Ozone is a colorless, naturally occurring gas found in two layers of the atmosphere. In the upper layers of the atmosphere, ozone protects life on Earth by absorbing some of the sun’s ultraviolet rays. In the layer closest to the ground, however, ozone is an air pollutant that is a key ingredient of smog and is a severe public health concern. It damages lung tissue, aggravates respiratory conditions and makes people more susceptible to respiratory infections. Children are especially vulnerable to ozone’s harmful effects.

Summer is Ozone Season

Ozone is formed from a chemical reaction in the lower atmosphere on hot, still, sunny days. The air pollutants contributing most to ozone formation are volatile organic compounds (VOCs) and nitrogen oxides (NOx). There are many sources of VOCs and NOx in Delaware, including large and small industrial facilities, gasoline vapors, vehicle exhaust, chemical solvents and natural sources. Many of these compounds are blown in from cities and states that are upwind of Delaware. Over 90 percent of the ozone levels in Delaware are created by the transport of air pollutants from upwind areas.

Monitoring for Ozone

DNREC has been measuring Delaware’s air quality for over 20 years. Federal law requires states to monitor levels of specific pollutants on an hourly basis. Of all the pollutants monitored in Delaware, ozone and particulate matter occurs at levels that are classified as “unhealthy.”

The number of days with unhealthy levels of ozone has been declining in the state for more than a decade. This is largely due to pollution control and prevention programs, including tougher emission controls on large industries, cleaner-running cars, vehicle emission inspection programs and reformulated gasoline.

But, ozone levels continue to be a problem in Delaware.

Air Quality Action Days

When ozone approaches unhealthy levels, an Air Quality Action Day is called. Air Quality Action Days for Ozone occur when the predicted weather the next day is ripe for ozone formation. On those days, the public is asked to curtail activities that could form ozone and avoid outdoor exposure. Air Quality Action Days are announced in the afternoon for the following day.

To find out what the ozone forecast is for the region, visit our Air Quality forecast page, check the weather segment of your local television news program, visit the EPA AirNow site, get the Delaware Valley Air Quality Forecast or call the Hotline at 1-800-872-7261.

You Can Help Reduce Ozone Pollution

You can take action to reduce the pollutants that contribute to the formation of ozone. Your actions are particularly important on hot, sunny days, but you can make a difference every day.

At Home

Use energy-efficient light bulbs and appliances.

Turn off any appliances and lights not in use.

Run dishwashers and clothes washers only when full, then let dishes and clothes air dry.

If you have air conditioning, set the thermostat at 72 or higher.

Insulate your home, caulk windows, and close off unused rooms.

Use a water-saving showerhead and limit shower to five minutes.

Use a push mower, when you can, and mow grass in the evening.

Paint only when necessary and keep the lids on tightly.

Use water-based or latex paints.

Don’t burn leaves, branches, or lawn trimmings — compost them.

Avoid using charcoal lighter fluid; use an electric probe.

Tightly seal household cleaners, workshop chemicals and solvents, and garden chemicals.

Plant trees!

On the Road

Limit daytime driving if possible.

Try to avoid driving during times of heavy traffic and roadway congestion.

Bike, walk, carpool, or take public transportation when you can.

Plan your travel and errands in advance and consolidate trips.

Drive the speed limit.

Avoid “jackrabbit” starts and sudden stops.

Keep your tires properly inflated.

Keep your vehicle and its emission control equipment tuned and well-maintained.

Avoid prolonged idling; turn off your engine when you stop your vehicle for a short time.

Be careful not to spill gasoline when filling up your vehicle (or your lawn equipment!).

Refuel your vehicle in the evening.

More About Ozone

Ozone is a naturally occurring gas that is found in two layers of the atmosphere. In the layer surrounding the Earth’s surface—the troposphere—ground level or “bad” ozone is an air pollutant that is a key ingredient of urban smog. The troposphere extends about 6 miles up to the stratosphere, which is where “good” ozone protects life on Earth by absorbing some of the sun’s UV rays. This layer ends at about 30 miles above the surface.

The stratospheric ozone layer acts as a shield, protecting life on Earth from the sun’s ultraviolet (UV) rays. In the 1980s, scientists began accumulating evidence that the ozone layer was being depleted by man-made chemicals. Depletion of the ozone layer results in increased UV radiation reaching the Earth’s surface, which in turn can lead to a greater chance of overexposure to UV radiation and the related health effects of skin cancer, cataracts, and immune suppression. To prevent further ozone depletion, the government has phased out the manufacture and use of ozone-depleting chemicals.

Ground-level (bad!) ozone (the primary constituent of smog) continues to be a pervasive pollution problem throughout many areas of the United States. Ozone is not emitted directly into the air but is formed by the reaction of VOCs and NOx in the presence of heat and sunlight. Ground-level ozone forms in the atmosphere all year round but most readily during hot summer weather. VOCs are emitted from a variety of sources, including motor vehicles, chemical plants, refineries, factories, consumer and commercial products (such as hair spray, cleaners and pesticides), and other industrial sources. Nitrogen oxides are emitted from motor vehicles, power plants, and other kinds of engines. Ozone and the precursor pollutants that cause ozone can be transported into an area from pollution sources found hundreds of miles upwind. Of the 492 tons per day of emissions, which form ozone, 27% come from stationary sources (e.g., stacks), 37% from mobile sources (i.e. vehicles), and the balance from natural emissions.

Short-term (1-3 hours) and prolonged (6-8 hours) exposures to ambient ozone have been linked to a number of adverse health effects. Repeated exposures to ozone can make people more susceptible to respiratory infection, result in lung inflammation, and aggravate pre-existing respiratory diseases such as asthma.

Ozone also affects vegetation and ecosystems, leading to reductions in agricultural and commercial forest yields, reduced growth and survivability of tree seedlings, and increased plant susceptibility to disease, pests, and other environmental stresses (e.g., harsh weather). Ground-level ozone damage to the foliage of trees and other plants also can decrease the aesthetic value of ornamental species as well as the natural beauty of our national parks and recreation areas. Ozone pollution is responsible for over $500 million a year in crop losses in the U.S.

Ambient ozone trends are influenced by year-to-year changes in meteorological conditions, population growth, loadings of VOC and NOx in the atmosphere, and by changes in emissions from ongoing control measures.

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