What Are "Short-Term Levels" of 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.
Bakersfield (CA) remains the city most polluted by spikes in particle pollution. Bakersfield has held this position for all but two years since the 2010 report covering data from 2006-2008. Twenty of the 26 most polluted cities had more days on average in the 2019 report. Many of these are due to wildfires.
Eight cities had their highest-ever weighted average number of days with spikes in particle levels: Fairbanks, AK; Missoula, MT; Yakima, WA; Spokane-Spokane Valley-Coeur d'Alene, WA-ID; Santa Maria-Santa Barbara, CA; Salinas, CA; Bend-Prineville, OR; and Bismarck, ND.
Showing the impact of wildfires, prior to this year's report, Santa Maria—Santa Barbara, CA had been on the list of cleanest cities for short-term PM for the past six years.
Twelve others also suffered from more days when particle pollution spiked into unhealthy levels. San Jose-San Francisco, Los Angeles, Seattle and Pittsburgh, Logan, UT; Phoenix; Sacramento; Medford-Grants Pass, OR; Eugene, OR; Reno, NV; Portland, OR; and Pocatello, ID.
Wildfire smoke shifted Santa Maria—Santa Barbara, CA off the list of cleanest cities for short-term particle pollution for the past six years to rank as the 17th most polluted city.
San Jose-San Francisco CSA added two counties from the former Modesto-Merced MSA, which had ranked fifth most polluted in the 2018 report. Their addition increased the weighted average, but, even without those counties, the metro area had more days.
Six of the 26 most-polluted cities improved and had fewer unhealthy air days on average than in the 2018 report. Despite being among the 10 most-polluted, Salt Lake City dropped to its fewest days ever reported on average. Five other cities had fewer unhealthy days on average: Bakersfield, CA; Fresno-Madera-Hanford, CA; Anchorage; Visalia, CA; and El Centro, CA.
In California, Washington and Oregon, extended wildfires increased the days when PM levels spiked. The Los Angeles metro area had two days when levels spiked to "hazardous," the highest "maroon" level in the Air Quality Index. Medford-Grants Pass, OR, Eugene, OR and Calaveras County, CA each had one maroon day.
Wildfires are not the only source of high particle pollution days. Other sources including wood stove use (especially in Fairbanks), older diesel vehicles and equipment, and industrial sources (as in Pittsburgh) contribute to a lot of particle pollution. Changes in weather patterns can create atmospheric inversions that trap particles in place, leading to days with spikes. Pittsburgh is the only city in the 25 most polluted that is east of the Mississippi River.