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

Abstract

Physical function disability is one of the most prevalent outcomes among older adults in the U.S. with considerable burdens. The process of physical disability starts when healthy people experience physical function limitations that then potentially deteriorate into having disabilities in performing activities of daily living (ADL). It is a dynamic process, where disability episodes can be followed by periods of recovery. Given that there is no cure for physical disability, identifying modifiable risk factors to delay the onset and lessen the severity of physical disability is valuable. The World Health Organization estimated that long-term outdoor ambient air pollution is associated with 4.2 million premature deaths, predominately attributable to aging-related chronic conditions such as chronic obstructive pulmonary diseases and cardiovascular diseases.1 Since physical disability is an overall clinical manifestation of series of physiological changes, it is reasonable to hypothesize that air pollution is a risk factor for developing physical disability. There are some weaknesses in the current knowledge in this field. First, many studies used city-level air pollution values as an indicator of individual-level exposures, or they only used single air pollutants without adjusting for a broader mixture of pollutants. Thus, the conclusions from the existing literature are likely to be subject to exposure measurement error and confounding biases. Second, prior studies have been conducted piecemeal, focusing on physical function limitations or ADL disability separately. Such segmented analysis has resulted in limited understanding of the impacts of air pollution on the continuum from health to physical function limitation and subsequent ADL disability. Finally, current studies mainly focus on the clinical state of physical disability, which is irreversible and leaves little opportunity for interventions to delay the process. The three aims of this dissertation were designed to address these gaps. These studies were conducted within the Health and Retirement Study (HRS). I used the spatiotemporal model to predict 10-year average concentrations 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 and 2016. In Aim 1 I quantified associations of these long-term exposures with the incidence of ADL disability. In Aim 2 I further assessed associations with transition hazards between healthy, physical function limitations, and ADL disability. In Aim 3 I explored the association between long-term exposure to air pollution and the biologic age acceleration, which reflects the overall changes in health at earlier subclinical stage. Over an average follow-up period of 8±5 years, in Aim 1, I observed that higher concentrations of PM10-2.5 were robustly associated with 5% greater hazards of incident ADL disability, whereas O3 was inversely associated with incident of ADL disability. In Aim 2, except for O3, higher concentrations of air pollutants were also associated with a 3%-8% greater hazard of transitioning from healthy to physical function impairment and subsequently to ADL disability. Additionally, I also found higher exposure to PM2.5 and NO2 were associated with a 3% lower likelihood of recovering from mild physical function limitations. Aim 3 found no associations between higher levels of air pollutants and biologic age acceleration. In summary, this dissertation underscores the potential benefits of reducing air pollution levels to delay the onset of disablement process and improve physical performance among elderly individuals with physical disabilities.