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Landfill Gas Primer - An Overview for Environmental Health Professionals

Chapter 4 Continued: Ambient Air Monitoring

Historical Document

This document is provided by the Agency for Toxic Substances and Disease Registry (ATSDR) ONLY as an historical reference for the public health community. It is no longer being maintained and the data it contains may no longer be current and/or accurate.

Unlike most soil gas monitoring data and emissions monitoring data, which do not characterize levels of contamination that residents are inhaling, ambient air monitoring data can provide a better characterization of gases in the breathing zone. These data, together with indoor air monitoring data (when available), are most useful for evaluating the inhalation exposure pathway at landfill sites. The following discussion presents important background information about this type of monitoring.

What is ambient air monitoring?

Ambient air monitoring measures levels of contamination in outdoor air, or in the air that people breathe. The levels of pollution measured in the ambient air reflect the combined influences of many different nearby sources, and even some distant ones.

Why is ambient air monitored at or near landfills?

The main reason ambient air monitoring is performed at or near landfills is to evaluate worker and community exposure concerns regarding releases of toxic chemicals to the air. However, because federal regulations currently do not require ambient air monitoring to be performed in the vicinity of municipal solid waste landfills, no ambient air monitoring data are available for many landfills. This is especially true for smaller landfills and those that have not generated extensive community health concerns.

In some cases, a landfill may be considered a hazardous waste site under federal and state regulations. At these sites, regulatory agencies or the landfill owner and operator may collect ambient air data. At other landfills, states may operate ambient air monitoring stations near landfills to measure concentrations of some or all of EPA's criteria pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide). If organic compounds are of concern, continuous monitoring for total hydrocarbons is also possible (but to obtain speciated data, sampling and analysis is usually needed). These pollutants, however, originate from many sources in addition to landfills, and their monitoring data often are viewed as an indicator of general air quality, rather than as the influence of any one particular source (e.g., a landfill).

How are ambient air concentrations measured?

Ambient air concentrations are generally measured according to specifications set forth in an ambient air monitoring plan. Though the content of these plans varies from project to project, the plans typically address at least the following critical elements of ambient air monitoring:

  • Chemicals selected for monitoring. One of the first decisions environmental professionals make when developing an ambient air monitoring plan is to select the chemicals to be monitored—a decision that is largely influenced by the purpose of conducting monitoring in the first place. For example, at sites where potential exposure to landfill gas is of concern, monitoring typically focuses on NMOCs, rather than on metals or particulate matter. At sites where windblown dust is an issue, monitoring would likely also consider particulate matter.

    Results from soil gas, near-surface, and emissions monitoring data, if available, may be a useful guide for selecting chemicals to consider in air monitoring programs. The programs should attempt to measure as many of the chemicals detected in the soil gas and emissions as possible, but especially the most toxic chemicals with the highest concentrations and emission rates. Table 2-1 (Chapter Two) lists some of the more prevalent NMOCs in landfill gas. The EPA's compilation of Air Pollutant Emissions Factors (known as AP-42), Section 2.4, provides typical concentrations of more than 40 NMOCs and inorganic compounds in MSW landfill gas. If no site-specific data are available, the substances on this list may provide a starting point. Realistically, however, ambient air monitoring for the scores of chemicals that landfills emit is a prohibitively expensive endeavor. From a practical standpoint, selection of chemicals for monitoring is determined by weighing several factors, such as cost, chemical toxicity, and the availability of sampling methods that can reliably measure ambient air concentrations of a given chemical.

  • Sampling methods. After sponsoring many years of research into ambient air monitoring, EPA has approved several different types of sampling and analytical methods for a long list of common air pollutants. For criteria pollutants (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide), EPA has published a list of sampling devices that are capable of measuring concentrations both accurately and precisely for comparison to its national ambient air quality standards (NAAQS). Similarly, EPA has published two "compendium documents" that describe in detail the Agency's approved methods for measuring ambient air concentrations of certain pollutants, including organic compounds. References to these EPA documents are listed at the end of this chapter. Optical methods, such as FTIR discussed in the Emissions Monitoring section, may also be useful for ambient monitoring near the boundary of a landfill.

    When possible, use of EPA-approved methods is encouraged, because the approval is based on extensive testing of the accuracy and precision of ambient air monitoring. In some cases, however, EPA-approved methods might not be available for certain chemicals or monitoring frequencies (e. g., the compendium documents do not address many of the continuous sampling devices that are available), and use of other methods might be necessary. In these cases, extra care should be taken to ensure that the selected methods are capable of generating high- quality data.

