May 27th EarthTalk


Dear EarthTalk: I know of issues associated with the Earth's ozone layer, but what is "ground level ozone" and why is that a problem? --D. Goetz, Palm Beach, FL(

Ozone (O3) is a colorless gas formed when three atoms of oxygen bond together. About 90 percent of the Earth's ozone forms naturally in the stratosphere, dozens of miles above ground. It forms the protective layer that shields us from overexposure to the sun's radiation, and is therefore considered "good" ozone.
The rest of the ozone found on Earth occurs at ground level, and forms when nitrous oxides and various "volatile organic compounds' (VOCs)-originating with car exhaust, industrial emissions, chemicals and gasoline vapors, as well as some natural sources-bond together in the presence of sunlight.
Ground level ozone, or "bad" ozone, is a key component of smog, which wreaks havoc on human health and the environment, especially in urban areas. The U.S. Environmental Protection Agency (EPA) reports that exposure to even relatively low concentrations of ground-level ozone for extended periods (several hours) can significantly reduce lung function and cause respiratory inflammation in normal, healthy people. Symptoms can include chest pain, coughing, nausea and congestion. For people with asthma and other respiratory illnesses, exercising in ozone-rich air can be deadly. Repeated exposure to high levels of ozone for several months or more can produce permanent structural damage in the lungs.
Beyond its effects on our health, the EPA estimates that pollution from ground-level ozone is responsible for nearly $2 billion in agricultural crop yield losses in the U.S. alone each year. The pervasive gas has also been shown to damage forests in California and the eastern U.S. and to contribute to global warming.
Under the mandate of the Clean Air Act, the EPA is charged with monitoring and limiting the amount of ground-level ozone in urban areas, and issuing warnings when smog levels are above its standard of 0.12 parts per million. But new studies indicate that ground-level ozone causes adverse health effects at even lower concentrations. And, according to the EPA, even rural areas suffer increased ozone levels, because wind carries ozone and the pollutants that form it hundreds of miles away from their original sources. As a result, the EPA is reviewing whether revisions to ozone standards and policies are warranted.
High concentrations of ground-level ozone are not as common in Canada, but three urban regions-British Columbia's Lower Fraser Valley, the Windsor-Qu─bec City Corridor and the Southern Atlantic Region that includes New Brunswick and Nova Scotia-do suffer several "bad air days" each year. The Canadian government, through its own Clean Air Act, has even stricter standards for exposure to ground-level ozone than in the U.S., though enforcement is not as big a priority given the smaller scope of the problem there.
To help minimize ground-level ozone, avoid car trips and the use of power lawn equipment during especially hot or windless days. Paints and solvents, most which off-gas VOCs that create ozone and form smog, are also best to steer clear of with hot summer temperatures coming on strong. Those concerned about their respiratory health should follow local weather sources, most which post smog alerts.
Contacts: U.S. EPA Ground-Level Ozone Information,; Canada's Clean Air Act,

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