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Landfill Gas Primer - An Overview for Environmental Health Professionals

Chapter 3: Landfill Gas Safety and Health Issues

Historical Document

This document is provided by the Agency for Toxic Substances and Disease Registry (ATSDR) ONLY as an historical reference for the public health community.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.

This chapter provides information about health and safety issues associated with landfill gas—specifically, possible explosion and asphyxiation hazards and issues related to odors emanating from the landfill and low-level chemical emissions. It also contains information about health and safety issues associated with landfill fires (which may or may not be the direct result of landfill gas). This chapter also describes the tools that can be used to help environmental professionals respond to community health concerns. It provides information about what is known and unknown about the short-term and long-term health effects associated with landfill gas emissions, which can be mixtures of hundreds of different gases.

When reading this chapter, keep in mind that if people are not being exposed to landfill gases, no adverse health effects are expected. Exposures occur only if the landfill is producing harmful levels of gases and if the gases are migrating from the landfills and reaching people. Responding to community concerns about the possible health impacts of known or potential landfill gas emissions can often be difficult. Data (at the point of exposure) are needed to fully evaluate exposures, and these data are often limited or not available (see Chapter Four).

How are people exposed to landfill gas?

People may be exposed to landfill gases either at the landfill or in their communities. As discussed in Chapter Two, landfill gases may migrate from the landfill either above or below ground. Gases can move through the landfill surface to the ambient air. Once in the air, the landfill gases can be carried to the community with the wind. Odors from day-to-day landfill activities are indicative of gases moving above ground. Gases may also move through the soil underground and enter homes or utility corridors on or adjacent to the landfill. Figure 3-1 illustrates the movement of landfill gases and potential exposure pathways. The levels of gases that migrate from a landfill and to which people are exposed are dependent on many factors, as described in Chapter Two. Landfill gas collection and control systems have the greatest impact on gas migration and exposures. If a collection or control system is in place and operating properly, migration and exposures should be minimal.

Explosion Hazards

Landfill gas may form an explosive mixture when it combines with air in certain proportions. This section provides information about:

  • The conditions that must be met for landfill gas to pose an explosion hazard.

  • The types of gases that may potentially pose explosion hazards.

  • What can be done to assess whether a landfill is posing an explosion hazard.

Figure 3-1: Potential Exposure Pathways to Landfill Gas
Figure 3-1: Potential Exposure Pathways to Landfill Gas

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When does landfill gas pose an explosion hazard?

The following conditions must be met for landfill gas to pose an explosion hazard:

  • Gas production. A landfill must be producing gas, and this gas must contain chemicals that are present at explosive levels.
  • Gas migration. The gas must be able to migrate from the landfill. Underground pipes or natural subsurface geology may provide migration pathways for landfill gas (see Chapter Two, "What factors affect landfill gas migration?"). Gas collection and treatment systems, if operating properly, reduce the amount of gas that is able to escape from the landfill. (See Chapter Five.)
  • Gas collection in a confined space. The gas must collect in a confined space to a concentration at which it could potentially explode. A confined space might be a manhole, a subsurface space, a utility room in a home, or a basement. The concentration at which a gas has the potential to explode is defined in terms of its lower and upper explosive limits (LEL and UEL), as defined at right.

Lower and Upper Explosive Limits (LEL and UEL)

The concentration level at which gas has the potential to explode is called the explosive limit. The potential for a gas to explode is determined by its lower explosive limit (LEL) and upper explosive limit (UEL). The LEL and UEL are measures of the percent of a gas in the air by volume. At concentrations below its LEL and above its UEL, a gas is not explosive. However, an explosion hazard may exist if a gas is present in the air between the LEL and UEL and an ignition source is present.

Landfill Gas Explosions

Although landfill gas explosions are by no means common occurrences, a number of incidents known or suspected to have been caused by landfill gas explosions have been documented.

1999 An 8-year-old girl was burned on her arms and legs when playing in an Atlanta playground. The area was reportedly used as an illegal dumping ground many years ago. (Atlanta Journal-Constitution 1999)
1994 While playing soccer in a park built over an old landfill in Charlotte, North Carolina, a woman was seriously burned by a methane explosion. (Charlotte Observer 1994)
1987 Off-site gas migration is suspected to have caused a house to explode in Pittsburgh, Pennsylvania.(EPA 1991)
1984 Landfill gas migrated to and destroyed one house near a landfill in Akron, Ohio.Ten houses were temporarily evacuated. (EPA 1991)
1983 An explosion destroyed a residence across the street from a landfill in Cincinnati, Ohio. Minor injuries were reported. (EPA 1991)
1975 In Sheridan, Colorado, landfill gas accumulated in a storm drain pipe that ran through a landfill. An explosion occurred when several children playing in the pipe lit a candle, resulting in serious injury to all the children. (USACE 1984)
1969 Methane gas migrated from an adjacent landfill into the basement of an armory in Winston-Salem, North Carolina. A lit cigarette caused the gas to explode, killing three men and seriously injuring five others. (USACE 1984)

See the box above for a few of many documented situations where all the conditions for explosions were met and explosions actually occurred.