    The monitoring methods selected for use in a given program determine the detection limits for each chemical. The detection limit is the lowest concentration at which the method can reliably measure a chemical's ambient air concentration. For ambient air monitoring data to be useful to the environmental health official, all efforts should be made to use methods with detection limits that are lower than or comparable to ambient air concentrations that would be of health concern.

  • Ambient air monitoring locations. One of the most important elements of developing an ambient air monitoring program is selecting monitoring locations. With strategically chosen locations, monitoring programs can generate data of great usefulness for the environmental health professional. Poorly chosen locations, in contrast, can cause monitoring programs to generate data that offer little insight into air quality in neighborhoods of concern. In general, monitoring locations are selected according to the goal of the sampling program. If the goal is to address community concerns, monitoring locations should include residential neighborhoods at downwind locations nearest the landfill and other places where people might be exposed to landfill gases (e.g., nearby parks, malls, and schools).

    Many additional concerns should be considered when selecting monitoring locations. For perspective on the extent to which landfill emissions affect air quality, simultaneous monitoring at locations upwind and downwind of the landfill of concern is advised. It is equally as critical to review the surroundings of monitoring stations to ensure that local sources of air pollution will not bias a monitor's readings. As examples, monitoring alongside busy roadways or atop industrial facilities will likely generate results indicative of emissions from these sources, even if a landfill is nearby. Schools, parks, and churches generally make excellent choices for monitoring locations because they have few sources of emissions on their premises, they often have sources of electricity readily available, and they typically are located in or near residential neighborhoods.

  • Monitoring schedules. Ambient air monitoring plans should specify both the frequency and duration of the proposed monitoring, and both factors should be considered when interpreting data. The frequency of monitoring is often determined by the available sampling methods. Continuous methods provide an ongoing account of air quality, but these methods usually measure levels of only one pollutant; periodic monitoring is typically, though not always, conducted by collecting 24-hour averaged samples on either a 6-day or 12-day cycle. These frequencies ensure that ambient air samples will be collected on every day of the week over a long-term program.

    Sometimes 8- or 12-hour sampling is conducted. Though useful for occupational exposures, such as on the landfill, such sampling may miss significant off-site releases affecting nearby residents during non-working hours, such as the predawn hour when landfill gas odors are not diluted and diffused by strong wind.

    The duration of monitoring is also an important consideration. Because landfill emissions might exhibit significant seasonal variations, monitoring for a year or longer is needed to accurately estimate the long-term average concentrations of air pollutants. Further, landfill emissions can change from year to year for various reasons, such as increases or decreases in daily disposal rates, changes in waste mix and moisture, landfill closure, and installation of pollution controls. As a result, monitoring results collected when a landfill actively received wastes might not be representative of air quality after the landfill closes. Use of long-term monitoring at fixed locations, when funds to conduct such monitoring are available, is the best approach for evaluating ongoing effects of landfill air emissions on local air quality.

  • Data quality parameters. In ambient air monitoring plans, data quality objectives will be specified for the program. Data quality objectives provide a goal for exactly how accurate, precise, and complete a data set must be. In general, ambient air monitoring programs should strive to collect and analyze air samples in accordance with their method's data quality specifications. Though these specifications vary from method to method, measurement accuracy and precision of better than 50% is usually feasible for most methods. A sampling completeness (defined as the percent of attempted sampling events that are successful) of better than 90% is desired.

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What do ambient air monitoring data tell you?

As noted earlier, ambient air monitoring data characterize levels of contaminants in the air that people breathe. Because these data are almost always the best metric for exposure concentrations at landfill sites, it is extremely important that environmental health professionals interpret ambient air monitoring data critically. At a minimum, you should ask yourself the questions below when reviewing these data to ensure that they are truly representative of exposure concentrations.

Questions To Consider When Reviewing Ambient Air Monitoring Data

Chemicals Selected for Monitoring
  • What chemicals were selected for monitoring?
  • Do these include the chemicals of concern identified by residents, regulators, and public health officials?
  • Do the chemicals selected for monitoring include those expected to be emitted in greatest quantities from the landfill and/or those that are most toxic chemicals in the emissions?
  • Are there any data gaps in the chemicals selected for monitoring?

Sampling Methods

  • Were EPA-approved sampling methods selected? If not, why?
  • Are the selected methods recommended for measuring the chemicals selected for monitoring?
  • Are the selected methods capable of achieving detection limits comparable to or lower than ambient air concentrations that would be of public health concern?

