What is smog? When public health officials or people in the media talk about smog, they are most likely referring to photochemical smog-the brown haze associated with automobile pollution. This type of smog was first recognized in Los Angeles, but it's common in urban and suburban settings throughout the United States. Though photochemical smog is a mixture of various chemical gases and particles, the primary component is ozone.

What is ozone?
Ozone (O3) is a toxic gas. Each molecule consists of three oxygen atoms that have bonded together chemically, resulting in a highly reactive and powerful oxidizing agent. These chemical properties make ozone an excellent disinfectant, and it is often used to sterilize medical equipment or as an alternative to chlorine in the purification of drinking water.

Doesn't ozone exist naturally in the atmosphere?
Ozone is found in two different layers of the Earth's atmosphere. In the stratosphere-the part of the atmosphere roughly between five and 30 miles above the planet's surface-ozone occurs naturally and is very beneficial. Here, it acts as a shield by absorbing ultraviolet radiation, thereby reducing our exposure to the sun's harmful rays. However, ozone is also found at ground level, in the section of the atmosphere called the troposphere, which extends approximately five miles up from the surface. It's this ground-level, or tropospheric, ozone that makes up smog.

How is ground-level ozone formed?
Ozone doesn't come out of automobile tailpipes or smokestacks directly. Rather, the emissions from such sources contain the raw ingredients for making it.

To create ozone, you need nitrogen oxides and hydrocarbons. These chemicals are emitted in motor-vehicle exhaust and by industrial factories and power plants. Hydrocarbons are also emitted by refineries, and are found in the vapors from gasoline, paint, and various solvents. Sunlight (hence the term photochemical) and warm temperatures drive the chemical reaction that creates ozone. The mixture of pollutants is literally cooked in the air.

In fact, you won't find much ground-level ozone forming on cold winter days when sunlight intensity is weak. On days when wind is minimal, or when air is trapped due to a temperature inversion (when a layer of warmer air rests over cooler air, it acts like a lid, trapping the air below it), ozone concentrations increase dramatically. All the conditions needed to generate ozone-lots of automobile emissions, temperature inversions, and plenty of hot, sunny days-come together perfectly in a place like Los Angeles, which is why the city is currently the most ozone-polluted city in the country (see table, page 2).

What are the adverse health effects of ozone?
Ozone is an irritant that can trigger or exacerbate asthma. Short-term exposure may result in coughing and throat irritation, while repeated exposure has the potential to permanently scar lung tissue and reduce lung function. Children, adults who are active outdoors, people with respiratory diseases, and anyone with high sensitivity to the pollutant are most at risk from ozone exposure.

Many studies have shown ozone's deleterious effects in humans. For example, a study conducted during the 1996 summer Olympic games in Atlanta demonstrated a correlation between ground-level ozone and asthma attacks in children. In an effort to stem traffic congestion during the games, the city closed a number of roads and beefed up its public transportation fleet by adding 1,000 city buses. Monitoring of air quality showed that ozone levels dropped nearly 28 percent during the Olympics compared with levels during the weeks before and after the games. This decrease was associated with a 41 percent drop in asthma attacks among children, as reported in a Medicaid database.

Does ozone harm the environment?
The short answer is yes. Ozone can damage and kill leaves, impairing a plant's ability to produce food and causing it to become more susceptible to pests and disease. Crop yields can be reduced, and forest growth and regeneration can be negatively affected by repeated exposure to ozone. It's important to realize that ozone can have an impact far beyond the area in which it's produced. Air-quality monitoring has shown that wind can carry the gas hundreds of miles. In fact, many of our national parks-including Yosemite and the Great Smoky Mountains-are adversely affected by smog that limits visibility and harms plant life.

What's being done to control ground-level ozone?
Thanks to the Clean Air Act of 1970, the Environmental Protection Agency is required to monitor and set maximum allowable concentrations of so-called criteria air pollutants, a group that includes ground-level ozone (the others are particulate matter, carbon monoxide (CO), sulfur oxides, nitrogen oxides, and lead). In 1997, the EPA lowered the allowable levels of ground-level ozone to a maximum of 0.08 parts per million (ppm) over an 8-hour period during a single day.

To be sure, the Clean Air Act has had an extremely positive impact on air quality since its inception. Regulating the maximum allowable concentration of ground-level ozone has led to a slow but steady decline of the pollutant. Most recently, the EPA reported that between 1990 and 2006, the national average for ground-level ozone has decreased by 9 percent. However, the American Lung Association estimates that one-third of Americans-approximately 99 million people-still live in communities with unhealthful levels of ozone.

What's more, scientists and public health advocates argue that the latest research strongly suggests that current levels don't go far enough to protect public health. In March 2007, the EPA's Clean Air Scientific Advisory Committee unanimously recommended that the current ozone standard should be lowered further, to 0.07 ppm. (It's worth noting that California, which often calls for stricter controls than the federal government on a variety of pollutants, already has a maximum allowable ozone level of 0.07ppm.)

In recent years, the American Lung Association pressed and won a lawsuit against the EPA, forcing the agency to review current ground-level ozone regulations. In June 2007, the EPA was expected to announce new recommendations for allowable levels. However, a final ruling is not expected until March 2008, to allow for public comment and review.

How can I find out about ozone pollution where I live?
There are two easy ways to obtain information about air quality in your community. First, local newspapers and TV and radio weather forecasters keep tabs on air pollution with the aid of the Air Quality Index (AQI). This system, which collects air quality data from 700 counties across the U.S., is used to warn the public about dangerous air pollution levels. Pollutants are monitored and measured daily and ranked on a scale of 0 (perfect air) to 500 (very polluted air). To see current national air quality conditions and learn more about the AQI, visit www.airnow.gov.

Another source of information is the American Lung Association's State of the Air: 2007 report, which provides information about ozone pollution in the U.S. from 2003 through 2005. A handy feature allows users to input their ZIP codes to learn about local air quality. Go to www.lungusa.org.

What can I do to decrease smog?
Cutting your emissions of nitrogen oxides and hydrocarbons is the best way to limit smog formation. For most people, that means driving less. Try walking, biking, carpooling, or using public transportation when it's available. Hand-powered or electric lawn-care equipment will also help reduce smog. Public health advocates encourage people to get involved with their local community boards to spur better smog monitoring and tighter controls on emissions that cause smog. You can find more detailed information about reducing your contribution to smog at www.lungusa.org and www.epa.gov.

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