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.

Fresno-Madera-Hanford, CA returned to the rank of most polluted by year-round particle pollution in 2015-2017. This metro area now officially includes Kings County, the county with the highest year-round levels of particle pollution in the nation. This ties the highest year-round levels ever for Kings County, and for the metro area.

Fourteen of the 25 cities most polluted year-round by particle pollution improved over the levels in the 2018 report. Bakersfield, Visalia, CA, and El Centro, CA, continued to improve as they had in the 2018 report. Last year's most-polluted city, Fairbanks, AK, dropped back to #3. Ten reached their lowest annual level ever: Cleveland; Detroit; Birmingham; Lancaster, PA; Houston, TX, Philadelphia; Chicago; Indianapolis; Harrisburg, PA; and Knoxville, TN. This is Chicago's first year back with complete data on particle pollution in Illinois.

Six others improved over the 2017 report: Visalia-Porterfield-Hanford, CA; Bakersfield, CA; El Centro, CA; San Jose-San Francisco; San Luis Obispo, CA; and Atlanta.

Fairbanks, AK, moved to the most-polluted city for the first time. Previously ranked as #17 most polluted, Fairbanks' improved monitoring in the borough now identifies that this problem is more severe than previously known. Six other cities in the 25 most polluted had higher particle levels year-round: Los Angeles; Pittsburgh; Lancaster, PA; Birmingham, AL; Harrisburg-York-Lebanon, PA; and Las Vegas.

Fourteen of the 25 cities with the highest year-round particle levels experienced lower levels in the 2019 report.

Eleven of the 25 cities suffered worse year-round levels. Two, Fresno-Madera-Hanford, CA and Missoula, MT, tied their worst annual average levels of particle pollution. Others that had higher levels were Los Angeles; San Jose-San Francisco; Pittsburgh; Medford-Grants Pass, OR; Cincinnati; Johnstown-Somerset, PA; Atlanta; McAllen-Edinburg, TX; and Shreveport, LA.

All the cities below the seven most polluted meet the current national air quality standards. Often annual levels vary in cities once they clean up enough to meet that standard. However, evidence shows that no threshold exists for harmful effects from particle pollution, even below the official standard.

California continues to dominate this list, with six of the 10 most-polluted, and five of the seven cities that fail to meet the annual standard. Pennsylvania has five cities on this list, although only Pittsburgh fails to meet the standard. Other areas with several cities on the list include the Midwest with five cities; Southeast with four cities; the Northwest with three cities; and Texas with two.

For San Jose-San Francisco, the higher levels came in the two counties added to the metro area by the Office of Management and Budget. Both had been part of the former Merced-Modesto, CA MSA. They were incorporated into the larger CSA because of increased integration with the larger metro area.

Cities with high power plant emissions as well as local, industrial sources continue to show up on the list. That list includes Pittsburgh; Philadelphia; Detroit; Cincinnati; Cleveland; Chicago; Birmingham; Atlanta; Indianapolis; Youngstown; and Shreveport, LA.

Fortunately, year-round particle pollution continues to decline across most of the nation, unlike the days with high ozone and high short-term particle pollutions.

Because of the high numbers and long duration, the western wildfires contributed to some of the elevated annual averages in western cities. That is especially true in Missoula, MT, Medford-Grants Pass, OR, and likely in Los Angeles as well.