Bad air day: more than 142 million Americans, or 50 percent of the U.S. population, breathe grade "F" air. The American Lung Association's state of the Air 2002 Report studied air quality in 678 counties, and 400 flunked. What's choking the air? An invisible gas--ground-level ozone. Read the graph below, then check out ozone chemistry, and test your pollution know-how. (Earth science: ozone/air pollution).

What's Bad Air? The U.S. Environmental Protection Agency (EPA) monitors air quality daily. And one major pollutant they keep an eye on is ozone. At unhealthy levels, ozone can trigger lung irritations and breathing problems in merely four hours of outdoor activities. Ozone is measured in parts per million (ppm)--the number of ozone molecules for every million molecules of air mixture. Here's how the EPA ranks ozone concentration in the air: good (0.0 to 0.064 ppm); moderate (0.065 to 0.084 ppm); unhealthy for sensitive groups (0.085 to 0.104 ppm); unhealthy (0.105 to 0.124 ppm); and very unhealthy (0.125 ppm and above). "To increase public awareness we've put EPA's findings in terms that more people can relate to, like school grades," says Janice Nolen, Director of National Policy at the American Lung Association. And grade "F" counties had at least 10 days with ozone levels above 0.085 ppm over three years.

How "Bad" Ozone Forms

Naturally occurring ozone gas, which resides 10 to 17 kilometers (6 to 10 miles) above Earth's surface, offers protection from the sun's harmful ultraviolet (UV) rays. Ground-level ozone, although composed of the same chemical formula (three atoms of oxygen), is a harmful air pollutant and lung irritant. It's also the primary ingredient of urban smog. Here's how "bad" ozone is made.

1 Motor vehicle exhaust and industrial emissions spew nitrogen oxides (NOx)-including nitrogen dioxide (NO)--into the air.

2 The sun's UV rays and air temperature above 20[degrees]C (68[degrees]F) convert NO2 into one molecule of nitric oxide (NO) and one atom of oxygen (0): NO2 + UV sunlight = NO + O

3 The single oxygen atom combines with molecular oxygen (O2) in the air to form three atoms of oxygen, or ozone (O3): O + O2 = 03

4 In clean air, the NO reacts with O3 to form NO2 and O2: NO + O3 = NO2 +O2. Ozone is broken down.

5 But in polluted areas, volatile organic compounds (VOCs)--emitted from automobile exhaust, power plants, solvents in paints, factories, dry cleaners, and even vegetation, like trees--interfere with Step 4. VOCs react with NO, converting it back to NO2:

VOC + NO = NO2

Excess NO2 means there's less NO in the air to break down ozone (step 4). The pollutant accumulates.

WHAT DO YOU KNOW ABOUT YOUR AIR?

Check out the diagram above and graph at left to correctly answer the following questions:

1. Which of the following describes lower atmospheric ozone?

a. It occurs at ground level.

b. It protects Earth against UV rays.

c. One molecule contains two oxygen atoms.

d. One molecule contains two nitrogen atoms.

2. Which of the following statements are true?

a. The combination of motor vehicle and industrial emissions alone are enough to form "bad" ozone.

b. Sunlight and air temperature play a role in converting car and factory emissions into "bad" ozone.

c. A molecule of ozone contains one atom of nitric oxide.

d. An excess of nitrogen dioxide in the air causes more "bad" ozone to form.

3. Ozone is measured in parts per

a. hundred.

b. thousand.

c. hundred-thousand.

d. million.

4. From 1998 to 2000, Fresno County had a total of 231 days with ozone levels above

a. 0.065 ppm.

b. 0.85 ppm.

c. 0.125 ppm.

d. 0.350 ppm.

5. Why do you think seven of the 10 most ozone-polluted counties in the U.S. are in California?

To learn more about ozone: www.epa.gov/airnow For more on the State of the Air 2002: www.stateoftheair.org

THE 10 MOST DZONE-POLLUTED COUNTIES IN THE U.S.

                                            TOTAL NUMBER OF DAYS WITH
                                            UNHEALTHY LEVELS OF OZONE
RANKING                        POPULATION   (1998-2000)

  1       San Bernardino, CA   1,669,934              270
  2       Kern, CA               642,495              250
  3       Fresno, CA             763,069              231
  4       Riverside, CA        1,530,653              199 *
  5       Tulare, CA             358,470              222
  6       Harris, TX           3,250,404              142
  7       Fulton, GA             744,827              117
  8       Los Angeles, CA      9,329,989              110 *
  9       Merced, CA             200,745              112
 10       Sevier, TN              65,783              111

* These counties rank as more polluted than the ones below them even
with fewer total days of high-level ozone. Their ranking combines fewer
days with much higher ozone levels.

Note: Table made from bar graph.

ANSWERS

1. a 2. b and d 3. d 4, b 5. Answers may vary, but may include California's high number of vehicles on the road and generally warm climate, a perfect combination to form excessive ozone.