Long-term Exposure to PM2.5 and Mortality Among Older Adults in the Southeastern US

Abstract

Background: Little is known about what factors modify the effect of long-term exposure to PM2.5 on mortality, in part because in most previous studies certain groups such as rural residents and individuals with lower socioeconomic status (SES) are under-represented.

Methods: We studied 13.1 million Medicare beneficiaries (age ≥65) residing in seven southeastern US states during 2000-2013 with 95 million person-years of follow-up. We predicted annual average of PM2.5 in each zip code tabulation area (ZCTA) using a hybrid spatiotemporal model. We fit Cox proportional hazards models to estimate the association between long-term PM2.5 and mortality. We tested effect modification by individual-level covariates (race, sex, eligibility for both Medicare and Medicaid, and medical history), neighborhood-level covariates (urbanicity, percentage below poverty level, lower education, median income, and median home value), mean summer temperature, and mass fraction of 11 PM2.5 components.

Results: The hazard ratio (HR) for death was 1.021 (95% confidence interval: 1.019, 1.022) per 1 μg m increase in annual PM2.5. The HR decreased with age. It was higher among males, non-whites, dual-eligible individuals, and beneficiaries with previous hospital admissions. It was higher in neighborhoods with lower SES or higher urbanicity. The HR increased with mean summer temperature. The risk associated with PM2.5 increased with relative concentration of elemental carbon, vanadium, copper, calcium, and iron and decreased with nitrate, organic carbon, and sulfate.

Conclusions: Associations between long-term PM2.5 exposure and death were modified by individual-level, neighborhood-level variables, temperature, and chemical compositions.

Figure 1

Effect modification of the association of long-term PM_2.5 exposure (hazard ratios for each one μg m^-3 increase) with mortality among older adults using a univariate interaction model by individual-level covariates including age group, sex, race, dual eligibility, previous hospitalizations due to congestive heart failure (CHF), myocardial infarction (MI), chronic obstructive pulmonary disease (COPD), and diabetes, and number of days staying in intensive care unit (ICU) and coronary care unit (CCU) (one day versus zero day).

Figure 2

Effect modification of the association of long-term PM_2.5 exposure (hazard ratios for each one μg m-3 increase) with mortality among older adults by neighborhood-level covariates including percentage of less educated (20th percentile versus 80th percentile), percentage below the poverty level (20th percentile versus 80th percentile), urbanicity, median income, and median home value.

Figure 3

Effect modification of the association of long-term PM_2.5 exposure (HRs per one μg m-3 increase) with mortality among older adults using simultaneous interaction models. The dataset was restricted to each of the sex and race combinations. Modifiers include individual-level covariates (dual eligibility, previous hospitalizations due to congestive heart failure (CHF), myocardial infarction (MI), chronic obstructive pulmonary disease (COPD), and diabetes), urbanicity, and mean summer temperature. The estimate labeled “average” shows the HR when all modifiers were set at their means. The hazard ratio (HR) for each modifier set that modifier at the corresponding value and set the remaining modifiers at their means.

Figure 4

Effect modification of the association of long-term PM_2.5 exposure with mortality among older adults by species-to-PM_2.5 ratios covariates including aluminum (Al), calcium (Ca), copper (Cu), elemental carbon (EC), iron (Fe), nitrate, nickel (Ni), organic carbon (OC), sulfate, vanadium (V), and zinc (Zn). The hazard ratios (HR) for each one μg m-3 increase at the 20th and 80th percentile of the modifiers were compared.