Year Round Particle Pollution

Nearly 24 million people in the United States live in counties with unhealthful year-round levels of particle pollution.

Exposure to particles over time can increase risk of hospitalization for asthma, damage to the lungs, lung cancer and, significantly, increase the risk of premature death. These 24 million people live in 24 counties where chronic levels are regularly a threat to their health. Although this totals 7.6 percent of the population, even more people may be at risk than we can know, because of an unexpected lack of data from Illinois, Tennessee and Georgia.

Overall, the best progress came in the continued reduction of year-round particle pollution in 2011-2013, especially in the eastern U.S., thanks to cleaner diesel fleets and cleaner fuels used in power plants

What Is Year-Round Particle Pollution?

Particle pollution is a mix of very tiny solid and liquid particles in the air. "Year-round" refers to an annual average level that represents the concentration of particles day-in and day-out. To learn more, go to Health Risks.

Fresno-Madera (CA) improved, but remained the most polluted metropolitan area for year-round particle pollution, as it was in the 2014 report.

  • Of the 25 metro areas with the worst year-round levels of particle pollution, fourteen reduced their annual levels compared to 2010-2013 and 13 cities recorded their cleanest years yet, some continuing to reduce levels for the 2nd report in a row. Eleven of the cities with the highest year-round particle levels now get a passing grade; that is, they meet the official health-based standard for year-round particle pollution. 1

Some of the biggest improvements have been in the East and South and Los Angeles, which had its lowest ever annual average level of particle pollution since our report began.

  • Five other cities also had their lowest year-round particle pollution, but, as with Los Angeles, they still had levels above the national air quality standard: Pittsburgh, Cleveland, Philadelphia, New York City, and Louisville (KY).
  • Six other cities had their lowest year-round particle pollution, but met the national air quality standard, a marker that they benefited decisively from the measures put in place to reduce this dangerous air pollutant. Those cities include: Lancaster (PA), Birmingham (AL), Indianapolis (IN), Houston, Macon (GA), Little Rock (AR), and Wheeling (WV).
  • Two other cities improved and also met the national air quality standard: El Paso-Las Cruces (TX-NM), and Shreveport (LA).

Many of the seven cities with more annual particle pollution compared to the 2014 report are in the West, especially in the central valley of California.

  • Six of those metro areas still fail the national air quality standard: Bakersfield (CA), Modesto-Merced (CA), El Centro (CA), San Jose-San Francisco, Cincinnati and Harrisburg-York (PA). Only Erie (PA) recorded higher annual particle levels and still met the national air quality standards. Johnstown (PA) had the same level as in the 2014 report, failing the standard, while new monitoring data added Altoona (PA) to the list, but at a level that passes the national test.

Data are missing for 4 cities that were in the 2014 most polluted cities list. Data are unavailable for St. Louis, Atlanta, Chicago and Columbus (GA) so it is impossible to know whether the air quality there improved or worsened. Problems with data processing in Illinois, most of Tennessee and part of Georgia prevented people in those states from having information on their particle pollution levels for this period.

Continued cleanup of power plants and the continued turnover of the diesel fleet have likely driven these reductions. These steps continue because of the Clean Air Act and U.S. Environmental Protection Agency steps to enforce it.

» 25 Most Ozone-Polluted Cities
» 25 Cleanest Cities for Ozone


  1. Note that the full metropolitan areas often include multiple counties, incorporated cities and counties in adjacent states, as the Office of Management and Budget defines them. Not all counties in any metro area will have monitors. These trends are based on prior available data. Not all cities had counties with complete annual averages posted for all prior years.
Did You Know?

Did You Know?Looking for a clean air city? Six cities made all State of the Air cleanest cities lists in 2011-2013.

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