Meteorologic Data

  • Is there an on-site meteorologic station providing data on wind direction, speed, rainfall, and atmospheric pressure?
  • If there is no on-site meteorologic station, how far away is the closest reporting station and how relevant is the data from that station to the site and community?
  • Are there known prevailing wind patterns at the site or in the community that may affect changes in contaminant flow pattern, such as the canyon winds in Southern California or seashore wind patterns?

Ambient Air Monitoring Location

  • Was monitoring performed at both upwind (sometimes labeled background) and downwind locations?
  • Do you have reason to believe that ambient air concentrations of certain pollutants were higher in residential areas other than those selected for monitoring?
  • Are the monitoring locations considerably removed from other emissions sources (e. g., industrial facilities or heavily traveled roadways) that might bias the air quality measurements?

Monitoring Schedules

  • Was monitoring continuous or periodic?
  • If periodic, what monitoring frequency was selected? Was this frequency sufficient for characterizing fluctuations in emissions from the landfill and other sources?
  • Over what duration was monitoring performed?
  • Is this duration sufficient for characterizing seasonal fluctuations in air quality?
  • Was monitoring performed at a time when landfill emissions were considered to be relatively high (e.g., when the landfill was active) or relatively low (e.g., after emissions controls were installed)?
  • Was monitoring performed at any period when people complained about odors, such as predawn hours or evenings?

Data Quality Parameters

  • What percent of attempted sampling events were successful?
  • How accurate were the reported sampling results
  • How precise were the reported sampling results?
  • EPA maintains an extensive database of ambient air monitoring results that have been submitted to the Agency over the last 30 years. This database, called the Aerometric Information Retrieval System (AIRS), might include ambient air monitoring data for landfill sites that you will review. General information about accessing this database can be found at
  • Data summary reports for two of EPA's nationwide ambient air monitoring programs can be found in the "technical guidance" section of the Web site ttn/amtic. Information in these reports can be useful for determining whether concentrations measured at a given site are unusually high or low when compared to concentrations at other locations, but these comparisons should be made with caution.

When reviewing monitoring data and considering the questions above, you should remember that ambient air monitoring data characterize levels of contamination that result from a combination of many nearby emissions sources, and these data do not characterize influences from any one source (e.g., a landfill) alone. In fact, ambient air monitoring conducted in urban environments will almost certainly identify elevated concentrations of many chemicals (e.g., benzene and 1,3-butadiene) that originate primarily from mobile sources and emissions from gasoline stations.

Failure to consider these other sources might cause you to reach biased conclusions regarding air quality near landfill sites. Perhaps the best way to determine whether a particular landfill is the primary source of a pollutant is to examine whether ambient air concentrations decrease markedly from a source. Chemicals with concentrations that vary little with changing wind directions or with increased distance from a landfill likely do not originate primarily from the site of concern, though exceptions may exist. As listed below, many sources of information are available to guide you in your efforts to make sense of ambient air monitoring data collected near landfills.

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Where can I get more information about ambient air monitoring?

EPA has published numerous references on ambient air monitoring data, a subset of which are listed below. Additionally, environmental health professionals should consult with local and state regulators for their insights on site-specific air quality.

  • EPA's list of approved sampling equipment for measuring concentrations of criteria pollutants is available in the document "List of Designated Reference and Equivalent Methods," which can be downloaded from the EPA Web site at:

  • EPA's compendia of approved sampling and analytical methods for inorganic and organic pollutants can be found in the documents "Compendium of Methods for the Determination of Inorganic Compounds in Ambient Air" (EPA document number EPA/625R-96/01a, which is available at the Web site and "Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air: Second Edition" (EPA document number EPA/625/R-96/01b, available at

  • Additional information about emerging sampling technologies, such as monitoring over an open path using a FTIR spectrometer, is also documented on the EPA Web site

  • EPA maintains an extensive database of ambient air monitoring results that have been submitted to the agency over the last 30 years. This database, called the Aerometric Information Retrieval System (AIRS), might include ambient air monitoring data for landfill sites that you will review. General information about accessing this database can be found at

  • Data summary reports for two of EPA's nationwide ambient air monitoring programs can be found in the "technical guidance" section of the Web site Information in these reports can be useful for determining whether concentrations measured at a given site are unusually high or low when compared to concentrations at other locations, but these comparisons should be made with caution.

Indoor Air Monitoring

This section defines indoor air monitoring and how it relates to landfills, discusses why indoor air monitoring might be performed in structures near landfills, and presents information that environmental health professionals should consider when reviewing indoor air monitoring data.

