Ellington

Certain ingredients of airborne particulates—especially carbon—apparently increase the risk of death in U.S. cities, according to a new nationwide study.

For decades, scientists have been trying to unravel why more people die of heart attacks, asthma and other health problems whenever fine particulate matter, known as PM2.5, increases. Particulates are a mix of substances emitted by sources of combustion, including cars, trucks, industrial plants and wood burning.
 


The new study of 72 urban areas builds on previous research that suggests the health effects depend on the pollution's composition. But it is the first on a national level to examine mortality risks of different particulate ingredients.
 


The researchers from Johns Hopkins, Harvard and Yale universities used air pollution and mortality data from all causes of death, except accidents, between 2000 and 2005. Seven compounds that, together, typically make up 79 to 85 percent of PM2.5 mass were analyzed.
 


Small increases in organic carbon matter, elemental carbon, silicon and sodium ion were associated with slightly more deaths the following day.
 


With a typical, day-to-day increase in pollutant levels, deaths rose by 0.39 percent for organic carbon, 0.22 percent for elemental carbon, 0.17 percent for silicon and 0.16 for sodium. The researchers found "little evidence" of variation by season or region.
 


Carbon comes largely from diesel and gasoline-powered vehicles, silicon from coal combustion and sodium from aerosolized sea salts. However, all of the pollutants have multiple sources.
 


Federal standards for PM2.5 are based on total concentrations of particles, not individual compounds.
 


"Our work indicates that some constituents of PM2.5 may be more toxic than others and therefore regulating PM total mass alone may not be sufficient to protect human health," the authors wrote in Environmental Health Perspectives.
 


Figuring out which ingredients are the most dangerous could help local air pollution agencies hone in on certain sources, making regulations more targeted.
 


"By identifying PM2.5 constituents that are more toxic, we can move towards developing source-specific air pollution regulation that may be more effective at protecting public health," the authors wrote.
 


The study was limited by infrequent measuring of the different compounds, as monitors typically measure them every third or sixth day.
 


It's also possible that certain compounds, such as nickel and vanadium, that are associated with carbon and silicon, may be the real culprits. Such metals are in low concentrations—sometimes too low to detect in particulate testing—but are more toxic than the major constituents, the authors wrote.
 


Some previous studies have found that certain PM2.5 compounds are more likely to cause health problems. A 2009 national study linked elemental carbon and organic carbon matter with heart and respiratory hospitalizations.
 


Two different regional studies—in 2011 and 2012—reported that elemental carbon, organic carbon, sulfate, silicon and nitrate were associated with increased emergency room admissions.