What types of gases can pose an explosion hazard?

  • Methane. Methane is the constituent of landfill gas that is likely to pose the greatest explosion hazard. Methane is explosive between its LEL of 5% by volume and its UEL of 15% by volume. Because methane concentrations within the landfill are typically 50% (much higher than its UEL), methane is unlikely to explode within the landfill boundaries. As methane migrates and is diluted, however, the methane gas mixture may be at explosive levels. Also, oxygen is a key component for creating an explosion, but the biological processes that produce methane require an anaerobic, or oxygen-depleted, environment. At the surface of the landfill, enough oxygen is present to support an explosion, but the methane gas usually diffuses into the ambient air to concentrations below the 5% LEL. In order to pose an explosion hazard, methane must migrate from the landfill and be present between its LEL and UEL.

  • Other landfill gases. Other landfill gas constituents (e.g., ammonia, hydrogen sulfide, and NMOCs) are flammable. However, because they are unlikely to be present at concentrations above their LELs, they rarely pose explosion hazards as individual gases. For example, benzene (an NMOC that may be found in landfill gas) is explosive between its LEL of 1.2% and its UEL of 7.8%. However, benzene concentrations in landfill gas are very unlikely to reach these levels. If benzene were detected in landfill gas at a concentration of 2 ppb (or 0.0000002% of the air by volume), then benzene would have to collect in a closed space at a concentration 6 million times greater than the concentration found in the landfill gas to cause an explosion hazard.

Table 3-1 summarizes the potential explosion hazards posed by the important constituents of landfill gas. Keep in mind that methane is the most likely landfill gas constituent to pose an explosion hazard. Other flammable landfill gas constituents are unlikely to be present at concentrations high enough to pose an explosion hazard. However, the flammable NMOCs do contribute to total explosive hazard when combined with methane in a confined space.

Table 3-1: Potential Explosion Hazards from Common Landfill Gas Components

Component Potential to Pose an Explosion Hazard
Methane Methane is highly explosive when mixed with air at a volume between its LEL of 5% and its UEL of 15%. At concentrations below 5% and above 15%, methane is not explosive. At some landfills, methane can be produced at sufficient quantities to collect in the landfill or nearby structures at explosive levels.
Carbon dioxide Carbon dioxide is not flammable or explosive.
Nitrogen dioxide Nitrogen dioxide is not flammable or explosive.
Oxygen Oxygen is not flammable, but is necessary to support explosions.
Ammonia Ammonia is flammable. Its LEL is 15% and its UEL is 28%. However, ammonia is unlikely to collect at a concentration high enough to pose an explosion hazard.
NMOCs Potential explosion hazards vary by chemical. For example, the LEL of benzene is 1.2% and its UEL is 7.8%. However, benzene and other NMOCs alone are unlikely to collect at concentrations high enough to pose explosion hazards.
Hydrogen sulfide Hydrogen sulfide is flammable. Its LEL is 4% and its UEL is 44%. However, in most landfills, hydrogen sulfide is unlikely to collect at a concentration high enough to pose an explosion hazard.

How can I assess whether a landfill in my community poses an explosion hazard?

The checklist on the following page can help determine if a landfill may pose an explosion hazard. If your evaluation identifies the potential for an explosion, several actions can be taken to prevent harm to the community. Measures and controls to prevent explosion hazards are discussed in Chapter Five. Possible public health actions are described in Appendix B.

