What is ground-level ozone?
Ozone gas occurs naturally in the sky about 10 to 30 miles above the earth's surface. It forms a layer that protects life on earth from the sun's harmful rays. However, when ozone is created at the ground-level, it can be harmful. Ground-level ozone is an odorless, colorless gas. It is created by a chemical reaction between two other pollutants in the presence of heat and sunlight. The primary chemicals that form ozone are nitrogen oxides (NOx) and volatile organic compounds (VOCs).
Why does MA EPHT track ground-level ozone?
Ground-level ozone is associated with increased rates of hospitalization for respiratory and heart conditions, adverse birth outcomes, and lung cancer. Tracking levels of ozone over time alongside health data can help us better understand the impact of ground-level ozone on health. It can also allow us to look at adverse health impacts in specific communities throughout the state and help us identify the best ways to protect those most at risk of harm.
MA EPHT tracks ground-level ozone with both modeled and monitored data. The EPA and the U.S. Centers for Disease Control and Prevention (CDC) use the downscaler model to combine measurements from the EPA's Air Quality System (AQS) with predictions from the Community Multi-Scale Air Quality Model (CMAQ). These data fill in the gaps that exist in the monitored data. In areas without outdoor air monitors, the value reported is the modeled estimate.
Both types of ozone data help us understand pollution trends. The combination of monitored and modeled data allows us to look at the frequency of poor air quality events and historical trends.
How can ground-level ozone affect my health?
Breathing air containing ground-level ozone can reduce lung function and aggravate asthma, COPD, or other respiratory ailments. Symptoms of exposure to high levels of ground-level ozone vary from one person to another but may include coughing, nose and throat irritation, chest pain, and shortness of breath.
Studies link prolonged exposure to ground-level ozone at levels measured in the ambient air with a wide range of health effects. This includes increased respiratory infections and medicine use by asthmatics as well as increased doctors' and hospital visits, and hospital admissions. Long-term exposure to ozone can result in permanent lung damage.
Some people may be more sensitive to ozone and may experience serious effects at lower concentrations. These include:
- Older adults age 65 and over
- People with pre-existing medical conditions such as heart or lung diseases
- Children
- People who are physically active outdoors
Older adults over age 65 are more likely to have pre-existing medical conditions or undiagnosed lung disease. Children are still developing, spend more time outdoors, and generally are more active. Adults who exercise or work vigorously outside have a faster breathing rate, which leads to a higher level of exposure to ground-level ozone than people who are less active. All these factors influence risk.
How can I be exposed to ground-level ozone?
You can be exposed to ground-level ozone by breathing ambient air in areas where it is present. Ground-level ozone is not directly emitted but instead forms when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of heat and sunlight.
Major sources of NOx and VOCs include emissions from:
- Power plants
- Industrial boilers
- Cars and trucks
- Off-road vehicles (e.g. construction equipment)
- Consumer and industrial products
Ground-level ozone is measured downwind of these sources of NOx and VOCs, which typically exist in urban areas. It is also measured at rural and suburban locations upwind because it can be transported regionally and come from out-of-state sources.
Ground-level ozone is often higher in rural areas and lower in urban areas. This is due to a process called “ozone scavenging.” When levels of VOCs are low, NOx can react with ozone itself and neutralize it. This occurs most often in urban areas where there are higher levels of NOx from vehicle exhaust and industrial emissions.
Changes in weather patterns can also affect the concentrations of ground-level ozone. High levels of ground-level ozone often occur during hot, sunny weather. Low levels are more likely to occur during cloudy, cool, or rainy weather.
How can I prevent exposure to ground-level ozone?
The Massachusetts Department of Environmental Protection (MassDEP) provides a daily air quality forecastto warn when ground-level ozone levels are expected to be unhealthy. The air quality forecast is based on U.S. EPA's Air Quality Index (AQI).
The AQI has a color-coded scale and is used to let people know when air pollution is expected to reach unhealthy levels in their area. The AQI can be used to plan daily activities. For example, if the AQI is high, you may want to avoid strenuous activity outside or shorten your time spent outdoors.
What are the air quality standards for ground-level ozone?
The federal Clean Air Act (CAA) establishes air quality regulations for ground-level ozone. The CAA sets timeframes and milestones for states to meet and maintain air quality standards. These standards are called the National Ambient Air Quality Standards (NAAQS). EPA sets the NAAQS levels to protect public health and the environment. Each state is required to monitor the ambient air to determine if it meets the NAAQS.
The NAAQS for ozone is 0.07 ppm (parts per million). It is measured by taking the 4th highest daily maximum 8-hour average ozone concentration and averaging it over a 3-year period. This is done each year using data from monitors within a specified area. Concentrations above the NAAQS are considered unhealthy, particularly for sensitive individuals.
If the air quality does not meet standards, the state must develop pollution control strategies. Once air quality meets a standard, the state must develop a plan to maintain that standard while accounting for future economic and emissions growth. Taken together, these plans and control strategies constitute the State Implementation Plans (SIPs). For more information, see the Massachusetts SIP.
Available data
Use the Explore outdoor air quality data link on this page to access the following measures for ground-level ozone modeled, monitored, and combined monitored and modeled data:
- Number of days over the NAAQS
- Number of person-days over the NAAQS
The calculations for the MA EPHT outdoor air quality measures for ground-level ozone are based on the number of days in a county that the maximum 8-hour average ozone concentration exceeded the NAAQS during the year. The outdoor air quality measures for all years are based on the current NAAQS of 0.07 ppm.
Number of person-days over the NAAQS is a measure used to estimate exposure. It accounts for the number of days that the maximum daily ground-level ozone concentration exceeds the NAAQS and the population within the county where monitors are located.
Combined and monitored data is available at the county level only. Modeled ground-level ozone data is available at the county, community, and census tract levels. The most current available data will be shown. Be sure to check the site periodically as new data is added.
Related links
Additional resources
Massachusetts
- MassAir Online at MassDEP: Current ground-level ozone levels and information for Massachusetts
- Massachusetts State Implementation Plans (SIPs): Information on how Massachusetts ensures that it meets the NAAQS and regional haze goals under the Clean Air Act
- DPH fact sheets on poor outdoor air quality: Learn about climate and poor outdoor quality. Available in 16 languages
National
- EPA - Ground-level ozone pollution: Information on ground-level ozone and how ozone standards are set and implemented
- EPA Air Quality System (AQS): EPA's repository of ambient air quality data that assists in air quality assessments, designations, modeling for permit review, and preparing reports as mandated by the Clean Air Act
- EPA Community Multiscale Air Quality (CMAQ) modeling system: Active open-source development project of the U.S. EPA that consists of a suite of programs for conducting air quality model simulations
- EPA - Air quality criteria for ozone : Information on outdoor air quality standards for ozone from the US EPA