What does an Air Quality Alert really mean?

Monday, July 5, 2010 at 11:45pm

Speeding down the highway on a blazing hot day, air conditioners cranked, at least some motorists will feel guilty as they pass under electronic signboards that advise, “Today: Air Quality Alert” and suggest drivers reduce trips, carpool or take public transit.

During these days of summer, harmful pollutants produced by motor emissions can collect in the air, creating a health hazard for many. Based on the weather forecast for the coming day and the current air pollutant levels measured by sensors throughout the region, local officials try to determine if a public health risk is imminent.

If levels of ozone or particulate matter creep high enough, an air quality alert is issued.

What we’re breathing

Nashville’s most frequent air offender, ozone, is a molecule composed of three oxygen atoms. Though beneficial in the upper atmosphere, ozone is harmful at ground level. It attacks lung tissue through a chemical reaction that can be likened to sunburn on your lungs, potentially causing or worsening asthma attacks and reducing lung function, among other effects.

Formed when emissions from cars, industrial sites, lawn equipment and other sources react with sunlight, ground-level ozone is most prevalent on hot, dry sunny days with light winds.

Particulate matter, as the name suggests, are minute bits of solid and liquid pollution present in the air. Created through the burning of fossil fuels or through mechanical processes like construction and agriculture, particulate matter is most prevalent on hot and humid sunny days. Its associated health risks include decreased lung function, aggravated asthma, chronic bronchitis and premature death, according to the American Lung Association.

During Nashville’s summers, the pollutants can rise to unhealthy levels as determined by the Air Quality Index, a color-coded scale created by the U.S. Environmental Protection Agency to evaluate how clean or polluted the air is. Green and yellow indicate good and moderate air quality, respectively, and few risks.

But when it hits orange, the air is harmful for sensitive groups, including those with heart and lung conditions, small children and the elderly, as well as those who are physically active outdoors. According to the ALA, in 2008 in Davidson County, there were 57,000 people with asthma, 167,000 people with cardiovascular disease, 20,500 people with chronic bronchitis 218,000 people either under the age of 18 or 65 and older.

Orange alerts make up the bulk of the alert days in the Nashville area, though the ALA reports two red-level alerts in Davidson County between 2006 and 2008, a level indicating the air is unhealthy for the general population.

Comparing the air

In total, the greater Nashville area experienced five alerts last year, down from 14 in 2008 and 20 in 2007, according to the Clean Air Partnership of Middle Tennessee, a nonprofit group focused on air quality in the eight-county Nashville region.

Though the EPA does not rank regions by air quality, the ALA uses the EPA’s data to make comparisons — and Music City is nowhere to be found among the areas with the dirtiest air. Our region doesn’t make any of the ALA’s cleanest lists, either.

One factor working against us is Nashville’s geography, said Pam Barkey, meteorologist for the Tennessee Department of Environment and Conservation. “Because Davidson County sits in a bowl — we have the ridge around us — sometimes stuff gets trapped until there’s enough wind to blow it out,” she said.

But because the majority of our air pollution comes from cars and trucks, according to Clean Air Partnership spokeswoman Melissa Stevens, anything people can do to reduce their emissions — carpooling, using public transit, reducing their trips — can help the region avoid alert days. So far in 2010, Metro Nashville has experienced four alert days, two of which came long before the typical June-to-August season.

“We were all surprised when we got those two in April,” Stevens said. “If the weather continues in a pattern like that, I guess we’d see some more.”