Landfill Gas Explosion Hazard Checklist
Is the landfill producing gas and if so, how much?
  Because methane and carbon dioxide are the main components of landfill gas and these chemicals are both odorless and colorless, monitoring data are necessary to answer this question. (See Chapter Four for information about how landfill gas is monitored.)
Is a landfill gas collection system in place?
  Landfill gas collection systems reduce levels of gas migrating from the landfill to surrounding areas. (See Chapter Five for information about collection systems.)
Is gas migrating from the landfill?
  Off-site monitoring data may be necessary to answer this question. (See Chapter Four.)
If gas is migrating from the landfill and reaching structures, are there places for gas to collect?
  Uncontrolled gases escaping from a landfill may migrate to structures on the landfill itself or in the surrounding area. However, the further a structure is from the landfill, the less likely it is that gases are migrating to it at concentrations great enough to pose an explosion threat. The most common places for gases to collect are basements, crawl spaces, or buried utility entry ports. Homes with basements, especially those with pipes or cracks in the basement that would allow gas to enter, are more likely to collect gases.
Is gas collecting at concentrations that are high enough to pose an explosion hazard?
  Monitoring data are needed to answer this question. Caution should be used in selecting sampling equipment to ensure that an ignition source is not introduced into the area. (See Chapter Four for information about monitoring.)
Is there an ignition source?
  Gases can be ignited by many different sources, such as a furnace in the basement or a pilot light on a gas stove. Other sources may include candles, matches, cigarettes, or a spark. Because there are so many ignition sources, it is safest to assume that the potential for an ignition source is always present.

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Asphyxiation Hazards

Landfill gas poses an asphyxiation hazard only if it collects in an enclosed space (e.g., a basement or utility corridor) at concentrations high enough to displace existing air and create an oxygen-deficient environment. The Occupational Safety and Health Administration (OSHA) defines an oxygen-deficient environment as one that has less than 19.5% oxygen by volume (OSHA n.d.a). Ambient air contains approximately 21% oxygen by volume. Health effects associated with oxygen-deficient environments are described in Table 3-2.

Any of the gases that comprise landfill gas can, either individually or in combination, create an asphyxiation hazard if they are present at levels sufficient to create an oxygen-deficient environment.

Carbon dioxide, which comprises 40% to 60% of landfill gas, may pose specific asphyxiation hazard concerns. Because it is denser than air, carbon dioxide that has escaped from a landfill and collected in a confined space, such as a basement or an underground utility corridor, may remain in the area for hours or days after the area has been opened to the air (e.g., after a manhole cover has been removed or a basement door opened). Carbon dioxide is colorless and odor-less and therefore not readily detectable. Carbon dioxide concentrations of 10% or more can cause unconsciousness or death. Lower concentrations may cause headache, sweating, rapid breathing, increased heartbeat, shortness of breath, dizziness, mental depression, visual disturbances, or shaking. The seriousness of these symptoms depends on the concentration and duration of exposure. The response to carbon dioxide inhalation varies greatly even in healthy normal individuals.

In assessing the public health issues of migrating landfill gas, environmental health professionals should investigate the presence of buried utility lines and storm sewers on or adjacent to the landfill. These structures not only provide a pathway for migrating gases, but also pose a special asphyxiation problem for utility workers who fail to follow confined space entry procedures prescribed by OSHA. On-site or adjacent residences and commercial buildings with basements or insulated (or sealed) crawl spaces should also be investigated for potential asphyxiation hazards.

Table 3-2: Health Effects from Oxygen-deficient Environments
Oxygen Concentration Health Effects
21% Normal ambient air oxygen concentration
17% Deteriorated night vision (not noticeable until a normal oxygen concentration is restored), increased breathing volume, and accelerated heartbeat
14% to 16% Increased breathing volume, accelerated heartbeat, very poor muscular coordination, rapid fatigue, and intermittent respiration
6% to 10% Nausea, vomiting, inability to perform, and unconsciousness
Less than 6% Spasmatic breathing, convulsive movements, and death in minutes
Source: OSHA n.d.b

Health Issues Associated with Landfill Gas Emissions - Odors and Low-Level Chemical Exposures

Landfill odors often prompt complaints from community members. People may also have concerns about health effects associated with these odors and other emissions coming from the landfill. This section contains information about

  • Symptoms possibly triggered by landfill gas odors.

  • What scientists know about the potential health effects of exposures to landfill gas emissions.

  • How environmental health professionals can assess whether landfill gas emissions may be posing a health threat.

Can the presence of odors trigger symptoms?

People in communities near landfills are often concerned about odors emitted from landfills. They say that these odors are a source of undesirable health effects or symptoms, such as headaches and nausea. At low-level concentrations—typically associated with landfill gas— it is unclear whether it is the constituent itself or its odors that trigger a response. Typically, these effects fade when the odor can no longer be detected. The box below describes the biology behind detecting odors.

