CLEARING THE AIR ON SMOG

Smog has been causing health problems for plants, animals and humans for decades, and the Environmental Protection Agency is raising the standards that will govern how much smog is too much from a health point of view.

Ground-level ozone, a principal component of smog, has been the most difficult air pollution problem in the United States. Nearly 100 cities across the country are currently in violation of federal standards for ground-level ozone, a status known as non-attainment. That number is expected to grow because EPA enacted stricter standards. Hundreds of communities are learning for the first time that they have a problem. Nitrogen oxides are also dangerous ingredients of smog.

Smog includes ozone, nitrogen oxides, and other dangerous ingredients. Here's a primer on what you need to know about ozone.

The standards

EPA's new standards say an area cannot exceed 80 parts per billion of ozone for any one-hour period.

What is ground-level ozone?

Ozone is a molecule made of three oxygen atoms linked together.

By itself it is odorless and colorless.

It is not emitted from smokestacks and tailpipes, but forms from pollutants emitted by them.

The pollutants, volatile organic compounds and nitrogen oxide, combine in the presence of sunlight and stagnant air to form ozone. Mix in sooty particles, known as particulates, and you have smog.

High in the atmosphere, the natural ozone layer is good, because it filters out harmful ultraviolet rays. But it is not meant to be breathed by humans, animals and plants, and it is damaging to all three at ground level.

Because it forms in warm sunlight, it is primarily a summertime problem, but can crop up in affected areas anytime between April and October.

Where does it come from, and who has it? In most areas, 1/3 to ½ of the emissions that form ozone smog come from cars, trucks and other internal combustion engines.

Ten metro areas, home to 57 million people, are severely polluted, exceeding EPA standards by 50 percent or more. They include Los Angeles, Chicago, Houston, Milwaukee, New York, New Jersey, Baltimore, Philadelphia, Sacramento and Ventura County, Calif. Seriously polluted areas include Atlanta; Dallas; Washington, D.C.; Boston; San Diego and nine others.

Why is it harmful? Ozone is a severe irritant that can cause choking, coughing and stinging eyes. It damages lung tissue, aggravates respiratory disease and makes people more susceptible to respiratory infections. On days when ozone levels are high, emergency room visits for asthma attacks have been shown to increase by as much as 36 percent.

Children and the elderly are especially vulnerable to ozone's harmful effects, as are adults with asthma and other lung and heart ailments.

People who exercise on high-ozone days may actually be damaging their lungs by breathing ozone.

Ozone also inhibits plant growth and can cause widespread damage to crops and forests.

The standards, which are controversial, were revised as the result of a court decision that required EPA to revise its standards. The new standard is based on the theory that prolonged exposure to lower levels of ozone is more harmful than brief exposure to high levels.