skip navigation
Health EffectsEnvironmental HazardsTopics A-ZTopics A-ZPublicationsResourcesFrequently Asked QuestionsAbout Us
NH Environmental Public Health Tracking Program

Ozone (O3)

Ozone is a gas that you cannot see or smell. Ozone occurs naturally in the sky about 10 to 30 miles above the earth's surface. Sometimes, this ozone is called 'good ozone' because it forms a layer that protects life on earth from the sun's harmful rays.

Ground-level ozone, on the other hand, can be bad for your health and the environment. Ground-level ozone forms when pollutants from cars and trucks, power plants, factories, and other sources come in contact with each other in heat and sunlight. Factors such as weather conditions and intensity of sunlight also play a part in how ozone is formed. Ground-level ozone is one of the biggest parts of smog, and it is usually worse in the summer months.

Many urban areas tend to have higher levels of ground-level ozone. However, rural areas have ground-level ozone, too. Wind carries ozone and the pollutants that form it hundreds of miles from their original sources, and rural areas have sources of ozone that contribute to this problem.

Ozone is a highly reactive molecule made of 3 oxygen atoms (O3) bonded together. Ground level ozone is an air pollutant, as opposed to stratospheric (high-altitude) O3 (the ozone layer) which is beneficial as it blocks the Sun's harmful ultraviolet rays from hitting Earth's surface (for more information on the difference between stratospheric and ground-level O3 see Ozone - Good Up High Bad Nearby.

Ozone molecules are highly reactive because they want nothing more than to give their 3rd oxygen atom to something else. The 3rd oxygen atom will react with anything that it comes in contact with, including lung tissue when inhaled.

Ozone is not directly released into the atmosphere. It forms from a secondary reaction between other pollutants already in the air when it is hot and there is sunlight (see figure below). The pollutants necessary for O3 formation are nitrogen dioxide (NO2-) and volatile organic compounds (VOCs). Nitrogen dioxide comes from the combustion of fossil fuels (vehicles, industry, power plants, etc.). Volatile organic compounds come from industrial emissions and plant life and because NH is rich with plant life, there are usually enough VOCs in the atmosphere to create O3 whenever NO2 and sunlight are also present. The figure below illustrates the sequence of factors that lead to ozone formation.

The figure below illustrates the sequence of factors that lead to ozone formation

Weather plays a large role in determining how much O3 builds in NH's air. In NH, O3 events usually occur on the afternoon of hot and sunny (or hazy) days. The worst events occur on hot, sunny (or hazy) days when the air is stagnant (no breeze), and has been stagnant for several days.

Ozone and Health

Some people are more sensitive to air pollutants than others. People with lung disease, children, older adults, and people who are active outdoors (athletes, workers, etc.) can be affected when O3 levels are unhealthy. Elevated O3 levels can limit a person's ability to take a deep breath and trigger symptoms such as coughing, throat irritation, and breathing discomfort. Ground-level O3 can also make it easier for you to get a cold or pneumonia. It can damage your lungs and make heart and breathing problems worse. Children are sensitive to the effects of O3, and even healthy adults engaged in moderate or strenuous outdoor activities can be affected.

When ozone levels are very high, everyone should be concerned about ozone exposure. But ozone bothers some people more than others, mainly when they are outside. People in these groups may feel the effects of ozone when they are outside for short periods of time, even if they are only doing light activities. Those most likely to be bothered by ozone include:

Many scientific studies have linked ground-level ozone contact to such varied problems as:

As a result of these studies, scientists know that breathing in too much ozone can increase events such as:

Reduce Your Risk

Being very active outside during peak O3 hours carries the biggest risk. If you start coughing, having trouble breathing, or get a sore throat, you should reduce your level of activity or reduce the time you are outside. People in the above groups should be especially careful to limit their outdoor activities during peak ozone hours, as they may feel the effects of ozone more quickly than others, even if they are only doing light activities.

EPA's Air Quality Index, or AQI, is a tool to help you quickly learn when air pollution is likely to reach unhealthy levels. Local TV stations, radio programs, and newspapers carry these air quality forecasts to tell you when particle levels are likely to be unhealthy. You can use the AQI to plan your daily activities to reduce exposure to ozone.

Your exposure to ozone depends mainly on where you live and work and how much time you spend outside. When ozone levels are high, you can:

For more information about ozone:

Environmental Public Health Tracking Program
NH Department of Health and Human Services, Division of Public Health Services
29 Hazen Drive, Concord, NH 03301
Tel: (603) 271-4988 800-852-3345 ext.4988

copyright 2009. State of New Hampshire