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Air Quality - Frequently Asked Questions

Fine Particles (PM2.5)

What is particulate matter?

Particulate matter, which is also known as particle pollution or PM, is a complex mixture of extremely small particles and liquid droplets. Particle pollution is made up of a number of components, including chemicals, metals, and soil or dust particles. The size of the particles is directly related to their potential for causing health problems. Particulate matter is grouped into three size categories: PM10, PM2.5, and ultrafine particles. Particles that are 10 micrometers or less in diameter (PM10) are of the most concern because they can pass through the nose and throat and enter the lungs. Fine particles, also known as PM2.5, are 2.5 micrometers in diameter and smaller. Ultrafine particles are smaller than 100 nanometers. Currently air quality regulations exist for PM10 and PM2.5.

What is PM2.5?

PM2.5 refers to "fine" particulate matter that is 2.5 micrometers in diameter and smaller; in comparison, the average human hair is about 70 micrometers. Sources of particulate matter are primarily the result of combustion activities (e.g., residential wood burning, fossil fuel combustion) or formed from chemical reactions of gaseous pollutants in the atmosphere. Fine particles can be a local pollutant but also are considered regional pollutants because they can be suspended in the atmosphere for long periods of time and transported over long distances. Fine particles are also the major cause of haze or reduced visibility.

Why is PM2.5 an important public health issue?

Fine particles pose the greatest risk to health because scientific studies have linked particle pollution, especially fine particles that can deposit deep into the lungs, with increased respiratory symptoms, such as irritation of the airways, coughing, or difficulty breathing; decreased lung function; aggravation of lung disease, causing asthma attacks and acute bronchitis; irregular heartbeat; nonfatal heart attacks; and premature death in people with pre-existing heart or lung disease.

Why is the Massachusetts Department of Public Health tracking PM2.5 as a public health issue in the state?

The U.S. Centers for Disease Control and Prevention (CDC) established nationally consistent environmental and health data measures (NCDM) for the environmental and health tracking network. Air quality NCDMs have been established for two pollutants: PM2.5 and ozone. Linking data on PM2.5 concentrations with health data may provide information on the degree to which particle 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 particle pollution in the state.

Where does PM2.5 come from?

Major sources of PM2.5 in Massachusetts include industrial process emissions, motor vehicles, incinerators, and power plants.

Who is likely to be affected by exposure to PM2.5, and why?

Older adults, individuals with heart or lung diseases, and children are more likely to be affected by PM2.5 exposure. People with heart and lung diseases are at increased risk because particles can aggravate these diseases. Older adults are at increased risk because they may have undiagnosed lung or heart disease. Children are likely at increased risk because they are still developing and spend more time outdoors and generally at high activity levels.

How can I reduce my exposure to PM2.5?

The Massachusetts Department of Environmental Protection (MassDEP) provides an air quality forecast on a daily basis to inform you when particle levels are expected to be unhealthy. The air quality forecast is based on U.S. EPA's Air Quality Index, or AQI. A semi-continuous, hourly measurement method for PM2.5 is used for real time reporting of PM2.5 values and to assist in developing daily PM2.5 predictions for the AQI. 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 PM2.5 levels are forecasted to be at the highest.

How is attainment of the PM2.5 National Ambient Air Quality Standard 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. The annual standard is met when the annual average of the quarterly mean PM2.5 concentrations is less than or equal to 15 µg/m3 (3-year average). If spatial averaging is used, the annual average from all monitors within the county may be averaged in the calculation of the 3-year mean. The 24-hour standard is met when the 98th percentile value is less than or equal to 35 µg/m3 (3-year average). 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). The Massachusetts SIP is available at http://www.mass.gov/eea/agencies/massdep/air/reports/state-implementation-plans.html

Are there differences between the calculations for determining NAAQS attainment and determining the EPHT air quality measures for PM2.5?

