The Associations of Long-term Ambient Air Pollution, Physical Disability, and Accelerated Aging in Later Life

Physical function disability is one of the most prevalent health outcomes among olderadults in the U.S. with considerable individual- and society-level burdens. The process ofphysical disability starts when healthy people experience physical function limitations that thenpotentially deteriorate into having disabilities in performing activities of daily living (ADL).Physical disablement is a dynamic process, where disability episodes can be followed by periodsof recovery. Given that there is no cure for physical disability, identifying modifiable risk factorsto delay the onset and lessen the severity of physical disability is valuable.The World Health Organization estimated that long-term outdoor ambient air pollution isassociated with 4.2 million premature deaths, predominately attributable to aging-related chronicconditions such as chronic obstructive pulmonary diseases, respiratory infections, cancers withinthe respiratory tract, and cardiovascular diseases.1 Since physical disability is an overall clinicalmanifestation of series of chronic conditions and physiological changes, it is reasonable tohypothesize that air pollution is a risk factor for developing physical disability. If air pollution isindeed a risk factor, then interventions aimed at reducing population-level air pollutionconcentrations could significantly contribute to controlling physical disability-associatedhealthcare expenses and achieving the goal of healthy aging.Despite of emerging studies linking air pollution to physical disability, there are someweaknesses in the current knowledge. First, many studies used city-level air pollution values asan indicator of individual-level exposures, or they only used single air pollutants withoutadjusting for a broader mixture of pollutants. Thus, the conclusions from the existing literatureare likely to be subject to exposure measurement error and confounding biases. Second, priorstudies have been conducted piecemeal, focusing on physical function limitations or ADLxidisability separately. Such segmented analysis has resulted in limited understanding of theimpacts of air pollution on the continuum from health to physical function limitation andsubsequent ADL disability. Finally, current studies mainly focus on the clinical state of physicaldisability, which is irreversible and leaves little opportunity for interventions to delay theprocess. To postpone the onset of physical disability, it is important to capture the impacts of airpollution at the early subclinical stage.The three aims of this dissertation were designed to address these gaps to inform potentialpolicies and regulations aimed at reducing air pollution to delay and mitigate physical disability.These studies were conducted within the Health and Retirement Study (HRS), a prospectivecohort of over 25,000 participants. I used the spatiotemporal model to predict 10-year averageconcentrations of outdoor particulate matter (PM2.5, PM10-2.5), nitrogen dioxide (NO2), and ozone(O3) at each respondent’s residential addresses preceding their data collection between 2000 and2016. In Aim 1 I quantified associations of these long-term exposures with the incidence of ADLdisability. In Aim 2 I further assessed associations with transition hazards between healthy,physical function limitations, and ADL disability; further, I also estimated the role of long-termair pollution on the rate of progression of physical function limitations and ADL disability. InAim 3 I explored the association between long-term exposure to air pollution and the biologicage acceleration, which reflects the overall subclinical changes in health.Over an average follow-up period of 8±5 years, in Aim 1, I observed that higherconcentrations of PM2.5, PM10-2.5, and NO2 were associated with 3%-5% greater hazards ofincident ADL disability, whereas O3 was inversely associated with incident of ADL disability.Accounting for confounding by co-pollutants, associations remained robust for only PM10-2.5(HR: 1.05, 95% CI: 1.00, 1.09) and O3 (HR: 0.95, 95% CI: 0.91, 1.00). In Aim 2, except for O3,higher concentrations of air pollutants were also associated with a 3%-8% greater hazard oftransitioning from healthy to physical function impairment and subsequently to ADL disability.Consistently, increased levels of these air pollution exposure were associated with fasterprogression rates of physical function limitations and ADL disability. Additionally, I also foundhigher exposure to PM2.5 and NO2 were associated with a 3% lower likelihood of recoveringxiifrom mild physical function limitations. Aim 3 found no associations between higher levels ofair pollutants and biologic age acceleration.In summary, this dissertation contributes to understanding the impacts of long-termoutdoor ambient air pollution on each stage of the dynamic physical disability process. Itunderscores the potential benefits of reducing air pollution levels to delay the onset ofdisablement process and improve physical performance among elderly individuals with physicalfunction limitations and ADL disabilities.