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Outdoor Air Quality

Modeled outdoor air quality measures provide estimates of air pollution concentrations for fine air particles (PM2.5) and ozone. The dataset can be used for understanding outdoor air quality trends over time and identifying areas where the potential for PM2.5 or ozone exposures may exist. There are uncertainties associated with the data. The estimates should not be used for determining regulatory compliance with NAAQS.

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In 1970, the federal Clean Air Act required the U.S. Environmental Protection Agency (U.S. EPA) to set limits called National Ambient Air Quality Standards (NAAQS) to protect public health, including the health of "sensitive" populations such as asthmatics, children, and the elderly. These standards specify limits for six air pollutants across the United States. These criteria air pollutants include: Carbon Monoxide (CO), Lead (Pb), Nitrogen Dioxide (NO2) Sulfur Dioxide (SO2), Ozone (O3), and Particulate Matter (e.g., PM10 and PM2.5).

State, local, and tribal air quality agencies operate and maintain a wide variety of ambient monitoring systems across the United States to determine compliance with the NAAQS. Environmental contaminants in outdoor air are monitored by the Massachusetts Department of Environmental Protection (MassDEP).

Outdoor air quality monitoring networks serve multiple environmental objectives including determining compliance with national air standards, informing the public about how clean or polluted the outdoor air is, forecasting daily pollution levels for the public, and tracking progress in reducing air pollution from significant sources including automobiles, trucks, and industrial sources (e.g., power plants, chemical manufacturing facilities).

In Massachusetts, the MassDEP Air Assessment Branch (AAB) operates an ambient air quality monitoring network of 22 monitoring stations located in 17 cities and towns in Massachusetts. The Wampanoag Tribe of Gay Head operates an ozone monitoring site on Martha’s Vineyard (Dukes County). Monitoring sites are chosen to meet U.S. EPA’s national monitoring objectives and siting criteria. For example, some sites are selected because they are probable “hot spots” for high levels of certain air pollutants while others are chosen to provide data that are representative of a wider area. The MassDEP provides a map of the “Air Quality Index” (AQI) that tells you how clean or polluted your air is and what associated health effects might be a concern for you to consider. (For more information, connect to: MassDEP Air Quality Online).

MassDEP submits quality-assured monitoring data to a central U.S. EPA repository database system called the Air Quality System (AQS), which is accessible to the public. The outdoor air quality measures for environmental public health tracking are based on monitoring data extracted from the U.S. EPA's AQS.

The map below shows Massachusetts communities where air monitors were located during 2017.

Monitoring data

Ambient monitoring data provide the best available information on the concentration of pollutant levels in the outdoor air at a given time and location since the devices measure air pollution concentrations in real-time. One disadvantage of using ambient monitoring data is that there are a limited number of monitors that can be sited and maintained. This limitation creates gaps in coverage across the state in areas that do not have ambient monitoring sites. Since some monitors do not operate daily, there is also a gap in time during which data is gathered. However, the U.S. EPA has been working with the CDC to develop models that predict air pollution concentrations in counties without monitors. This modeled data are available on EPHT at finer-scale geographies than air monitoring data.

Modeled data

The outdoor air pollution modeling approach developed and tested by EPA and CDC is a statistical method that combines AQS monitoring measurements with air pollution predictions from the Community Multi-Scale Air Quality Model (CMAQ). This approach, known as Downscaler, fills in the substantial time and geography gaps that exist in the monitored data. In counties where monitors exist, the combined “monitored and modeled” pollutant dataset predicted by the Downscaler model become useful for understanding pollution trends that may occur at the local and community levels. The dataset allows researchers, scientists, and state and local health practitioners to examine PM2.5 and ozone pollution patterns and their possible relationship with health outcomes, such as asthma and heart disease. Because modeled data are estimates of air pollution, there are uncertainties associated with the data. Therefore, modeled estimates are intended for planning and investigative purposes only and should not be used for determining regulatory compliance with NAAQS. View additional information about the EPA and CDC’s methodology for the Downscaler modeling approach.

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