What is ozone and what causes it?

Ozone is a different form of oxygen: an oxygen molecule has two oxygen atoms, an ozone molecule has three. Ozone is a toxic gas. It occurs naturally in the stratosphere from solar-induced chemical reactions, and at that altitude it protects us and our environment from harmful ultraviolet rays. But when ozone forms at ground level, it becomes the unnatural and predominant ingredient of smog.

Ozone near the ground is created through a series of chemical reactions involving sunlight, oxides of nitrogen - a product of fossil fuel combustion in motor vehicles, power plants, industrial boilers and so on - and volatile organic compounds also emitted during fossil fuel combustion and

What happens when we inhale ozone molecules?

Ozone molecules are highly reactive oxidizing agents that tend to stick to large molecules lining our respiratory tracts. They react with these large molecules, breaking their chemical bonds and adding oxygen atoms to their structures. These new oxidized compounds damage the cells in our airways; this can lead to respiratory tract inflammation, causing irritation and burning, and can reduce lung function.

Do ozone levels remain constant throughout the day?

No. In fact, they can be as different as, well, night and day. Because sunlight activates the ozone-forming chemistry, we typically see a 24 hour pattern of buildup and reduction. Overnight and early morning concentrations are usually quite low. A rapid increase normally follows until about noon, when concentrations tend to level off. Then, a second, rapid rise can occur later in the afternoon, with levels dropping sharply after sunset.

What's weather got to do with it?

A lot. Although ozone forms throughout the year, ozone production peaks during the warmest months of the year. The higher the sun is in the sky and the warmer the temperature, the faster ozone-producing chemical reactions occur. Wind direction is also important; when the wind blows across areas where many people live - and drive cars, and rely on power plants to generate a lot of electricity - the atmosphere becomes laden with the ingredients that produce ozone. When winds blow from less populated, less industrialized areas, much less ozone forms, no matter how sunny and hot it gets.

Also, winds high above us can carry ozone and ozone-forming pollutants long distances. Not only does the poorly maintained car right in front of you foul the air you breathe, so do power plants hundreds of miles away. A "pool" of ozone and ozone-forming pollutants starts to build over the industrialized midwest. That pool then flows eastward with the wind, passing through the heavily populated corridor stretching north from Washington, D.C., through Baltimore, Philadelphia, New Jersey, and New York City. All that human activity generates more pollution and adds to the pool. When a broad southwesterly airflow develops over the eastern United States, it turns that pool northeastward toward Massachusetts. And when that happens in summer, the weather is usually sunny and hot - perfect conditions for ozone production.

Meteorologists at the Massachusetts Department of Environmental Protection (MassDEP) get concerned when a certain summertime weather pattern appears: a large high-pressure system centered over the Atlantic Ocean, well south of New England, that expands westward across the eastern United States. This "Bermuda high" is responsible for some of our hottest and most humid weather - and for our worst ozone episodes.

During the cooler months of the year, lower temperatures and weaker sunlight keep ozone concentrations down. New England's ozone season, then, runs from April through September.

Is all this related to the "ozone hole" we hear so much about?

No. Why? Location, location, location. Ground-level ozone is "bad" ozone, harmful to living things, including vegetation. Take it away and we breathe easier.

Ozone in the stratosphere is "good" ozone, protecting us from the sun's ultraviolet radiation. Take it away and we're cooked, literally. The ozone hole refers to the relative absence of ozone in the stratosphere primarily above the Earth's polar regions, which has been linked to our use of chlorofluorocarbons and other chemicals.

Do I have to worry about ozone if I don't live in the city?

Yes, because it's an equal opportunity contaminant, often hitting rural and suburban areas harder than it does cities. And, depending on wind direction, some parts of the state may get higher ozone levels than others. MassDEP measurements also indicate that ozone severity is often somewhat less across areas of the state that lie farthest from the usual ozone path, areas including northern tier towns from central Massachusetts to the state's western border. Still, even these sections of the state are subject to occasional bouts of elevated ozone.

Is air quality improving?

Yes. While measured concentrations of ozone are still too high, they nevertheless confirm that we're breathing cleaner air now than we did a decade ago, and we should be breathing still cleaner air in the years ahead. Much of the thanks goes to tougher government regulation and industrial cooperation, which have led to less-polluting cars, industrial boilers, utilities, paints, and other household products.

What should I do on bad ozone days?

Plan exercise and other outdoor activity during times when ozone concentrations are expected to be lowest; generally, this means mornings before about 10:00 and evenings after 7:00. If that's not possible, then try to limit the intensity and duration of activity. Let common sense prevail: ozone is usually highest on hot, humid days when most adults limit activity, anyway. Parents should take special care to limit outdoor activities of children on high ozone days, because children can be most severely affected.