Smog rises slightly in NC cities. Here's how Charlotte ranks in the U.S.

Charlotte was among N.C. cities that have seen a small increase in unhealthy-air days due to ozone pollution, the main ingredient in smog, the American Lung Association said Wednesday.

Air quality has improved in Charlotte, North Carolina and the nation in recent years as state and federal laws limited emissions from sources such as coal-fired power plants and motor vehicles.

But the number of unhealthy days due to ozone ticked up slightly in most U.S. cities from 2014 to 2016, the period assessed in the Lung Association report. The association attributes the rise to warmer temperatures in 2016. Ozone forms in the atmosphere on hot, sunny days.

The report ranks Charlotte as the nation's 41st most-polluted city from ozone, the report says. That's an improvement from a decade ago, when the Lung Association regularly ranked it among the 25 most ozone-plagued cities.

The report lists 17 potentially unhealthy days, known as Code Orange days, in Mecklenburg County from 2014 to 2016, far more than any other N.C. county. The Charlotte region recorded two potentially unhealthy days in 2017, state data shows.

Mecklenburg had no unhealthy days in 2014 to 2016 due to a different form of air pollution, airborne particles from diesel exhaust and other sources.

The N.C. Department of Environmental Quality offers daily air quality forecasts for Charlotte and other metro areas.

Emissions from Charlotte traffic are the biggest drivers of ozone formation in the atmosphere. At high levels, the invisible gas can trigger asthma attacks, aggravate people with lung diseases such as emphysema and heart disease, and cause premature death. Children, seniors and people who work outside are particularly at risk.

The Charlotte metro area met federal clean air standards for ozone in 2015.

This story was originally published April 18, 2018 at 10:35 AM with the headline "Smog rises slightly in NC cities. Here's how Charlotte ranks in the U.S.."