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.

Learn more on EPA’s website.

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

The primary health-based National Ambient Air Quality Standard (NAAQS) for ozone is 0.070 parts per million (ppm) averaged over an 8-hour period.  The secondary welfare-based standard is set at the same level.  EPA set this standard in October 2015 based on a review of updated scientific information that indicated that the 2008 ozone standard of 0.075 ppm was not health protective.  MassDEP’s statewide network of air quality monitors show that the air in Massachusetts meets the 2008 ozone standard and likely will meet the more stringent 2015 ozone standard when EPA formally designates states in 2017 as either attaining or not attaining the standard.  

MassDEP has monitored air concentrations of ozone for several decades and provides daily air quality forecasts for ozone from the beginning of April through the end of September. At some monitoring locations across the state, ozone levels still exceed the EPA eight-hour standard a few days each summer.

While measured concentrations of ozone can still reach unhealthy levels a few days each summer in Massachusetts, they confirm that we're breathing cleaner air now than we did years ago, thanks in large measure to tougher government regulation and steps taken by industry aimed at reducing pollution from vehicles, power plants, factories and consumer products.

For more information on ozone monitoring, standards, and trends see MassDEP’s Annual Air Quality Reports.