Air Quality Metadata: Ozone


Air quality data are from the US Centers for Disease Control and Prevention (CDC) and U.S. Environmental Protection Agency (EPA); population data are from the US Census Bureau.

Dates Available:

1999 - 2018

Geographic Resolution:



This dataset contains the annual number of days that the maximum 8-hour ambient ozone concentration is above the previous U.S. standard (National Ambient Air Quality Standard, NAAQS) of 75 parts per billion (ppb), by year and county. The dataset also contains the annual number of person-days over the standard, per year and by county, based on the total county population multiplied by the annual number of days over the standard. The current air quality standard for ozone is 70 ppb as an 8 hour average. The CDC and EPA worked together to develop a statistical model, called Downscaler, that combines predictive air pollutant models with measurements from continuous air monitors.

The dataset contains the following measures:

  1. Number of days with maximum 8-hour average ozone concentration over the National Ambient Air Quality Standard (NAAQS). Monitor only measures, 1999-2018; Downscaler measures, Monitor + Modeled, 2001-2016.
  2. Number of person-days with maximum 8-hour average ozone concentration over the National Ambient Air Quality Standard (NAAQS). Monitor only measures, 1999-2018; and Downscaler measures, Monitor + Modeled, 2001-2016.


This data set supports efforts to improve public health in Maine and contributes to the U.S. CDC’s National Environmental Public Health Tracking (EPHT) Network. A key activity of participants in this network is to track and make available environmental health measures on state and national data portals. Measures derived from the data set described here can be used to compare levels of ambient ozone across the state, between groups of people, over time, and in relation to other risk factors, exposures, and health outcomes.

The Maine Tracking Network, a member of the National EPHT Network, connects communities, public health professionals, policy makers, state agencies, and others to the data they need to monitor public health, respond to health concerns, prioritize resources for public health action, and evaluate prevention activities. Maine tracks certain health effects, exposures, and environmental hazards that have known relationships, as well as some health effects and environmental hazards that have suspected relationships. By making health and environmental data available through the Maine Tracking Network, more people have access to data they need to think critically and hypothesize about health outcomes and their relationships to conditions in the environment.

Supplemental Information:

Ground-level, or ambient, 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. Studies have linked ground-level ozone to such varied health issues as: aggravation of asthma, bronchitis, and emphysema; respiratory illness such as pneumonia or bronchitis; and wheezing or breathing difficulties during exercise or outdoor activities. The number of days in which the daily maximum 8-hour average ozone concentration exceeds the national standard provides an indication of short-term spikes in ambient ozone concentrations.

Ozone air quality data come from Federal Reference Method monitors (FRM). These monitors are placed in regions where ozone levels tend to be higher. Ozone concentrations are measured continuously and averaged over each 8-hour period. In Maine, ozone is monitored from May through September of most years. EPA retains data from ozone monitors that meet the minimum data completeness criteria set forth in the national ambient air quality standards (NAAQS). In order for the daily 8 hour maximum value to be included, ozone data must be available for 75% of the possible hours in the day. Data are also included if any of the maximum 8 hour average value is above the level of the standard, regardless of completeness.

Not all counties have air quality monitors, and air monitors do not produce usable data every hour of every day. The CDC and EPA worked together to develop a model, called Downscaler, that generates ambient air ozone concentration measures for areas of the contiguous United States that do not have monitors, as well as fills in time gaps when monitors are not recording data.

Entity and Attribute Overview:

This dataset includes: county, year, state, number of days above with maximum 8-hour average ozone above 75 parts per billion (ppb), and person-days above same threshold (population in county x days above). 

Data Limitations:

  • The measures provide a general indication of the overall trend in annual ozone concentrations; they do not directly reflect personal exposure. The relationship between ambient concentrations and personal exposure is subject to many factors, including individual activity patterns and microenvironments.
  • Air monitors provide concentration data around the specific location of each monitor. Intra-county variation in concentrations will likely exist but cannot be captured in this measure.
  • The Downscaler model (“Monitor + Modeled”) is used to fill in air quality estimates in areas and time periods without monitoring data, but for counties without any monitors, temporal (seasonal) and spatial (regional) factors can lead to systematic under- or over-prediction. 

Access Constraints:


More Information:

Suggested Citation for Data Displays:

Maine Center for Disease Control and Prevention, Maine Tracking Network. Air Quality: Ozone. Available online: Accessed on [Date].