Ozone (O3) is a colorless gas made up of three oxygen atoms. In the upper atmosphere, ozone exists naturally and shields the earth from the sun’s ultraviolet rays. Near the ground, however, ozone is a pollutant and the primary component of summertime smog. Ozone is a major focus of air quality regulatory agencies across the country.

diagram of ozone in the atmosphere

For general information about ground-level ozone, including health and environmental impacts, please read the NHDES fact sheet Smog and Ground-Level Ozone and EPA’s Ozone Information page.

For more about the protective ozone layer, please read the NHDES fact sheet Stratospheric Ozone Protection – The CFC Issue; also see Frequently Asked Questions on CFCs.

Health Concerns
Ozone in the air where we breathe can cause coughing, throat irritation, and breathing difficulty or discomfort. Some people are more sensitive to the harmful effects of ozone, including children, those with pre-existing lung conditions such as asthma, and even healthy adults who are active outdoors.

diagram of ozone formationSources of Ozone
Ozone is not directly emitted but is formed in the air from other “precursor” pollutants: nitrogen oxides (NOx) and volatile organic compounds (VOCs). Please also visit the NHDES NOx page and find information about where NOx and VOCs come from on EPA’s Air Emissions Sources page.

Air Quality Forecasts
Ozone concentrations exceeding the “good” classification are common enough in New Hampshire that NHDES provides daily air quality forecasts so you can stay informed and take action to protect your health. When ozone or other pollutant levels are expected to reach unhealthy levels, NHDES calls an Air Quality Action Day (AQAD). AQAD alerts are most likely to occur during the “ozone season,” which spans April through September when the air is warm and the energy from the sun is high.
map of ozone monitoring stations in NH
New Hampshire air quality forecasts and current air quality levels are available on the NHDES website, and EPA provides this information nationwide on AirNow. Read more in the NHDES fact sheet Air Quality Information in New Hampshire.

Ozone Monitoring
NHDES measures hourly ozone concentrations at several air monitoring stations throughout the state (see map at right). Please visit NHDES’ Air Quality Monitoring and Data page for more information.


For additional topics related to ozone, please click on the links below:

Ozone Attainment Status and Trends
New Hampshire is currently in attainment statewide for ozone. However, EPA lowered the 8-hour ozone standard to 70 ppb in 2015, and new attainment designations have not yet been finalized.

High Ozone Days
A few days per year, ozone can reach unhealthy concentrations for sensitive populations in New Hampshire; find out when and where.