Subtle but Potentially Serious: Long-Term Ambient $\mathbf{P}{\mathbf{M}}_{\mathbf{2}\mathbf{.5}}$ Exposure and Risk of Cardiopulmonary Mortality

Abstract

Fossil fuel combustion emits a slew of air pollutants, including fine particulate matter. These minute particles cause human disease and mortality worldwide,1^,2^,3^,4 but studies tend to be nonrepresentative of entire populations.5 Using data for nearly 1.6 million adults in the U.S. general population, a recent study published in Environmental Health Perspectives reported evidence that ${\mathrm{PM}}_{2.5}$ at ambient concentrations increases mortality rates across diverse populations.5

${\mathrm{PM}}_{2.5}$ can penetrate deep into the lungs.1 The particles, which may include any combination of soot, metals, and an array of chemical compounds, can directly affect lung tissue as well as directly and indirectly inflict damage throughout the body.6^,7 Long-term ${\mathrm{PM}}_{2.5}$ exposures have been associated with respiratory disorders as well as cardiovascular and metabolic diseases.6 In the most recent figures available (2016), an estimated 92% of the world’s population lived in areas where ambient ${\mathrm{PM}}_{2.5}$ exceeded the World Health Organization’s annual mean guideline of ${10\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$.8

Risk factors such as smoking, obesity, and poverty are strongly associated with an increased risk of cardiopulmonary-related death. At the individual level, the estimated excess risk from ${\mathrm{PM}}_{2.5}$ exposure is far smaller. At the population level, however, the ubiquity and unavoidability of ${\mathrm{PM}}_{2.5}$ may add up to a substantial public health burden over time.5 Image: © iStockphoto/fotoguy22.

In the current study,5 the researchers used information collected from participants 18–84 years of age as part of the National Health Interview Surveys9 from 1986 to 2014. The analysis used data on 1,599,329 individuals who were representative of the U.S. population in terms of age, sex, education, socioeconomic status, race/ethnicity, and region. Additional data on body mass index and smoking status were available for a subset of 635,539 people.

“Whenever you do an analysis like this, what you really want is a representative cohort so your results can be generalizable,” says lead author C. Arden Pope III, a professor of economics at Brigham Young University. “You want to be able to say, ‘We think this is relevant to a general population.’”

The investigators obtained mortality data for participants who died before the end of 2015 through linkages with the National Death Index, and deaths were categorized by cause: cardiopulmonary (subdivided into cardiovascular disease, cerebrovascular disease, chronic lower respiratory disease, and influenza/pneumonia), any malignant cancer, lung cancer, and other/unknown. The team estimated ${\mathrm{PM}}_{2.5}$ concentrations for each person’s census tract using a model based on local traffic, land use, emissions sources, satellite-based estimates, regulatory monitoring data, and other factors.

Estimated ${\mathrm{PM}}_{2.5}$ exposures and risk of mortality were significantly associated, particularly for deaths due to cardiovascular and cerebrovascular diseases and influenza/pneumonia. Overall, the associations were consistent across subgroups based on age, sex, race/ethnicity, region, and other characteristics.

One implication of these results, Pope says, is that reducing exposures to air pollution is likely to improve many people’s health. “The evidence here is that continued efforts to reduce air pollution will continue to reduce the risk of cardiovascular and related disease,” he says.

The study’s strengths include the use of high-quality publicly available data on a large, representative population, with sufficient numbers to control for individual risk factors. Potential limitations are the lack of directly measured exposures and the possibility that unknown and unmeasured factors influenced the conclusions.

“The results are in keeping with what is been found before, so I think of this as another piece of evidence from a well-designed, well-conducted study that adds a lot of support,” says Helen Suh, a professor of civil and environmental engineering at Tufts University, who was not involved with the work.

Suh also noted that the study is quite timely, given that the ${\mathrm{PM}}_{2.5}$ National Ambient Air Quality Standards are due for review.10^,11 The primary (health-based) standard currently stands at an annual mean of ${12\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$, averaged over 3 years.12 “It is really important that new studies come out,” Suh explains, “especially those that investigate the impact of exposures at low levels that are perhaps lower than the standard or at the standard.” Such studies will enable regulators to assess whether or not the standard is sufficient and whether there’s new evidence that suggests the standard should be changed.

Biography

Julia R. Barrett, MS, ELS, a Madison, WI–based science writer and editor, is a member of the National Association of Science Writers and the Board of Editors in the Life Sciences.