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Outdoor Air:
Ozone
Ground-level ozone (the "bad" ozone) generally forms in on hot sunny days from interactions between
two classes of chemicals: nitrogen oxides (NOx), which are formed during
combustion, and reactive volatile organic compounds (VOCs), such as
ethylene and propylene. Ozone also exists high in the atmosphere (the "good" ozone) where
it helps to protect us from overexposure to ultraviolet rays.
 Ground-level
ozone is measured in parts per billion (ppb). Starting around 60 ppb, people who
are very sensitive to ozone, such as young children with asthma, may begin
having ozone-related symptoms. In the Houston area, warnings are issued when any
of the region's monitors reach or exceed 125 ppb, averaged over one hour. The EPA uses a
system that color codes pollution levels. We receive an orange warning if one-hour ozone levels
are between 125-164 ppb, a red warning between 165-204 ppb, and a purple warning
between 205-404 ppb.
To see a map of current ozone levels at our
monitors, go to the Texas Commission on Environmental Quality (TCEQ)
Houston ozone map. To see a summary of high one-hour and eight-hour ozone averages in the metropolitan areas
where the TCEQ monitors ozone, go to High Ozone in Your Metro Area.
In 2005, the Houston area had 33 days on which the one-hour average for ozone reached or exceeded 125 at one or more of the area's monitors. The area had 52 days on which the eight-hour average for ozone reached or exceeded 85 at one or more of the area's monitors.
Why is ground-level ozone bad?
- Ozone is a highly reactive and corrosive gas that, at high
concentrations, can literally "burn" the cells lining your airways.
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At levels commonly found in the Houston area on "bad air" days, ozone
irritates airways and can cause inflammation, mucus formation and may cause
some of the airways to constrict.
- Burning eyes, sinus headaches
and irregular heartbeats have also been associated with elevated levels of
ozone outdoors.
- Individuals with asthma or other respiratory or
allergic diseases generally experience symptoms at lower levels of ozone
than do people without these conditions.
- Recent evidence also
suggests that ozone not only exacerbates asthma, but may actually cause it,
especially in exercising children (McConnell,
2002).
- Numerous studies have linked increased visits to
hospital emergency rooms and increased hospital admissions to elevated
levels of ozone outdoors (Brunekreef,
2002).
- People are more likely to get a cold or other
respiratory infection during or shortly after high ozone days.
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Ozone also causes considerable damage to agricultural crops and to surface
coatings on buildings and cars.
Does Houston have an ozone problem and if yes, why?
In most years, Houston is second only to Los Angeles in the number of days that
ozone levels reached or exceed 125 ppb. A large industrial base that emits large amounts of
highly reactive VOCs and NOx, extensive use of the automobile, a long ozone
season and relatively lax regulations contribute to Houston's ozone problems.
Because of the complex chemistry of ground-level ozone formation, suburbs
downwind of the NOx and VOC sources sometimes have higher levels of ozone
pollution than do highly populated or industrial-source areas. (Brunekreef,
2002). How do I know when ozone levels are high?
Ozone forecasts can be found on the internet at the Texas Commission on Environmental
Quality’s site and on local broadcasted news weather forecasts. You can also call toll free
1-888-994-9901 for hourly updated air pollution warnings. To receive e-mails notifying you
that ozone levels are high in your community, sign up at the
Harris County Office of
Emergency Management.
What should I do when ozone levels are high?
Ozone levels are generally much lower indoors (the only exception is if
ozone-producing equipment, such as copiers and some air filters, is used indoors). On high ozone days, susceptible
populations should stay indoors and avoid outdoor exercise. To learn more,
download Ozone: Your 2004 Lung
Protection Guide, a four-page brochure produced by St. Luke's Lung
Institute. BACK TO OUTDOOR AIR
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