Yes. The calculation for the EPHT air quality measures for PM2.5 are based on the exceedances of daily or annual PM2.5 NAAQS in a county over a one year period whereas the calculation for determining NAAQS attainment is based on the 3-year annual average of the quarterly mean PM2.5 concentrations or a 98th percentile value for the daily PM2.5 NAAQS over a three year period. The air quality measures are based on monitoring data submitted by the MassDEP to U.S. EPA's Air Quality System, or AQS (http://www.epa.gov/ttn/airs/airsaqs/). EPHT data may also include 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). For example, data for several 2002 PM2.5 exceedances was flagged as an "exceptional event", because they were caused by emissions from a forest fire which occurred in early July of that year in Quebec.

How is the maximum daily and annual average PM2.5 concentration calculated?

The maximum PM2.5 daily concentration is determined by retaining the maximum concentration reported for each monitoring site for each monitored day (i.e. if valid 24-hour averages are available for at least 75% of possible hours in a day). For the annual PM2.5 concentration, a complete (valid) quarterly average must have at least 11 observations (11 valid 24-hour values). For each valid quarter, a quarterly average is calculated. The annual average for each monitor is the average of four valid quarterly averages. The PM2.5 annual average measure is the maximum annual average among monitors with complete (4 valid quarters) data in that county. The values that exceed the NAAQS (i.e., 15.5 µg/m3 and higher for annual or 35.5 µg/m3 and higher for daily) are considered above the NAAQS.

How is the number of percent of population and person-days calculated?

The air quality measures takes into account both the number of days that the maximum daily PM2.5 concentrations exceed the NAAQS and the population within the county where monitors are located. The maximum daily and annual PM2.5 concentrations are calculated based on the methodology presented above. The number of persons in each county 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 at http://www.census.gov/popest/methodology/2012-nat-st-co-meth.pdf.

What are limitations of the data?

There are several limitations associated with the EPHT PM2.5 air quality measures that need to be considered:

  • The relationship between ambient concentrations and personal exposure is difficult to estimate and varies depending upon pollutant, activity patterns, and microenvironments.

  • The percent of days that exceed the U.S. EPA NAAQS or other health benchmarks does not provide information regarding the severity (maximum concentrations) of potential exposures. The data for this measure only represent counties that have air monitors and tend to reflect urban air quality (where most people live). Thus, although populations in areas without monitors may also be exposed to PM2.5 that exceed the standard, they are not counted.

  • The percent of the state's population exceeding annual PM2.5 NAAQS is based on measurements every third day and, therefore, may not necessarily be comparable when sampling is conducted on the different day.

  • 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 then smaller counties with fewer monitors.

  • The percent of the state's population living in counties with no PM2.5 measurements must always be considered when attempting to estimate the proportion of population at risk.

Where can I get additional information on fine particles?

The following websites offer information on fine particles:

Information on particulate matter in Massachusetts: MassDEP Particulate Matter FAQ
Current PM2.5 levels and health implications in Massachusetts: MassAir Online at MassDEP
Information on air quality standards for PM2.5: US EPA "Air Quality Criteria for Particulate Matter"
Information on particulate matter: US EPA Particulate Matter Web Site

Ozone

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. 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 0.075 ppm (effective May 27, 2008). 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). The Massachusetts SIP is available at http://www.mass.gov/eea/agencies/massdep/air/reports/state-implementation-plans.html

Are there differences between the calculations for determining NAAQS attainment and for determining the EPHT air quality measures for ozone?

Yes. The calculation for the EPHT 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 air quality measures are based on monitoring data submitted by the MassDEP to U.S. EPA's Air Quality System, or AQS (http://www.epa.gov/ttn/airs/airsaqs/). EPHT data may also include 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 0.075 ppm (i.e., 0.076 and higher) are considered above the NAAQS. The 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 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 at http://www.census.gov/popest/methodology/2012-nat-st-co-meth.pdf.

What limitations of the air quality measures for ozone need to be considered?

There are several limitations associated with the EPHT ozone 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.

  • The data for these measures only represent counties where monitors are located and tend to reflect urban 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.

  • 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.
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