About Ozone

Ozone is a highly reactive, colorless gas pollutant that is not typically emitted directly into the air by any one source.  Instead, ozone is considered a secondary pollutant, which means that it is formed through complex chemical reactions of molecules in the air. Specifically, ground level ozone is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) chemically react in the presence of sunlight. While ozone high up in the atmosphere protects us from harmful ultraviolet rays, exposure to high concentrations of ground level ozone can be quite harmful to our health and environment. That is why we say ozone is “good up high, but bad nearby”.

High concentrations of ground-level ozone can create breathing problems, especially for children, people with asthma or other respiratory problems, and adults who work or exercise outdoors. According to the U.S. Environmental Protection Agency, ozone can also cause tree and crop damage. Mobile sources of air pollution such as cars, trucks, and lawn equipment, contribute to nearly half of the ozone formed in South Carolina.

High ozone concentrations generally occur on hot, sunny days in the spring and summer when the air is stagnant, and the sun’s rays shine more directly on the earth’s surface. Like the weather, ozone and its precursor pollutants can be transported from place to place by wind, resulting in significant changes in ozone concentrations from day to day and even hour to hour.

Ozone Monitoring

DHEC monitors ozone levels each year from March 1st through October 31st as mandated by the U.S. Environmental Protection Agency ozone regulations. The March through October monitoring ozone season that is chosen for South Carolina is based on when the atmosphere is more conducive to ozone formation, which is highly dependent on weather conditions. Monitors help us understand the actual amount of ozone occurring at a particular location as well as the distribution of ozone concentrations across the state.

Ozone Forecasting

DHEC meteorologists provide next-day ground level ozone forecasts from March 31st to September 30th to allow the public to be proactive about protecting their health by reducing outdoor activities and by reducing their own contributions to emissions when ozone concentrations are expected to be high. If ozone levels are forecast to reach unhealthy levels (exceeding the ozone National Ambient Air Quality Standard), DHEC will declare an 'ozone action day' advising people to reduce their activity levels outdoors, especially for those with respiratory conditions, such as asthma. Historical monitoring data indicates that unhealthy ozone concentrations are extremely rare during fall and winter months due to beneficial weather conditions and the lower sun angle that typically keeps ozone concentrations lower than in spring and summer months. Since the vast majority of days in March and October exhibit “code green” ozone concentrations (i.e., low concentrations similar to other fall and winter months), forecasters currently do not issue daily ozone forecasts for these two ozone monitoring season months. Forecasters keep an eye on ozone observations and weather conditions each day during these two months and issue an 'ozone action day' in the rare event that is necessary. Additionally, historical monitoring data shows that ozone forecasts are not necessary in the Coastal Plain region of South Carolina outside of the Charleston area (shown by the “no forecast” white area on the ozone forecast map) due to lower emissions and more favorable weather conditions for cleaning the atmosphere of ozone precursor pollutants at locations closer to the coast.

Visit DHEC's forecast page


Ways to get the South Carolina ozone forecast:

Other Ozone Forecasts

Don't live in South Carolina? Here are some useful links to find the ozone forecast in other areas:

Ways to help reduce ozone pollution:

  • Drive less by walking, riding your bike, or using alternative transportation.
  • Reduce idling - turn off your engine if you expect to be stopped for long periods of time (except in traffic).
  • Register for the Breathe Better (B2) for Businesses program and receive anti-idling signs and learn more about air pollution reduction strategies for your business.
  • Keep to the speed limit. It saves gas and reduces emissions.
  • Keep your vehicle tuned up and your tires properly inflated. Both help save gasoline and improve air quality, as well as make your car safer.
  • Learn more

What can I do to avoid exposure?

There are safe ways to limit your exposure to ozone without reducing healthy physical activity.

  • Sign up for our ozone forecast so you can plan outdoor work or exercise during your day to avoid possible high levels of ozone.
  • Use these recommendations for schools and outdoor activities to modify plans for outdoor activities such as recess, lunch, and physical education class.
  • If you're involved in an activity that requires heavy exertion, you can reduce the time you spend on this activity or substitute another activity that requires more moderate exertion(e.g., go for a walk rather than a jog).
  • No matter how fit you are, cutting back on the level or duration of exertion when ozone levels are high will help protect you from ozone's harmful effects.
  • Plan outdoor activities when ozone levels are lower, usually in the morning or evening.