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Last Edited: 9/28/2010

Air Quality: Measures and Limitations

Learn more about the how air quality is measured and some of the current limitations in using this data to estimate air pollution exposure.  Scroll down or select from a topic, below.

Ways to estimate air pollutant levels

Scientists have different strategies for calculating air pollution measures. These strategies include:

  • Reporting of direct measurements from monitoring stations
  • Reporting of sources, such as the locations of industrial facilities or traffic levels on roadways
  • Models that may consider either or both of the above types of data along with considerations of weather, topography, and dissipation patterns

None of these directly correspond to population exposure (i.e. the amounts of pollutants that are taken in by people day-to-day), although all can provide useful information.  In places where no monitoring data exist, pollutant levels need to be estimated using statistical modeling methods.  These methods commonly use known values at nearby locations to estimate pollution levels for locations without data.  

Another way of measuring air pollution is to look at the amount which is emitted into the air from:

  • Mobile sources (e.g. cars, trucks)
  • Stationary sources (e.g. industrial facilities)
  • Area sources (e.g. fireplaces, road dust)
  • Natural sources (e.g. wildfires, windblown dust)

Estimates for these categories can be found in the California Air Resources Board (ARB) California Emission Inventory.  Data from stationary sources are reported by facility owners to its local air district.  Emissions from the other sources are estimated by statistical modeling. 

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Advantages and limitations of using air monitoring data

Air quality data are essential for:

  • Assessing public health impacts caused by poor air quality
  • Determining whether an area is meeting the standards
  • Evaluating changes in air quality as a result of state implementation plans 

The challenge is to get measurements of air quality in time and space that are useful for public health activities.  The advantages of using ambient data collected through the Environmental Protection Agency (EPA), state, and local monitoring networks include:

  • These measurements of pollution concentrations are the best characterization of the concentration of a given pollutant at a given time and location
  • The data are supported by a comprehensive quality assurance program, ensuring good data of known quality

However, to compare ambient concentrations with acute health effects, daily local air quality data is needed.  Geographic or spatial gaps exist in the air quality monitoring network, especially in rural areas, since:

  • The air quality monitoring network is designed to focus on measurement of pollutant concentrations in high population density areas
  • There are a limited number of monitors that can be sited and maintained
  • The priority is for monitors to be located in areas to determine whether an air basin is in compliance with State and Federal regulations
    • So, the resulting measurements are not always a good indication of the burden of air pollution in a specific area
    • For example, air monitors are not located near sources of high air pollution, since the goal is to obtain a picture of the ambient background levels

Temporal limits also exist. 

  • PM2.5 monitors generally collect samples only once every three days, due in part to the time and costs involved in collecting and analyzing the samples
    • However, monitors that can automatically collect analyze, and report PM2.5 measurements on an hourly basis have been introduced over the past several years
    • These monitors are available in most of the major metropolitan areas
  • Ozone is monitored daily, but mostly during the ozone season (the warmer months, approximately April through October)
    • However, year-long data would be extremely usefu for evaluating whether ozone is a factor in health outcomes during the non-ozone seasons

The challenge of using ambient monitoring data is that the limited number of air monitors create gaps in space and time.

  • The gaps in temporal and spatial coverage limit the complete assessment of air pollution exposure that is needed to assess health outcomes
  • This spatial and temporal ‘misalignment’ between air quality monitoring data and health outcomes is influenced by the following key factors:
    • The air quality monitor may not be in the same location as where a person lives, works, or plays
    • The air quality monitor may not have sampled the air at the time a person experienced a health outcome (e.g. an asthma attack)

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Choosing which pollutant to measure

Just as there is no single air pollutant relevant to every discussion, there is no single way to express air quality.  Generally, the measure we choose at any given time depends on what our concerns may be.  Since frequently we do not know which compounds have the greatest health effects, it can be difficult to know which measure is most important. 


Considerations that may go into one’s choices of air pollution measures include:

  • Short-term versus long-term health effects
    • For example, day-to-day variations in levels of some air pollutants have been shown to correlate with emergency room visits by children with asthma, so number of exceedences may be of interest
    • By contrast, if the concern is with long-term cancer risk, annualized average concentrations may be more informative
  • Single pollutant versus multi-pollutant measures
    • When our discussion is driven by specific health concerns, we may choose to discuss all of a certain category (such as cancer-causing compounds) together

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Choosing which measure to use

There are a number of measures that can be used to examine air quality.


  • Single pollutant-- For a single pollutant, one can examine:
    • Maximum levels of pollutant in a given time period
    • Averages of pollutant concentrations in a given time period
    • Number of days the pollutant exceeds a standard in a given time period
  • Multiple pollutants
    • The EPA has an Air Quality Index which incorporates multiple criteria pollutants to give one an overall indication of air quality
  • Person-days--One can examine the number of person-days that a region has unhealthy air
    • Person-days are the number of persons living in an exposed region times the number of days the pollutant exceed a health standard
    • Person-days gives an indication of the population burden of air pollution exposure
  • Geography
    • One can choose to examine pollution in a county or in an air basin

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