Oklahoma’s air may be getting worse.

The newest data from the Environmental Protection Agency shows air quality throughout much of the state was poorer during each of the past two years. That bucked a trend in which Oklahoma, like most of the country, had seen significant strides in making the air healthier during much of the past decade.

An Oklahoma Watch analysis of the federal statistics shows that in the 13 Oklahoma cities with air quality monitoring stations, air quality was classified as “good” on 78% of days in 2018. That’s down from a record high of 83% in 2016.

The number of days considered unhealthy for sensitive groups climbed to 45, up from 20 in 2017, 12 in 2016 and five in 2015.

Oklahoma isn’t alone. The Associated Press recently reported that across the country, there were 15% more days with unhealthy air in each of the past two years than there were on average from 2013 through 2016. During those four years, the country had its fewest number of unhealthy air days since at least 1980.

Experts caution that in addition to man-made pollutants, many factors, including temperature, drought and wildfires, can affect air quality. More data are needed to determine whether the decline in air quality is a trend.

But environmental and health-policy advocates say the data present a warning sign. They say efforts to reverse Obama-era environmental regulations could undo years of progress and that more should be done to protect public health.

“Really, in the last 50 years we have seen so much improvement, but we are starting to see things turn,” said JoAnna Strother, regional director of public policy for the American Lung Association. “I don’t know (that) we are calling it a trend yet, but I definitely think this should serve as a call for immediate action.”

Tulsa experienced an increase in poor air-quality days last year, with alerts on 10 days that the air was unhealthy for sensitive groups and on one day that it was unhealthy for the entire population. That was the most alerts the city has had since 2013.

Despite the 2017-18 dip in air quality, regulators and environmental activists agree that the state’s air is still significantly better than it was decades or even several years ago.

As early as 2011, air monitoring stations across the state recorded some of the highest numbers in modern history on the Air Quality Index. The EPA uses that measure to rank each day, taking into account the most common air pollutants, such as ground-level ozone, particle pollution, carbon monoxide and sulfur dioxide.

Days with a score of 100 or below are considered “acceptable,” although some unusually sensitive people are warned to reduce prolonged outside exposure or heavy exertion when the score is above 50. The Oklahoma Department of Environmental Quality sends alerts when the score rises above 100, a “code orange” meaning unhealthy for sensitive groups, or 150, a “code red” meaning unhealthy for the entire population.

Almost all air monitoring stations in Oklahoma recorded a double-digit number of “code orange” days in 2011. In both Oklahoma City and Tulsa, more than 10% of the year was classified as unhealthy for sensitive groups.

From 2011 to 2016, EPA data show that air quality conditions steadily improved each year.

That changed in 2017, when the number of code orange or red days increased slightly for several major cities, including Oklahoma City.

Last year that downward trend continued as the number of code orange or red days was the highest since 2013 or 2012.

DEQ data show that high levels of ground ozone pollution and fine-particle pollution were the most common culprits on Oklahoma’s poor air-quality days.

According to the EPA, high levels of ozone occur when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants and other sources chemically react in the presence of sunlight. The EPA states that “ozone is most likely to reach unhealthy levels on hot sunny days in urban environments but can still reach high levels during colder months.

Particle pollution, meanwhile, can be caused directly by sources such as construction sites, unpaved roads, fields, smokestacks and fires. Particle pollution can also be emitted from power plants, industries and automobiles.

Curt Goeller, environmental programs manager with the state DEQ’s Air Quality Division, was surprised at first when Oklahoma’s air quality data showed a decline last year.

“We were scratching our heads about that for a little bit,” he said.

But after studying the data, Goeller concluded that the most likely culprit was the severe drought and fire season that afflicted much of the West, including parts of Oklahoma, last year. (He acknowledged that many factors can affect air quality, including automobiles and power plants.)

Smoke from large fires can increase the amount of microscopic particles in the air that are dangerous when inhaled, Goeller said. On top of that, he said fire emits carbon monoxide and other chemicals that can cause hazardous ground-level ozone levels to rise, he said.

With a much wetter winter and spring so far this year, Goeller said he hopes the air-quality numbers will improve.

“I’m anxious to see if this whole concern is a lot more weather-related than pollutant-related,” he said.

Others are more skeptical that fire and weather played the dominant role in worsening Oklahoma’s air quality.

Johnson Bridgwater, director of the Oklahoma chapter of the Sierra Club, said the drop in air quality here and elsewhere might reflect initial impacts of President Donald Trump’s continuing moves to roll back dozens of environmental regulations.

The Trump administration announced in June that it will scrap the Clean Power Plan, one of President Barack Obama’s signature environmental rules aimed at reducing the country’s energy-generation emissions by nearly a third by 2030.

“For the environment, that’s where the Obama administration truly changed the United States, and now (the Trump administration) is systematically cracking back at it and trying to destroy it,” he said. “Now we are seeing the results of that.”

DEQ spokeswoman Erin Hatfield said the agency is still studying the possible effects of Trump’s decision to replace the Clean Power Plan with a stripped-down policy called the Affordable Clean Energy rule, which calls for reducing emissions by only 0.7% to 1.5% by 2030.

She added that it’s still too early to say whether Trump’s other deregulation moves have caused the drop in air quality.

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