Significant associations were observed between exposure to all air pollutants except for PM_2.5–10, and increased risks of dementia. Moreover, an increasing air pollution score was associated with higher risks of all-cause dementia as well as the 2 major subtypes including Alzheimer’s disease and vascular dementia (8). This study benefited from the prospective study design, the large study population with extensively collected data (eg, a wide range of potential confounders) and the long-term follow-up, and therefore adds robust evidence to the existing literature (9) to further support the adverse effects of air pollution exposure on the development of dementia. In addition to mental or cognitive decline, air pollution has also been associated with declines in physical functioning in older adults. For example, studies from the United States (10) and the Netherlands (11) found that long-term exposure to air pollution was associated with lower levels of physical functioning indicated by worse lower-extremity physical ability or performance (eg, walking speed and balance test) in seniors. Consistent findings have also been reported in LMICs. Based on the Study on Global AGEing (SAGE) led by the WHO, which was conducted in 6 LMICs, Lin et al. found that long-term exposure to ambient PM_2.5 was associated with higher levels of disability (12). To be noted, disability in the SAGE was assessed using the WHO-developed Disability Assessment Schedule, which incorporated different domains of disability including physical, cognitive, and social functioning.

Despite the growing number of studies reporting adverse effects of air pollution on functioning in the older adults, most of them focused on ambient or outdoor air pollution, whereas less efforts have been put on indoor air pollution. In general, indoor pollution concentrations largely depend on the amount of air pollution penetrating from outdoor environments, but indoor emissions are also important contributors. Tobacco smoking and combustion of fuels for cooking and heating are the 2 most common sources of indoor air pollution, with the latter being more of concern in LMICs as polluting or unclean fuels (eg, kerosene, coal, and biomass such as wood and dung) are more commonly used in these regions. The recently published study by Smith et al. in the journal investigated the associations between unclean cooking fuel use and gait speed, a widely used measure of frailty, among 14 585 adults aged ≥65 years from 6 LMICs (China, Ghana, India, Mexico, Russia, and South Africa), which participated in the aforementioned SAGE (13). As a surrogate measure of indoor air pollution, unclean cooking fuel use was defined as the use of kerosene/paraffin, coal/charcoal, wood, agriculture/crop, animal dung, and shrubs/grass based on self-report. The pooled results of 6 countries showed that, compared with clean cooking fuel use, unclean cooking fuel use was significantly associated with higher odds for slow gait speed (odds ratio = 1.45 [95% confidence interval = 1.14–1.85]). For specific types of unclean fuels, stronger associations were observed for agriculture/crop and shrubs/grass (13). Despite certain limitations such as the cross-sectional study design, this study adds valuable evidence on the detrimental effects of indoor air pollution on physical dysfunction in the older adults.

Taken together, a growing body of evidence from large population-based studies has demonstrated the adverse effects of exposure to air pollution, both outdoors and indoors, on functioning in older adults. Given the ubiquitous nature of air pollution exposure, reductions in air pollution would result in substantial health benefits including the promotion of healthy aging. Undoubtedly, the mitigation of air pollution is a complex and ambitious process, which requires continuous involvement of the whole society, especially the governments and industry. Individual engagements are also essential for reducing personal exposure to air pollution, such as the replacement of unclean fuel with clean energy and the use of air purification device. A recently published trial showed promising results of using air purification in long-term care facility (14). The installation of an air purification device significantly reduced the levels of indoor air pollutants, particularly the volatile organic compounds and airborne pathogens. Moreover, a dramatic reduction (39.6%) was observed in health care-associated infections (14). These findings support the use of air purification equipment as an effective solution to remediating indoor air pollution, which may be of greater value for old people as they generally spend more time indoors and are more susceptible to air pollutants. Nonetheless, more studies are warranted to directly assess the effectiveness of the employment of air purification technology as well as other potential interventions on improving older adults’s functioning levels.

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