What is indoor air monitoring?

Indoor air monitoring is the measurement of air concentrations of contaminants in indoor or enclosed locations. Sampling locations for indoor air monitoring efforts include, but are not limited to, basements of buildings (residential, commercial, and industrial), living spaces in homes, and office spaces at landfills.

Why is indoor air sampled at or near landfills?

Near some landfills, property owners have expressed concern over indoor air contamination primarily because chemicals in landfill gas can transport directly into structures built on top of areas where soil gas contamination exists. As a result, the reasons for conducting indoor air sampling at or near landfills are generally identical to those for conducting soil gas monitoring (e.g., to meet regulatory requirements, to characterize risks for explosions, and to characterize potential exposures to toxic chemicals). These reasons are briefly reviewed below.

According to EPA's RCRA regulations, owners and operators of landfills subject to these requirements must ensure that the concentration of methane gas does not exceed 25% of the LEL for methane (1.25% by volume) in indoor air samples collected in the facilities' structures. This requirement reflects the fact that methane is explosive within the range of 5% to 15% concentration in air. If methane emissions repeatedly exceed the allowed limit, regulators might require that corrective action be taken, such as landfill gas control measures discussed in Chapter Five. Note, however, that this requirement applies only to on-site structures, and some landfills might be exempt from this requirement.

Though not required by law, indoor air monitoring at structures at or near landfills has been conducted for two other reasons. First, some studies have measured concentrations of methane in off-site structures to characterize the risks for explosion as a result of migration of soil gases beyond landfill property lines. If methane is found in a building, continuous indoor air monitors are available to measure methane concentrations and sound an alarm when methane concentrations approach dangerous levels. However, off-site migration of landfill gases at most municipal solid waste landfills is now detected and corrected as a result of the required perimeter soil gas monitoring, so that this type of indoor air monitoring is not necessary at structures located near many landfills. Second, some studies have measured concentrations of many different NMOCs that are suspected of migrating with landfill gas and into residential properties. These studies are rarely performed at landfills, however, and typically only in cases where evidence of off-site migration of landfill gases is well documented. One instance where this may be true is a landfill that may be designated as a hazardous waste site based on federal or state regulations.

How are indoor air concentrations measured?

Though EPA has conducted research on indoor air monitoring technologies, the agency has not issued recommended or approved methods for such monitoring programs. In theory, the sampling and analytical methods listed for the ambient air monitoring sections can also be used to measure indoor air concentrations, but some of these methods involve the use of bulky equipment that is sometimes not suitable for indoor environments, particularly homes. In these cases, environmental officials might rely on hand-held monitoring devices, surveying methods, or monitoring guidance provided by the National Institute for Occupational Safety and Health (NIOSH) or OSHA for use in occupational settings. Whatever the basis for selecting a particular monitoring method, you should take time to review its technical approach to ensure that it is capable of generating high-quality data.

Other features of indoor air monitoring programs—chemicals selected for monitoring, monitoring locations, monitoring schedules, and data quality parameters—should be reviewed in the same manner as the features of ambient air monitoring programs.

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What do indoor air monitoring data tell you?

Indoor air monitoring data characterize levels of contamination in indoor environments. The significance of these data depends largely on the scope of the monitoring program. In cases in which only methane is monitored, for example, the data are useful only for evaluating risks of explosion. In cases in which other chemicals are monitored, the data can be used for evaluating potential health risks. The questions in the box earlier on this page should be considered when making this evaluation.

Additionally, you will need to consider the extent to which other indoor sources (e.g., cigarette smoke, losses from cleaning supplies, and emissions from stoves and furnaces) might have contributed to the measured concentrations. Because indoor sources of contaminants can differ considerably from one house to the next, indoor air monitoring data from a given residence should not be viewed as representative of other residences in the area.

Where can I get more information about indoor air monitoring?

Various federal agencies have published references on indoor air monitoring data, a subset of which are listed below. Additionally, environmental health professionals should consult with local and state regulators for their insights on this issue.

  • EPA and NIOSH together published a two-part guidance document on indoor air quality issues. The reports are called "Building Air Quality: Action Plan" (EPA document number 402-K-98-001) and "Building Air Quality: A Guide for Building Owners and Facility Managers" (EPA document number 400-1-91-033). Though these documents primarily include information about managing air quality in large buildings (e.g., office buildings), they also include general information about indoor air quality and considerations for conducting indoor air sampling.

