Study Finds Link Between Elderly Mortality Rates, Warm-season Pollution

Qian Di, MS

Short-term exposure to air pollution has a distinct effect on older American's mortality risk.

A case-crossover study of more than 22 million deaths showed a correlation between daily fine particulate matter increase and ozone exposures, to deaths per persons at risk per day. The findings may call for a reevaluation of US air quality standards.

The study, from a team of researchers at the Harvard T.H. Chan School of Public Health in Boston, MA, sought to estimate the association between short-term exposures to ambient fine particulate matter (PM_2.5) and ozone, with areas of air pollution levels between current daily National Ambient Air Quality Standards (NAAQS), and mortality in the US.

The NAAQS is a set-standard of PM_2.5 and ozone levels that aim to provide a margin of safety for at-risk populations, as set by the administrator of the US Environmental Protection Agency (EPA). It is required to be reevaluated once every 5 years, with its most recent public comments being shared in 2012. At the time of the study’s publication, the 2017 reevaluation of standards were under review.

In 2012, the annual and 24-hour NAAQS for PM_2.5 were set to 12 μg/m³ and 35 μg/m³, respectively. Ozone levels, which has no annual standard, were set to 70 parts per billion (ppb) for 8-hour NAAQS. However, researchers chose to restrict analysis to days with daily air pollution concentrations below 25 μg/m³ and 60 ppb because levels on most days included in the study were already below the current safety standards.

Led by author Qian Di, MS, researchers used the US Medicare population from 2000 to 2012 as its study population — meaning it generally consisted of individuals older than 65 years old. Using a crossover design and conditional logistic regression to estimate associations, researchers reported 22,433,862 case days and 76,143,209 control days. Of all days, 93.6% reported PM_2.5 levels below 25 μg/m³, during which 95.2% of deaths occurred.

Of the total days, 91.1% reported ozone levels below 60 ppb, during which 93.4% of deaths occurred.

Per 1 million persons at risk per day, baseline mortality rates were 137.33 for the entire year, and 129.44 for the warm season. Each short-term increase of 10 μg/m³ in PM_2.5 and 10 ppb in warm-season ozone were associated a statistically significant relative increase of 1.05% (95% CI; 0.95-1.15) and 0.51% (95% CI; 0.41-0.61) in daily mortality rate, respectively.

Researchers emphasized the inclusion of smaller city, town, and rural residents for the study, noting they were populations previously excluded from US pollution-based studies. They reported no significant differences in mortality risk association with pollution among urban and rural area residents.

Though the study is novel in its large-scale analysis of daily air pollution exposure and mortality rates, it’s among much more research into the link between death and pollution.

“The association of mortality and PM_2.5 exposure is supported by a large number of published experimental studies in animals and in humans exposed to traffic air pollution, diesel particles, and unfiltered urban air,” researchers wrote.

In late 2017, a study reported that deaths from ambient air pollution have risen 20% in 25 years, from 3.5 million in 1990 to 4.2 million in 2015. Barring “aggressive intervention,” the researchers projected deaths related to pollution would increase by more than 50% by 2050.

The study, "Association of Short-term Exposure to Air Pollution With Mortality in Older Adults," was published online in JAMA last week.