What conditions create poorest air quality during the summer

Huntsville
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HUNTSVILLE, Ala. (WHNT)-- Hazy, hot, humid. These three short words are used often during the summer to describe the weather.

The 22-day stretch of excessive summer heat took a toll on air quality-- and many people are now feeling the effects.

"The combination of light winds and high temperatures is a recipe for high ozone and high particulate county," says University of Alabama in Huntsville Atmospheric Science Professor Richard McNider.

Ozone poses one of the biggest risks to people's health on hot summer days.

Ozone is essentially an unstable molecule made up of three oxygen atoms.  It is created in the lower atmosphere by a combination of pollutants called reactive organic compounds and oxides of nitrogen that get “cooked” by the sun’s ultraviolet rays in a photochemical reaction.

Ozone-forming pollutants emitted in one part of the region can affect other areas as they get blown around by local air currents and trapped by geographical features like low-lying valleys.

McNider says the topography of the Tennessee Valley may play a role in where air quality is worst on stagnant summer afternoons. "You have less ventilation through those valleys, so you'll probably have a little bit higher concentrations there," he says.

Ozone's effect on the lining of the lung is comparable to the effect of sunburn on the skin. Ozone damages the cells that line the air spaces in the lung. Within a few days, the damaged cells are repaired, just as our skin recovers from sunburn naturally.

On typical summer afternoons, the air quality is usually poorest during the middle of the day. As the humidity drops, the particles in the atmosphere become smaller and easier to get deep into the lungs. So although visibility may improve, "the particles themselves are still there," says McNider.

In a few cases each summer, a layer of dust from northern Africa-- the Saharan Air Layer-- can migrate thousands of miles across the Atlantic and wind up right over our heads. McNider says the Saharan Air Layer usually only affects visibility, and doesn't have a significant impact on air quality.