A major new international study shows independent associations between short-term exposure to PM10 and PM2.5 and daily all-cause, cardiovascular, and respiratory mortality in more than 600 cities across the globe. These data reinforce the evidence of a link between mortality and PM concentration established in regional and local studies, according to the researchers.
Their paper, published in the New England Journal of Medicine (NEJM), is the largest international study to investigate the short-term impacts of air pollution on death, conducted over a 30-year period.
The study, led by Dr. Haidong Kan from Fudan University in China, analyzed data on air pollution and mortality in 652 cities across 24 countries and regions, and found increases in total deaths are linked to exposure to inhalable particles (PM10) and fine particles (PM2.5) emitted from fires or formed through atmospheric chemical transformation.
They found that on average, an increase of 10 μg per cubic meter in the 2-day moving average of PM10 concentration was associated with increases of 0.44% (95% confidence interval [CI], 0.39 to 0.50) in daily all-cause mortality; 0.36% (95% CI, 0.30 to 0.43) in daily cardiovascular mortality; and 0.47% (95% CI, 0.35 to 0.58) in daily respiratory mortality.
The corresponding increases in daily mortality for the same change in PM2.5 concentration were 0.68% (95% CI, 0.59 to 0.77), 0.55% (95% CI, 0.45 to 0.66), and 0.74% (95% CI, 0.53 to 0.95).
These associations remained significant after adjustment for gaseous pollutants. Associations were stronger in locations with lower annual mean PM concentrations and higher annual mean temperatures. The pooled concentration–response curves showed a consistent increase in daily mortality with increasing PM concentration, with steeper slopes at lower PM concentrations.
Associate Professor Yuming Guo from Monash University’s School of Public Health and Preventive Medicine in Australia, said as there’s no threshold for the association between particulate matter (PM) and mortality, even low levels of air pollution can increase the risk of death.
In an accompanying editorial—“Do We Really Need Another Time-Series Study of the PM2.5–Mortality Association?”—in the journal, Dr. John Balmes, of the Department of Medicine, University of California, San Francisco, and the School of Public Health, University of California, Berkeley, noted that:
Given the abundance of evidence in support of an association between short-term PM2.5 exposure and mortality, what is the contribution of the time-series study by Liu et al. in this issue of the Journal? First, this study included almost 60 million deaths from 652 cities in 24 countries, thereby greatly increasing the generalizability of the association and decreasing the likelihood that the reported associations are subject to confounding bias. In observations consistent with previous studies, all-cause (non-accidental), cardiovascular, and respiratory mortality were associated with short-term exposures to both PM10 and PM2.5. The strength of the associations was reduced but remained significant in two-pollutant models that addressed potential confounding by gaseous pollutants.
Perhaps the most interesting result of the study by Liu et al. is from their concentration–response analysis. On the basis of studies of exposure to multiple combustion sources of PM2.5 (outdoor air pollution, secondhand tobacco smoke, and active tobacco smoking) and cardiovascular mortality, Pope et al. proposed that the shape of the concentration–response relation is curvilinear, with a lesser slope at higher exposure levels. Although other studies have reported evidence of such curvilinearity, the current study of PM data from many regions around the world provides the strongest evidence to date that higher levels of exposure may be associated with a lower per-unit risk. Regions that have lower exposures had a higher per-unit risk. This finding has profound policy implications, especially given that no threshold of effect was found. Even high-income countries, such as the United States, with relatively good air quality could still see public health benefits from further reduction of ambient PM concentrations (i.e., below the current NAAQS).
Cong Liu, M.S., Renjie Chen, Ph.D., Francesco Sera, Ph.D., Ana M. Vicedo-Cabrera, Ph.D., Yuming Guo, Ph.D., Shilu Tong, Ph.D., Micheline S.Z.S. Coelho, Ph.D., Paulo H.N. Saldiva, Ph.D., Eric Lavigne, Ph.D., Patricia Matus, Ph.D., Nicolas Valdes Ortega, M.Sc., Samuel Osorio Garcia, Ph.D., et al. (2019) “Ambient Particulate Air Pollution and Daily Mortality in 652 Cities” N Engl J Med 2019; 381:705-715 doi: 10.1056/NEJMoa1817364