The National Weather Service issued an air quality alert for southern Fairfield and New Haven counties on Friday.

Officials said there could be poor air quality from 11 a.m. to 11 p.m. Friday.

“An Air Quality Action Day means that Ground Level Ozone within the region may approach or exceed unhealthy standards,” the NWS said.

Often called smog, ozone pollution is harmful to breathe. Ozone aggressively attacks lung tissue by reacting chemically with it. When ozone is present, other harmful pollutants can be created.

Ozone develops in the atmosphere from gases that come out of tailpipes, smokestacks and many other sources. When these gases come in contact with sunlight, they react and form ozone smog.

The American Lung Association “State of the Air Report” has given Connecticut an F for high levels of ozone pollution. The annual air quality report card tracks Americans’ exposure to unhealthful levels of ozone and particle pollution, both of which can be deadly.

The methodology of the report determined a “weighted average” of ozone pollution based on the number of “orange” and “red” days.

Orange days are when the air quality index is between 101 and 150, on days when temperatures are in the 80s and 90s and winds are light. Red days are when air quality index is between 151 and 200, on days that are hot, hazy and humid, the air is stagnant and skies are sunny with little chance of precipitation.

Fairfield County has the highest number of high ozone days in Connecticut with an average of 25.8 days. Last year, it had 42 orange days and 18 red days.

For additional information, visit the Connecticut Department of Energy and Environmental Protection website.