The Environmental Protection Agency (EPA) announced Friday that Manitowoc and Door counties in Wisconsin have been redesignated as attainment areas under current smog standards. The agency didn’t say whether the counties would still be considered attainment if the EPA adopts updated smog standards proposed in January.

Between 2006 and 2008, EPA air-quality monitors in the counties measured smog levels that met the 1997 eight-hour standard, which limits atmospheric ozone to 0.085 parts per million. The counties maintained those standards through 2009, the EPA said.

Ozone is one of six pollutants considered harmful to human and environmental health regulated under the EPA’s National Ambient Air Quality Standards (NAAQS). It’s created when volatile organic compounds (VOCs) and nitrogen oxides (NOx) react with sunlight. Common sources of VOCs and NOx are industrial emissions and car exhaust.

Health problems associated with smog include aggravation of asthma and an increased risk of death for people with heart and lung disease, according to the EPA. Additionally, smog can stymie tree growth, damage leaves and weaken vegetation’s resistance to disease.