Landfill gas odors are produced by bacterial or chemical processes and can emanate from both active or closed landfills. These odors can migrate to the surrounding community. Potential sources of landfill odors include sulfides, ammonia, and certain NMOCs, if present at concentrations that are high enough. Landfill odors may also be produced by the disposal of certain types of wastes, such as manures and fermented grains.

How Do People Detect Odors?

Humans can distinguish between 3,000 and 10,000 different odors. Although people commonly believe that they smell with their noses, the nose is only one part of the olfactory system that allows humans to distinguish smells. The nose serves as a vacuum that pulls in air and odorous chemicals, such as hydrogen sulfide. The air and odorous chemicals are warmed in the nasal cavity and then trapped in mucus surrounding the olfactory membrane. The olfactory membrane is an area smaller than 1 square inch located deep in the nasal cavity between the eyes. Odorous chemicals interact with receptors (chemoreceptors) found on small hairs (cilia) on the olfactory membrane. The receptors send messages about the odorous chemicals to the brain through the olfactory bulb. The brain then interprets and identifies the odor (Pacific Science Center 1999; Jacobs 1999).

The sense of smell, just like the other senses of sight, hearing, taste, and touch, varies from person to person. One person may be able to smell an odor like hydrogen sulfide at extremely low concentrations, while another person in the same community or home cannot. Because of this variation, there is no true odor threshold value above which odors are unpleasant and below which odors are not noticeable. Any odor threshold values reported in the literature, such as those provided in Table 3-3, are only estimates of concentrations that the average person may detect (AIHA 1989). Therefore, health professionals should be cautious when citing established threshold values or discussing them with community members.

Anatomy of Smell
Anatomy of Smell

  • Sulfides. Hydrogen sulfide, dimethyl sulfide, and mercaptans are the three most common sulfides responsible for landfill odors. These gases produce a very strong rotten-egg smell—even at very low concentrations. Of these three sulfides, hydrogen sulfide is emitted from landfills at the highest rates and concentrations.

    Humans are extremely sensitive to hydrogen sulfide odors and can smell such odors at concentrations as low as 0.5 to 1 part per billion (ppb). At levels approaching 50 ppb, people can find the odor offensive. Average concentrations in ambient air range from 0.11 to 0.33 ppb (ATSDR 1999a). According to information collected by the Connecticut Department of Health, the concentration of hydrogen sulfide in ambient air around a landfill is usually close to 15 ppb (CTDPH 1997; ATSDR 1999a).

  • Ammonia. Ammonia is another odorous landfill gas that is produced by the decomposition of organic matter in the landfill. Ammonia is common in the environment and an important compound for maintaining plant and animal life. People are exposed daily to low levels of ammonia in the environment from the natural breakdown of manure and dead plants and animals. Because ammonia is commonly used as a household cleaner, most people are familiar with its distinct smell.

    Humans are much less sensitive to the odor of ammonia than they are to sulfide odors. The odor threshold for ammonia is between 28,000 and 50,000 ppb. Landfill gas has been reported to contain between 1,000,000 and 10,000,000 ppb of ammonia, or 0.1% to 1% ammonia by volume (Zero Waste America n.d.). Concentrations in ambient air at or near the landfill site are expected to be much lower.

  • NMOCs. Some NMOCs, such as vinyl chloride and hydrocarbons, may also cause odors. In general, however, NMOCs are emitted at very low (trace) concentrations and are unlikely to pose a severe odor problem.

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Table 3-3 lists some of the common landfill gas components and their odor thresholds.

Table 3-3: Common Landfill Gas Components and Their Odor Thresholds
Component Odor Description Odor Threshold (parts per billion)
Hydrogen Sulfide Strong rotten egg smell 0.5 to 1
Ammonia Pungent acidic or suffocating odor 1,000 to 5,000
Benzene Paint-thinner-like odor 840
Dichloroethylene Sweet, ether-like, slightly acrid odor 85
Dichloromethane Sweet,chloroform-like odor 205,000 to 307,000
Ethylbenzene Aromatic odor like benzene 90 to 600
Toluene Aromatic odor like benzene 10,000 to 15,000
Trichloroethylene Sweet, chloroform-like odor 21,400
Tetrachloroethylene Sweet, ether-or chloroform-like odor 50,000
Vinyl Chloride Faintly sweet odor 10,000 to 20,000

Many people may find the odors emitted from a landfill offensive or unpleasant. In reaction to the odor, some people may experience nausea or headaches. Although such responses are undesirable, medical attention is usually not required. Often, symptoms such as headaches and nausea fade when the odor goes away. However, the effects on day-to-day life can be more lasting. Families living close to a landfill in Connecticut described frequent odor events as overwhelmingly disruptive. One family reported being awakened during predawn hours by a flood of nauseating air that persisted for 2 or more hours. The loss of sleep and the frustration from the frequent odor events greatly added to the level of stress in the family's life. Although landfill odors may not associated with long-term adverse health effects or illness for most people, the added disruption and stress of day-to-day activities can greatly impact quality of life. The story below describes how environmental and health professionals addressed odor problems in Danbury, Connecticut.

