RALEIGH -- Statewide monitoring shows that North Carolina continues to be in full compliance with national air quality standards as the 2017 ozone season ends.

Since March 1, the state recorded just four unhealthy ozone days with concentrations higher than the 70 parts per billion (ppb) ozone standard set by the Environmental Protection Agency in 2015.

It’s the best outcome since the previous record low of five unhealthy ozone days in 2013.

“This is a positive indicator that our partnerships focused on curbing air pollution are working,” said Mike Abraczinskas, director of the Division of Air Quality (DAQ) in the N.C. Department of Environmental Quality. “This year’s ozone data provides clear evidence that a robust economy and healthy environment can thrive at the same time.”

Ozone forms in the air when nitrogen oxides, or NOx, react with hydrocarbons on warm, sunny days with little wind. Ozone can be unhealthy to breathe, particularly for children, people with respiratory problems, and even healthy adults who work or exercise outdoors. Exposure to high ozone levels may cause previously healthy individuals to develop asthma over time. Ozone also causes millions of dollars in tree and crop damage each year in the U.S.

The 2017 unhealthy ozone days were reported May 16 in Mecklenburg and Guilford counties; July 20-21 in Mecklenburg County; and Sept. 28 in Union County.

The National Ambient Air Quality Standards for ozone was strengthened from 75 ppb to 70 ppb in 2015.

Air quality monitors in North Carolina are in compliance with the 2015 ozone standard, according to analyses by DAQ and local air programs in Buncombe, Forsyth and Mecklenburg counties. Based on this analysis of data from 2014-2016 and preliminary data from 2015-2017, the state has recommended that EPA designate the entire state as attainment.

While North Carolina data demonstrates full compliance, the EPA has delayed making designations nationwide under the new standard. The EPA designations were anticipated earlier this month. The official EPA determination will be based on a three-year average of the fourth-highest ozone values at each monitor for the 2014-2016 period.

In the early 2000s, about one-third of the state’s counties were classified as non-attainment for ozone, and Code Red and Orange ozone alerts were a frequent occurrence during summer months. However, ozone levels during the past few years have been the lowest since the state began monitoring the air in the early 1970s.

North Carolina’s air quality has improved due to declining emissions from motor vehicles, power plants and other industrial sources. The Clean Smokestacks Act that North Carolina adopted in 2002 required the state’s coal-fired power plants to reduce their emissions by about 75 percent. EPA requirements have led to lower emissions from other industrial sources, cars and trucks, as well as cleaner gasoline and diesel fuel.