Ozone is a colorless gas pollutant found in the air we breathe. It is a major part of urban smog. There are two types of ozone:
- Ozone that occurs naturally in the sky about 10 to 30 miles above the earth's surface forms a layer that protects life on earth from the sun's harmful rays.
- Ground-level ozone is an odorless, colorless gas that is created by a chemical reaction when pollutants that come from cars, power plants, and other sources come in contact with each other in the presence of heat and sunlight. Ground-level ozone is a regional concern, since the pollutant and its precursors can be transported by wind and travel long distances. Furthermore, it is frequently measured at higher concentrations in rural rather than urban locations because of “ozone scavenging.” Urban environments typically have higher concentrations of nitrogen oxides (NOx) from vehicle exhaust or industrial emissions. NOx is highly reactive and “scavenge” or readily react with ground level ozone, thereby reducing ozone concentrations in urban areas.
Breathing air containing ground-level ozone can reduce lung function and aggravate asthma or other respiratory ailments. Ozone exposure has also been linked with more respiratory infections, medicine use by asthmatics, doctors' visits, emergency department visits, and hospital admissions. Ozone exposure also may contribute to premature death in people with heart and lung disease. People with lung disease, children, older adults, and people who are active outdoors can be affected when ozone levels are unhealthy.
Studies link a wide range of health effects (e.g., lung function decrements, pulmonary inflammation) with prolonged exposures (e.g., 6- to 8-hr) to ozone at levels measured in the ambient air. The current National Ambient Air Quality Standards (NAAQS) for ozone is an 8-hour average ozone concentration of 0.070 ppm. Concentrations above the NAAQS are considered unhealthy, particularly for sensitive groups including people with lung disease, children, older adults, and people who are active outdoors. The outdoor air quality measures for all years are based on the current NAAQS of 0.070 ppm.
Ozone outdoor air quality measures are available in a number of formats, including monitored (only) in counties where monitors are located, as well as monitoring and modeled (combined) and modeled (only) for all counties, communities, and census tracts. The outdoor air quality measures for ozone include the number of days in a county that the maximum 8-hour average ozone concentration exceeds the NAAQS during the year and over multiple years (trends) as well as the number of people potentially exposed to levels over the NAAQS. The person-days measure provides information on the number of people exposed to ozone pollution above the NAAQS. When comparing across counties, the highest number of person-days indicates areas with both a large exposed population and a larger number of high pollution days. The number of days when ozone concentrations exceed the NAAQS also provides an indication of short-term "spikes" in ozone concentrations.
What is ozone?
Ozone is an odorless, colorless gas that is not emitted directly into the air, but created by a chemical reaction between two types of air pollutants - nitrogen oxides (NOx) and volatile organic compounds (VOCs) - in the presence of heat and sunlight. Many urban areas tend to have lower levels of ground-level ozone compared to rural areas because ozone is destroyed in a reaction with nitric oxide, which is released primarily from motor vehicles. Ozone is a regionally transported pollutant usually measured at rural and suburban locations upwind and downwind of sources of ozone precursors (NOxand VOCs). Ground-level ozone is different from "good" ozone, which occurs naturally in the sky about 10 to 30 miles above the earth's surface and forms a layer that protects life on earth from the sun's harmful rays.
Why is ozone an important public health issue?
Symptoms of exposure to increased ozone concentrations vary among individuals and may include: coughing, nose and throat irritation, chest pain, aggravation of asthma, shortness of breath, increased susceptibility to respiratory infection, decreased lung function, and other respiratory ailments. Long term exposure to ozone can result in permanent lung damage.
Why is the Massachusetts Department of Public Health tracking ozone as a public health issue in the state?
The U.S. Centers for Disease Control and Prevention (CDC) worked with EPHT state partners to develop nationally consistent data measures (NCDM) for environmental and health data. Outdoor air quality NCDMs have been established for two pollutants: PM2.5 and ozone. Linking data on ozone concentrations with health data may provide information on the degree to which ozone pollution may be contributing to adverse health impacts in communities throughout the state. It will also be useful in tracking the benefits of emission control efforts to reduce air pollution in the state.
Where does ozone come from?
The primary ozone precursors, NOx and VOCs, are emitted into the air from sources including power plants, industrial boilers, mobile on-road (e.g. cars and trucks) and off-road (e.g. construction equipment) sources, and consumer and industrial products. Much of the ozone precursors, as well as ozone, are transported into Massachusetts from out-of-state sources. Massachusetts sources also contribute to ozone formation at downwind locations. Variability in weather patterns contributes to yearly differences in the magnitude and frequency of ozone concentrations.
Who is likely to be affected by exposure to ozone, and why?
People with respiratory disease, children, older adults, and people who are active may be most affected when ozone levels are high. Sensitive people who experience effects at lower ozone concentrations are likely to experience more serious effects at higher concentrations. Active children are the group at highest risk from ozone exposure because they spend more time outdoors. Ozone can aggravate symptoms for children with asthma. Active adults who exercise or work vigorously outside have a higher level of exposure to ozone than people who are less active.
How can I reduce my exposure to ozone?
The Massachusetts Department of Environmental Protection provides air quality forecasts on a daily basis from April through September when ozone levels may be unhealthy. The air quality forecast is based on U.S. EPA's Air Quality Index, or AQI (MassAir Online at MassDEP). The AQI has a color-coded scale so that people know when air pollution is expected to reach unhealthy levels in their area. The AQI may be used to plan daily activities such as avoiding strenuous activity and shortening time spent outdoors when ozone levels are forecasted to be at the highest.