The Danbury Landfill—One Community's Story

Danbury, Connecticut, is a community that successfully tackled a landfill odor problem. In the Spring of 1996, a 100-year-old landfill in Danbury caught fire. Water used to extinguish the fire promoted bacteria growth and increased the production of odor-causing sulfides, especially hydrogen sulfide.

The increase in odors prompted public concerns and questions. Though hydrogen sulfide levels in the air were well below concentrations that might affect human health, the odor caused nausea and headaches in some residents. Local and state health authorities and environmental agencies worked together to address the odor problem. They took the following actions to alleviate community concerns and address the odor problem:

  • Landfill operators controlled sulfide releases and odors by adding lime as a short-term solution and by installing a fabric landfill liner, a gas collection system, and a flaring system as the long-term solution.

  • Local health departments produced and distributed a newsletter to educate community members about landfill odors and what was being done to reduce them.

  • Hydrogen sulfide concentrations were measured with monitoring devices located in areas where exposure might occur, such as nearby residential homes and retail areas. Concerned parties developed a four-tiered response plan based on measured hydrogen sulfide concentrations in the nearby community:

    • At concentrations greater than 100 ppb for 15 minutes, landfill operators would take immediate action to reduce emissions.

    • At concentrations greater than 100 ppb for 2 hours, medical personnel would be notified that sensitive individuals (e.g., children, elderly, or asthmatics) might be affected.

    • At concentrations greater than 500 ppb for 2 hours, sensitive individuals would be advised to stay indoors or leave the area.

    • At concentrations greater than 5,000 ppb for 30 minutes, all residents would be advised to leave the area.

The first action level was triggered a few times during the period when odor control measures were being installed. The other action levels were never triggered. Once odors were controlled, community complaints decreased markedly.

The impact of landfill gas odors on sensitive populations such as people with pre-existing respiratory illnesses is not well documented or understood. A study conducted on Staten Island showed an increase in self-reported wheezing among asthmatics living near a landfill on days when they reported odors. Ambient air measurements, however, showed levels of hydrogen sulfide and other emissions much lower than levels known to be associated with adverse health effects (ATSDR 1999b). The box below provides more information about this study. The study suggests that odors in and of themselves may trigger respiratory effects among asthmatics. This preliminary conclusion may be confounded by other environmental triggers for respiratory response in asthmatics, such as dust mites, animal dander, tobacco smoke, and outdoor air pollution. The American Lung Association Web site ( provides more information about possible environmental triggers for asthma. EPA provides information about asthma itself at

Fresh Kills Landfill—A Case Study

ATSDR conducted a health study of communities near the Fresh Kills Municipal Landfill in Staten Island, New York. The study was undertaken to gain a better understanding of the possible health risks posed by the landfill to residents in nearby communities. ATSDR designed the study to assess how hydrogen sulfide concentrations, odors, and proximity of residence to the landfill might affect respiratory function. The focus was on asthma. A group of more than 150 community residents reported as having asthma volunteered to participate in the study. For a 6-week period in July through September 1997, when annual landfill emissions tend to be at their peak, study participants completed a daily diary to record perceived odors, respiratory symptoms, and daily activities. Participants also measured their lung function each morning and evening with a peak expiratory flow meter. During this same period, ATSDR conducted continuous monitoring in the study area to assess ambient air concentrations of hydrogen sulfide, ozone, and particulate matter. Meteorologic data as well as pollen and fungi counts were collected; these are confounding factors that can trigger asthma. ATSDR also conducted a separate odor impact survey to provide an independent odor assessment around the landfill.

ATSDR concluded that the measured levels of hydrogen sulfide and other parameters were not high enough to cause adverse health effects. However, when study participants reported that they smelled rotten eggs or garbage, they also reported that they were more likely to wheeze or experience difficulties in breathing. A moderate decline in lung function was also documented on days when participants reported these odors. These results varied throughout the study group by factors such as the participant's age and how long he or she had suffered from asthma (ATSDR 1999b).

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