Ozone is a gas composed of three oxygen atoms that can be either beneficial or harmful to our health depending on where it forms:

  • Good Ozone. Many miles above ground in the Earth's upper atmosphere, ozone occurs naturally and provides a protective layer that shields us from the sun's harmful ultraviolet rays.
  • Bad Ozone. Near ground level, ozone is formed when oxides of nitrogen (NOx) and volatile organic compounds (VOCs) emitted by cars, power plants, factories and other sources react chemically in the presence of heat and sunlight. Ozone at ground level is a harmful air pollutant and the chief ingredient in "smog."

Concentrations of ozone tend to be highest in the summer and during the day, when the conditions required for its formation - sunlight and warm temperatures - are most prevalent. New England's ozone season typically runs from April through September.

Although smog-forming pollutants tend to originate in urban areas, they can be carried by the wind for hundreds of miles and form ground-level ozone in less populated areas. In Massachusetts, for example, it is not unusual for smog to be a bigger problem in the rural west-central part of the state or over Cape Cod than in larger cities.

See What You Should Know About Ozone for additional information.

Health Effects of Ozone & Who is Most at Risk

Ozone affects everyone, but some people are more sensitive to its impacts than others. Numerous scientific studies have linked ground-level ozone exposure to a variety of problems. Specifically, breathing ozone can:

  • Irritate the respiratory system and cause coughing, throat irritation and uncomfortable sensations in the chest.
  • Reduce lung function, leading to more rapid and shallow breathing that may limit a person's ability to engage in vigorous activities.
  • Heighten sensitivity to allergens such as pet dander, pollen and dust mites that commonly trigger asthma attacks, leading to more doctor and emergency room visits and greater use of medication.
  • Inflame the lung lining. Typically, damaged cells are shed and replaced much like skin peels after sunburn, but studies suggest that repeated inflammation over long periods of time can result in permanent scarring and loss of lung function.
  • Contribute to premature death in people with heart and lung disease.

When ground-level ozone reaches unhealthy levels, children are the group at highest risk because they tend to spend much of the summer playing outdoors and they also are more likely to have asthma. People with asthma or other respiratory diseases also are vulnerable, even at lower ozone levels.

In general, as concentrations of ground-level ozone increase, more people experience health symptoms, the effects become more serious, and hospital admissions for respiratory problems increase.

Environmental Effects of Ozone

Ground-level ozone can have detrimental effects on plants and ecosystems. It can interfere with the ability of sensitive plants to produce and store food, making them more susceptible to certain diseases, insects, other pollutants, competition from other plants and harsh weather. Ozone reduces forest growth and crop yields, and can potentially affect the diversity of species in ecosystems.

Ozone Standards

Two decades after establishing a health-based primary National Ambient Air Quality Standard (NAAQS) for ozone of 0.12 parts per million (ppm) averaged over a one-hour period, the U.S. Environmental Protection Agency (EPA) in 1997 set a stricter standard of 0.08 parts per million (ppm) averaged over an eight-hour period. Legal challenges delayed implementation for several years.

In separate 2004 decisions, EPA classified Massachusetts as failing to meet both the old one-hour ozone standard and the new eight-hour standard in all 14 of its counties, although the agency recognized that the state was making important progress.

The one-hour standard was revoked in June 2005. In March 2008, EPA lowered the health-based primary ozone standard to 0.075 ppm (from 0.08 ppm) averaged over an eight-hour period. The new secondary standard for protection of crops, vegetation and buildings was set at the same level. In March 2009, Massachusetts recommended to EPA that the entire state be designated as nonattainment of the new ozone standard.

Long-Term Ozone Trends

The Department of Environmental Protection (MassDEP) has monitored air concentrations of ozone for more than 20 years and issues a for ozone from the beginning of April through the end of September. At some monitoring locations across the state, ozone levels routinely exceed the federal eight-hour standard each summer.

One way to look at ozone pollution over the long term is to chart the number of times that the EPA standard has been exceeded at individual monitoring locations across Massachusetts. If one monitor exceeds the standard for any rolling eight-hour period, that counts as one "exceedance."

Figure 1 below tracks the total number of exceedances and exceedance days recorded in the state over the last two decades. Although today's eight-hour EPA standard for ozone took effect only in 1997, MassDEP has used this stricter limit to calculate exceedances dating back ten years earlier to provide a consistent basis for comparison over time. The data for 2008 reflect the new standard of 0.075 ppm in effect since March 2008.

While measured concentrations of ozone are still too high in Massachusetts, they nevertheless confirm that we're breathing cleaner air now than we did years ago, thanks in large measure to tougher government regulation and voluntary steps by industry aimed at reducing pollution from vehicles, power plants, factories and consumer products.


Figure 1
8-hr Ozone Exceedance Days and Total Exceedances 1987-2008
8-hour standard = 0.08 ppm 1987-2008; 0.075 ppm for 2008


8-hr Ozone Exceedance Days and Total Exceedances 1987-2008