  • EPA maintains a Web site ( dedicated to indoor air quality issues. This site includes information about air quality issues within a wide range of buildings, including homes, schools, and office buildings.

Air Modeling

What is air modeling?

Over the years, scientists have developed a number of mathematical models that can be used to evaluate how chemical emissions disperse in air. Different models may be used to answer a number of different questions about available data, such as how do contaminants disperse from the source or what is one source's contribution to area-wide contamination. These models may be screening models that with little information can provide very conservative estimates, or they may be refined models that require detailed information to provided more accurate estimates. The level of model uncertainty varies from model to model; however, uncertainties always exist with any model.

Another type of model is an emissions estimation model. If emission monitoring data are not available for a landfill, models may be used to estimate emissions. Models of various complexities exist. EPA's Landfill Gas Emissions Model (LandGEM) is one model that is commonly used to estimate year-by-year landfill gas emission rates (in Mg/year) over the life of the landfill and after closure. It can estimate methane, NMOC, and individual organic compounds including many hazardous air pollutants. Landfill owners use this model to determine if the NSPS/EG rules apply to them, and states use this model for emission inventories. The model requires basic information such as the dates the landfill opened and closed, the amount of waste in place, annual waste acceptance rate, and whether the landfill is in an arid or non-arid climate. Default factors can be used for other model parameters, or site-specific methane generation rate constants and organic compound concentrations can be input if site-specific measurements are available. The results from this or other emission models may be used in air dispersion models to predict exposures concentrations in ambient air as described below.

How can models be used at landfill sites?

At landfill sites where no off-site monitoring data are available, emissions data (measured or calculated) from the landfill may be input into a mathematical model to estimate potential contaminant concentrations in surrounding neighborhoods. Models may also be used to estimate the landfill's contribution to measured air pollution, as was done by ATSDR during its study of the Fresh Kills Landfill in Staten Island, New York. This modeling is most applicable in urban areas where multiple sources may be present. Also, if an emission rate model such as EPA's LandGEM is used, it can predict the increase in emissions over time as more waste is added to an open landfill or the decrease in emissions after a landfill is closed. These values could be input into air models to predict increases and decreases in ambient air concentrations and exposures over a period of years.

What factors should be considered when reviewing models?

It is possible (though not likely) that air modeling was conducted for a landfill site and you may need to review and understand the model results. More likely, you may want to consider conducting air modeling for sites under your review, for example, to estimate exposure doses in surrounding communities. In either instance, there are several factors to consider about models:

  • Are adequate data available to input into a model? At a minimum, landfill gas emission data and on-site meteorologic data should be available for air modeling.

  • Does the model provide an answer to your questions about landfill gas, its migration, or exposures?

  • What are the uncertainties associated with the model? Results from screening models may have limited use. Results from models with supporting experimental or measured data are more reliable.

Where can I get more information about models?

A good source of general information is EPA's Support Center for Regulatory Air Models at http://www This Web site includes links to EPA's latest version of the Guideline on Air Quality Models; to user guides for different types of models (e.g., screening and refined, simple terrain and complex terrain, and mobile sources and stationary sources); and to meteorologic data sets for locations across the country.

As described above, EPA has developed an air model specifically for estimating the emission rate of gases from landfills. EPA's LandGEM is available at the EPA Unified Air Toxics Web site for the Standards or Performance for Municipal Solid Waste Landfills:

Additional Resources

  • State air pollution agencies or hazardous waste management agencies.
  • California Air Resources Board- Ambient Air Quality Monitoring (
  • EPA's Office of Solid Waste and Emergency Response ( 1999. Municipal Solid Waste Landfills, Volume 1: Summary of the Requirements for the New Source Performance Standards and Emission Guidelines for Municipal Solid Waste Landfills. Office of Air Quality Planning and Standards. Research Triangle Park, NC. EPA- 453R/96-004.
  • EPA. 1994. Seminar Publication: Design, Operation, and Closure of Municipal Solid Waste Landfills. Office of Research and Development. Washington, DC. EPA/625/R-94/ 008.
  • MDNR 1999. Missouri Department of Natural Resources, Solid Waste Management Program. Sanitary landfill gas monitoring, technical bulletin. September 1999.
  • SWANA (Solid Waste Association of North America) ( (Note: The previous link opens a new window, as the website does not permit you to return to this location.)
  • SWANA. 1997. Landfill Gas Operation and Maintenance Manual of Operation. Available by searching the DOE Information Bridge at the Web site

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