What is

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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).
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Why is it called bad ozone?
High levels, or concentrations of ozone at the
earths 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).
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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.