What is “good” and “bad” ozone? Ozone occurs in two layers of the atmosphere. The layer surrounding the earth's surface is the troposphere. Here, ground-level or "bad" ozone is an air pollutant that damages human health, vegetation, and many common materials. It is a key ingredient of urban smog. The troposphere extends to a level about 10 miles up, where it meets the second layer, the stratosphere. The stratospheric or "good" ozone layer extends upward from about 10 to 30 miles and protects life on earth from the sun's harmful ultraviolet rays (UV-b)

(EPA, Good up High, Bad Nearby).

Why is it called “bad” ozone?High levels, or concentrations of ozone at the earth’s causes

damage to people, plants, and animals. Ozone at the ground level occurs naturally but activities by humans can cause concentrations to increase. In humans, when air with high concentrations of ozone is inhaled it adversely affects the respiratory system. The most sensitive people are children, the elderly, and those with asthma or other respiratory illnesses. In vegetation, changes their color, injures the plant (sometimes visibly seen by changes in leaf color) and decreases their yields.

Ozone effects human health mainly as an irritant to the lungs (Bates, 1994; Horvath and McKee, 1994). Populations sensitive to increased ozone levels are people with pre-existing conditions such as asthma, and those involved in outdoor activities. Symptoms include decreased lung function, chest pains, and coughing. Additionally, studies indicate that increased levels of ozone contribute to decreased crop yield and forest decline (Office of Technology Assessment, 1989; Lefohn et al., 1987).

When are ozone concentrations highest?

Ozone concentrations vary over different scales of time. In urban areas where ozone is generated, ozone concentrations tend to peak in the middle of the day and dip to their lowest concentrations during the middle of the night. This type of cycle is called diurnal.

Ozone concentrations vary from day to day depending on emissions and meteorology but sometimes under certain meteorological conditions episodes occur. Episodes are periods of a few days to a couple of weeks when ozone concentrations are unusually high.For U.S. southern urban areas, episodes are caused by local stagnation, when wind speeds are slow. This allows ozone concentrations to build up. In northern U.S. cities, episodes are more heavily influenced by transport of ozone and precursors from other areas. Episodes are also associated with high temperatures and high humidity.

Because ozone concentrations are dependent on high temperature and sunlight, the highest concentrations during the year occur in the summer months. The lowest concentrations, or the cleanest air, occurs during the winter months. This type of cycle is called seasonal.