Regardless of where people live in the world, air pollution is linked to increased rates of cardiovascular disease, respiratory problems, and all-cause mortality, according to one of the largest studies ever to assess the effects of inhalable particulate matter (PM), published Aug. 21 in the New England Journal of Medicine.
“These data reinforce the evidence of a link between mortality and PM concentration established in regional and local studies,” reported Cong Liu of the Huazhong University of Science and Technology in Wuhan, China, and an international team of researchers.
“Many people are experiencing worse allergy and asthma symptoms in the setting of increased heat and worse air quality,” Caren G. Solomon, MD, of Harvard Medical School, Boston, said in an interview. “It is often not appreciated that these are complications of climate change.”
Other such complications include heat-related illnesses and severe weather events, as well as the less visible manifestations, such as shifts in the epidemiology of vector-borne infectious disease, Dr. Solomon and colleagues wrote in an editorial accompanying Mr. Liu’s study.
“The stark reality is that high levels of greenhouse gases caused by the combustion of fossil fuels – and the resulting rise in temperature and sea levels and intensification of extreme weather – are having profound consequences for human health and health systems,” Dr. Solomon and colleagues wrote (N Engl J Med. 2019;381:773-4.).
In the new air pollution study, Mr. Liu and colleagues analyzed 59.6 million deaths from 652 cities across 24 countries, “thereby greatly increasing the generalizability of the association and decreasing the likelihood that the reported associations are subject to confounding bias,” wrote John R. Balmes, MD, of the University of California, San Francisco, and the University of California, Berkeley, in an editorial about the study (N Engl J Med. 2019;381:774-6).
The researchers compared air pollution data from 1986-2015 from the Multi-City Multi-Country (MCC) Collaborative Research Network to mortality data reported from individual countries. They assessed PM with an aerodynamic diameter of 10 mcg or less (PM10; n = 598 cities) and PM with an aerodynamic diameter of 2.5 mcg or less (PM2.5; n=499 cities).
Mr. Liu’s team used a time-series analysis – a standard upon which the majority of air pollution research relies. These studies “include daily measures of health events (e.g., daily mortality), regressed against concentrations of PM (e.g., 24-hour average PM2.5) and weather variables (e.g., daily average temperature) for a given geographic area,” Dr. Balmes wrote. “The population serves as its own control, and confounding by population characteristics is negligible because these are stable over short time frames.”
The researchers found a 0.44% increase in daily all-cause mortality for each 10-mcg/m3 increase in the 2-day moving average (current and previous day) of PM10. The same increase was linked to a 0.36% increase in daily cardiovascular mortality and a 0.47% increase in daily respiratory mortality. Similarly, a 10-mcg/m3 increase in the PM2.5 average was linked to 0.68% increase in all-cause mortality, a 0.55% increase in cardiovascular mortality, and 0.74% increase in respiratory mortality.
Locations with higher annual mean temperatures showed stronger associations, and all these associations remained statistically significant after the researchers adjusted for gaseous pollutants.
Although the majority of countries and cities included in the study came from the northern hemisphere, the researchers noted that the magnitude of effect they found, particularly for PM10 concentrations, matched up with that seen in previous studies of multiple cities or countries.
Still, they found “significant evidence of spatial heterogeneity in the associations between PM concentration and daily mortality across countries and regions.” Among the factors that could contribute to those variations are “different PM components, long-term air pollution levels, population susceptibility, and different lengths of study periods,” they speculated.
What makes this study remarkable – despite decades of previous similar studies – is its size and the implications of a curvilinear shape in its concentration-response relation, according to Dr. Balmes.
“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,” Dr. Balmes wrote. “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.”
The policy implications, however, extend well beyond clean air regulations because the findings represent just one aspect of climate change’s negative effects on health, which are “frighteningly broad,” Dr. Solomon and colleagues wrote.
“As climate change continues to alter disease patterns and disrupt health systems, its effects on human health will become harder to ignore,” they wrote. “We, as a medical community, have the responsibility and the opportunity to mobilize the urgent, large-scale climate action required to protect health – as well as the ingenuity to develop novel and bold interventions to avert the most catastrophic outcomes.”
The new research and associated commentary marked the introduction of a new NEJM topic on climate change effects on health and health systems.
SOURCE: Liu C et al. N Engl J Med. 2019;381:705-15.
This article was updated 8/22/19.