How is attainment of the National Ambient Air Quality Standard for ozone determined using monitoring data?
The federal Clean Air Act (CAA) established timeframes and milestones for states to meet and maintain National Ambient Air Quality Standards (NAAQS) for criteria pollutants. U.S. EPA sets the NAAQS levels to protect public health and the environment. Each state, including Massachusetts, is required to monitor the ambient air to determine whether it meets each standard. Each monitoring site with Federal Reference Method (FRM) monitors is required to achieve 75% or greater data capture for the year in order for the data from the monitoring site to be considered valid in the attainment demonstration. To attain the ozone standard, the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentration measured at each monitor within an area over each year must not exceed 0.070 ppm (effective December 28, 2015). If the air quality does not meet a standard, the state must develop and implement pollution control strategies to attain that standard. Once air quality meets a standard, a 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 Plan (SIP). For more: The Massachusetts SIP
Are there differences between the calculations for determining NAAQS attainment and for determining the EPHT outdoor air quality measures for ozone?
Yes. The calculation for the EPHT outdoor air quality measures for ozone (i.e., the number of days that the ozone NAAQS is exceeded and the number of total number of persons living in a county when the ozone NAAQS is exceeded over a one year period) is based on the number of days that the highest daily maximum 8-hour ozone concentration exceeds the NAAQS in a county over a one-year period whereas the calculation for determining NAAQS attainment is based on the 3-year average of the fourth-highest daily maximum 8-hour average ozone concentrations measured at each monitor within an area. Both the NAAQS and the EPHT outdoor air quality measures are based on monitoring data submitted by the MassDEP to U.S. EPA's Air Quality System, or AQS. EPHT data may also include outdoor air quality monitoring results during "exceptional events", whereas the U.S. EPA allows states to exclude certain "exceptional events" when assessing NAAQS attainment. Exceptional events may include chemical spills and industrial accidents, structural fires, and natural events (volcanoes and earthquakes, high wind events, wildfires).
How is the highest daily maximum 8-hour ozone concentration calculated for each county?
The daily maximum 8-hour daily concentration for ozone is determined by retaining data reported for each monitoring site that has valid observations for 75% of the days in an ozone season. The values that exceed the NAAQS of 0.070 ppm (i.e., 0.071 and higher) are considered above the NAAQS. The outdoor air quality measure is the number of days that the highest daily maximum 8-hour maximum ozone concentration exceeds the NAAQS.
How are the number of person-days exceeding the NAAQS calculated?
The person-days outdoor air quality measure takes into account both the number of days that the maximum 8-hour ozone concentration exceeds the NAAQS and the population within the county where monitors are located. The maximum 8-hour daily concentration for ozone is calculated based on the methodology presented above. The number of persons is derived from the Census Bureau population estimates. The Census Bureau typically updates the county-level population estimates for the years between censuses on an annual basis. The census county-level population is adjusted annually by considering births, deaths, immigration and other information sources. Documentation on the methodology used by the Census Bureau in estimating county-level can be found in this PDF.
What is modeled outdoor air quality data?
Modeled data are estimates of ozone data generated by a model called Downscaler, which was jointly developed by EPA and CDC. Downscaler uses a statistical approach to combine data collected from air monitoring stations with the statistical output from Community Multiscale Air Quality (CMAQ) model estimates. For additional information about Downscaler and CMAQ, refer to EPA documents on these topics found here.
How is modeled outdoor air quality data used?
In areas where outdoor air monitors are not present, the value reported is the modeled estimate. The Downscaler output provides estimated daily measurements of ozone on days when monitoring data isn’t collected or in locations where monitors are not available.
What limitations of the outdoor air quality measures for ozone need to be considered?
There are several limitations associated with the EPHT ozone outdoor air quality measure that need to be considered:
- The relationship between ambient concentrations and personal exposure is difficult to estimate and variable depending upon pollutant, activity patterns, and microenvironments.
- Environmental measures do not quantify exposure or the magnitude of risk associated with ozone levels in a given area. Variations that exist across a geography may not always be captured. Furthermore, the data does not provide information on the severity of potential exposures.
- The data for these measures only represent counties where monitors are located and tend to reflect urban outdoor air quality where most people live. Thus, although populations in areas without monitors may also be exposed to ozone at levels that exceeds the standard, they are not counted.
- Outdoor air quality monitoring data may not coincide with health outcome data because of differences (time, space, or other) between the measurement and the exposure or the lag time between the exposure and the symptoms.
- Person-day estimates for larger, highly populated counties may be biased higher than estimates for smaller less populated counties. The measure uses the highest value of all monitors in the county, so larger counties with more monitors may have a broader range of pollution values and greater potential to measure a high day than smaller counties with fewer monitors.
- The percent of the state's population living in counties with no ozone measurements must always be considered when attempting to estimate the proportion of population at risk.
- The model predictions are used to fill-in outdoor air quality estimates in areas and at times without monitoring data. For counties without monitoring data, temporal (seasonal) and spatial (regional) biases in the modeled estimates, can influence the accuracy of the measures.
- Modeled outdoor air quality estimates should not be used for NAAQS compliance determination.
Available data on Ozone
Use the Explore Maps & Tables link on this page to access the following measures for outdoor air quality for your community:
- Monitored Ozone (O3), and Particulate Matter (PM2.5) – county only
- Modeled Ozone (O3), and Particulate Matter (PM2.5) – county, community, census tract
- Combined monitored and modeled Ozone (O3), and Particulate Matter (PM2.5) – county only
The most current available data will be shown. Be sure to check the site periodically as new data are added each year.