Short Term Particle Pollution

Fifteen percent (15%) of people in the United States live in an area with too many days with unhealthful levels of particle pollution. More than 47.7 million Americans live in 58 counties that experienced too many days with unhealthy spikes in particle pollution, a decrease from the last report.

What is Short-Term Particle Pollution?

Particle pollution can be harmful even if it is inhaled over just a few hours or days, even if the year-round averages are low. “Short-term levels” refers to just such spikes. These represent levels averaged over a 24-hour period. Those days or weeks of high levels can be dangerous, even deadly. To learn more, go to
Health Risks.

Short-term spikes in particle pollution can last from hours to several days and can increase the risk of heart attacks, strokes and emergency room visits for asthma and cardiovascular disease, and most importantly, can increase the risk of early death.

One disturbing trend is the increase in the number of days of unhealthy particle levels in many cities. Although year-round average levels for particles are steadily dropping, the reverse is true for short-term spikes in days with high particle counts. These periods often occur in the winter, as has happened in Fairbanks (AK) and Salt Lake City in recent winters. In some cities, the increase comes from increased burning of wood and other fuels in the winter for heat, often in highly polluting indoor wood stoves or outdoor wood boilers. 

    "For six cities, this period held their worst record for short-term days since the data started to be collected."
  • Of the 25 cities with the worst problem with spikes in particle pollution,1 fourteen had more days or worse problems in 2009-2011 than in the previous report. For six cities, this period held their worst record for short-term days since the data started to be collected: Hanford (CA); Modesto (CA); Merced (CA); Fairbanks (AK); Stockton (CA), and Las Cruces (NM).  Other cities with more days of unhealthy particle pollution levels include: Los Angeles, Salt Lake City, Chicago, Seattle, Indianapolis, Phoenix, Allentown (PA), and Portland (OR).
  • Nine cities on the most polluted list continued to improve in 2009-2011. Five cities reported their best records yet: Fresno; Pittsburgh; Visalia (CA); Eugene (OR); and San Diego. Other cities improving include: Bakersfield (CA); Logan (UT); Provo (UT); and Green Bay (WI). Two cities had the same number of days as in the last report: Harrisburg (PA) and South Bend (IN).

» 25 Cities Most Polluted by Short-Term Particle Pollution
» 25 Cleanest Cities for Short-Term Particle Pollution


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.