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Unlike many air pollutants which are emitted directly from a specific source, such as sulfur dioxide from a power plant, ozone is a regional pollutant and is created in a series of complex chemical interactions in the atmosphere. Nitrogen oxides from burning automobile fuel and hydrocarbons from other petroleum based fuels or natural sources, react in the presence of sunlight and high temperatures, to create ozone. This pollutant can drift with polluted air masses several hundred miles from source areas, causing a regional problem. Ozone is a major component of urban smog. In humans, ozone can lower resistance to diseases such as colds and pneumonia, damage lung tissue, intensify heart and lung disease, and cause coughing and throat irritations. Even healthy adults who perform heavy physical exercise or manual labor outdoors experience the unhealthful effects of ozone.

What is ozone?

Each molecule of ozone is composed of three atoms of oxygen, one more than the oxygen molecule which we need to breathe to sustain life. The additional oxygen atom makes ozone extremely reactive. Ozone exists naturally in the earth's upper atmosphere, the stratosphere, where it shields the earth from the sun's ultraviolet rays. However, ozone found close to the earth's surface, called ground-level ozone, is considered an air pollutant.

Where does ground level ozone come from?

Ozone is formed by a chemical reaction between volatile organic compounds (VOCs) and oxides of nitrogen in the presence of sunlight. The primary sources of VOCs and oxides of nitrogen are automobile and industrial emissions. Other sources of VOCs include consumer products such as paints, insecticides, and cleaners. Ozone concentrations can reach unhealthful levels when the weather is hot and sunny with little or no wind. High ozone levels usually occur between 1:00 p.m. and 7:00 p.m. from May through September.

How does ozone affect human health?

Ground-level ozone is a serious public health problem. High concentrations of ozone may cause inflammation and irritation of the respiratory tract, particularly during heavy physical activity. The resulting symptoms may include coughing, throat irritation, and breathing difficulty. It can damage lung tissue, aggravate respiratory disease and cause people to be more susceptible to respiratory infection. Children and senior citizens are particularly vulnerable. Inhaling ozone can affect lung function and worsen asthma attacks. Ozone also increases the susceptibility of the lungs to infections, allergens, and other air pollutants. Medical studies have shown that the ozone damage lung tissue and unhealthful effects may continue for days after expose has ended.

Because ozone is such a common problem, many cities issue a daily pollution forecast. EPA's AirNow - site uses real-time data from monitoring stations to create color-coded animated maps to show how ozone forms during hot summer days and can be transported downwind. The ultimate objective is to provide citizens nationwide with real-time air quality data so that they can make informed decisions to protect their health as well as the health of the environment.

Here are some links to forecasts centers around the country.

  • SkyCast is such a forecast for the Kansas City area. It is an air quality forecasting system that predicts likely ground-level ozone concentrations based on current and prospective weather conditions. SkyCast indicates the potential ozone pollution threat for the next day and is updated at 3 p.m. each day.

  • EPA Bay Area OZONE MAP ARCHIVE - You may view animated San Francisco Bay Area ozone maps for previous days by visiting the map animations. You will also find static maps for each day showing the peak 1-hour and peak 8-hour spreads.

  • Ozone for New Jersey, Pennsylvania, Delaware, Maryland Areas

  • New England - EPA Region 1

  • IndianaThe mission of OAM's Ozone Web page is to provide Indiana citizens with information and data concerning ozone levels across the state.

How does ozone affect plant life?

There is clear evidence that ozone harms vegetation. Conifers and broad-leaf trees, shrubs, herbs, domesticated crops, and grasses all have member species susceptible to ozone injury. A 1988 study conducted by the EPA found that ozone pollution was reducing American crop yields by 3 billion dollars each year. The study showed that in the hot summer of 1988, ozone reduced crop yields in experimental plots by as much as 30 percent. Ozone pollution also harms forests because it causes early leaf drop and lower growth rates. Ozone reacts with photosynthetically active cells on the upper surface of leaves typically causing an interveinal injury which is dark purple to black in color with distinct edges. Plants sensitive to ozone injury can be used as biological indicators (biomonitors) of air pollution stress in both urban and remote rural ecosystems.

Common Milkweed As A Bioindicator Of Ground-Level Ozone

Common milkweed (Asclepias syriaca) is one such ozone sensitive plant. Common milkweed usually has a solitary, simple stem (1.5-6 feet tall) though clumps of multiple stems can be found. Leaves are opposite, oval, and 2-10 inches long. The surface of the leaf is hairy beneath and smooth above. The leaf stem is short and thick. Milkweed exudes a thick white sap from any cut or broken surface. It also appears that milkweed may send up stems intermittently through the growing season. This may result in stems being at different life stages throughout the summer. Milkweed is common in fields, meadows, and along roadsides. This species is found from New Brunswick to Saskatchewan, south to Georgia and Tennessee and west to Iowa and Kansas.

Milkweed flowers from June to August. The flower actually consists of many flowers in a large cluster on a stalk. A flower stalk may originate from the tip of a branch or stem or at the junction of a leaf with the stem. The flowers are usually pollinated by insects because the pollen is lumped in waxy masses not conducive to wind fertilization. Very few of the flowers produce large, healthy pods. The distinctive seeds of the milkweed are dispersed by the wind catching the long silky hairs. Studies of seed dispersal indicate seeds typically drift 25 to 100 feet before settling to earth.

Milkweed also propagates from underground rhizomes (root structures) which may be many feet in length. Each rhizome may produce multiple stems. For example, numerous stems within a relatively short distance (e.g., 2-3 feet) of each other may all originate from the same rhizome. These multiple stems are genetically identical and the clump of milkweed can be referred to as a clone. Milkweed may live as little as 2 to 3 years or as long as 25 years.

For more information about common milkweed you may want to refer to a field guide such as the Peterson or Audubon Field Guides or online, the Monarch Watch has a Photo Guide to the Milkweeds. The monarch butterfly has a close ecological relationship with the milkweed plant.

Ozone injury on milkweed leaves is unique and relatively easy to diagnose. This injury typically results in sharply defined, small dot like lesions (stipples) on the upper surface of the leaves. These lesions are observed only on the upper leaf surface and are black-dark' purple. Veins are usually not affected. If injury is severe, it may produce an overall dark discoloration of the upper leaf surface as the lesions coalesce. The small purple dots may be observed on the damaged leaf below.

View a leaf showing ozone damage.

Injury on the leaves may vary considerably! In general, the location of ozone injury on a leaf is determined by the maturity of the leaf. Acute ozone injury tends to develop towards the tip of young leaves, in the center of fully grown leaves, and at the base of the oldest leaves. Foliage frequently exposed to ozone may exhibit injury symptoms all over the upper leaf surface. Ozone damage appears as sharply defined, small dot-like lesions (stipples) on the upper surface of the leaves. These lesions are observed only on the upper leaf surface and are black-dark purple. Veins are usually not affected. If the injury is severe, it may produce an overall dark discoloration of the upper leaf surface as the lesions coalesce.

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