COLUMBUS — The difference between unhealthy, smoggy air and a clear sky in Ohio often is a matter of degrees.
Last summer’s hot, still days helped create one of the Columbus region’s worst summers for smog.
Warm, breezy days so far this year largely have kept the haze at bay.
There have been four days — June 12, Thursday, Friday, and Saturday — when the air has been considered unhealthy.
The Mid-Ohio Regional Planning Commission, which forecasts air quality, issued warnings only for Friday and Saturday.
By this time last year, the commission had issued six unhealthy-air alerts.
“It’s been a little more windy, too,” this year, said Evelyn Ebert, a commission spokesman.
“So there is less stagnant air that lets those pollutants form.”
Smog forms in the air on hot, still days when the sun cooks pollutants from cars, trucks, power plants, and factories.
It can irritate lung tissue, trigger asthma attacks, and worsen other chronic breathing problems.
The forecast has been for unhealthy levels for sensitive groups — children, older people, and those with lung disease.
There were 21 smog alerts last year, the most since 30 were issued in 2006.
National Weather Service data show that daily high temperatures in 2012 exceeded 85 degrees on 16 days from May 1 through June 19. That included seven days when temperatures were at least 90 degrees or higher.
“It’s been considerably cooler,” said Mike Kurz, a weather-service forecaster. “There wasn’t a day above 90 in May.”
Heidi Griesmer, spokesman for the Ohio Environmental Protection Agency, said a steady decrease in pollution from cars, trucks, and power plants also has contributed to this year’s decrease.
The U.S. EPA sets limits on smog levels. The current limit is a three-year average of no more than 75 parts per billion.
Ohio has until 2015 to meet that limit. As of now, the three-year average is about 76 parts per billion. Mr. Kurz said wind patterns in the upper atmosphere might mean a hotter July and August.
“Right now, they are favoring the potential for above-normal temperatures for the rest of the summer,” he said.
And that could mean an increase in smog warnings.
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