Many fire departments have recently equipped their emergency response units with various types of portable atmospheric monitors to detect and measure the presence of gases and vapors that could be hazardous to their personnel and the public. Included in this new arsenal are devices that can measure one type of hazard, such as the concentration of an explosive or toxic gas, and multigas atmospheric monitors that can simultaneously measure these hazards along with oxygen content.

If you recently had an opportunity to use one of these devices, consider the following questions:

  • Was the device correctly calibrated?
  • If the device was designed to measure combustible gases, what type of gas was used to calibrate it?
  • What was the vapor density of the gas or vapor you were attempting to measure? Was this substance heavier or lighter than air?
  • What was the device’s response time—i.e., how long did it take to see results?
  • Did you need to use a relative response curve or correction factor to determine accurate readings? If so, where did you find this information?
  • Based on the results that the device provided, how did you determine which actions to take? What decisions did you make, and why?

Although portable atmospheric monitors provide valuable information, it is vital to use each instrument according to the manufacturer’s instructions and fully recognize each device’s capabilities and limitations to ensure the safety of personnel and the public. Furthermore, atmospheric monitors generally do not suggest actions users should take when confronted with a dangerous environment (e.g., whether to evacuate and, if so, how far; the type of personal protective equipment required; or whether a building is safe for reentry following an incident). Instead, the information available is often nothing more than a simple numerical value. Firefighters must compare this value with a known standard, which is the basis for effective decision making. Failure to interpret atmospheric monitor results properly could prove disastrous.

Before using one of these devices, consider basic operation; calibration and repair; general use; and interpretation of readings, including decision making, multiple on-scene hazards, inaccurate readings, and “zero” readings.


Gases and vapors. Most types of atmospheric monitors fire departments use generally operate according to the electric currents produced or altered through the device’s exposure to various gases and vapors. The monitor then interprets and displays this change in the electric current as a numerical value indicating the concentration of gases and vapors present. Sensors are the individual monitor components directly exposed to gases and vapors; a small electric pump draws the atmosphere into each sensor. Remember, however, that atmospheric monitors may not detect the presence of dusts or mists.

Toxic gases and vapors. An electrochemical sensor determines the concentration of toxic gases and vapors present. Each type of sensor is designed to detect a different type of gas or vapor; most contain a special chemical substance that, when combined with a specific gas or vapor in the atmosphere, produce an electrical current.

Combustible gases and vapors. A sensor containing an electric circuit with a heated filament is often used to determine the concentration of combustible gases and vapors. When a combustible gas or vapor is exposed to the heated filament, the gas or vapor burns and produces heat in the combustion process, which changes the resis-tance in the circuit. The amount of change in the resistance determines the concentration of gas or vapor.

Batteries. Portable atmospheric monitors operate using either nonrechargeable or rechargeable batteries. Consider purchasing both when available, since rechargeable battery problems can often be resolved on-scene quickly by inserting a nonrechargeable battery pack. Remember always to install batteries in an area known to be safe from airborne hazards and to check the battery charge level before making entry with the monitor. Also, if you are able to use nonrechargeable batteries, keep fresh supplies on hand so that you don’t have to make an emergency visit to the local convenience store at the height of an incident.


Calibration. Atmospheric monitors are calibrated by exposing a sensor to a known concentration of a certain type of gas. This ensures that the concentration reading displayed on the monitor is similar to the actual concentration of the calibration gas. For example, if a monitor with a hydrogen sulfide sensor is exposed to a calibration gas known to contain 50 parts per million (ppm) of this chemical, the numerical reading displayed by the monitor should be the same or within a certain percent of deviation as allowed by the manufacturer.

A number of factors determine the calibration frequency, including the manufacturer’s instructions, specific fire department policies and procedures, and regulations that may apply to specific tasks performed while using the device. Regardless, perform calibrations on a regular schedule and whenever the device is repaired. Properly trained personnel should conduct calibrations, and calibration test results should be thoroughly documented.

The concentration of a particular calibration gas depends on the manufacturer, so if you switch suppliers for this gas, be sure to check the concentration and adjust the monitor accordingly. In addition, most calibration gas is sold with an expiration date; using the gas for calibration after that date may not only produce inaccurate results but may also subject your organization to legal problems should the device’s performance be questioned. If the gas cylinder does not display an expiration date, it likely will display a production date; the manufacturer should be able to advise you on how to calculate the expiration date.

Repair. Only properly trained and equipped personnel should repair atmospheric monitors. Although some repairs require returning the monitor to a factory-authorized site, in-house personnel should be able to perform routine maintenance—e.g, sensor replacement. Therefore, when purchasing atmospheric monitors, consider the degree of difficulty involved in replacing sensors; the average life expectancy of sensors; and their cost, delivery time, and shelf life.


Personnel precautions. When using atmospheric monitors, personnel must first protect themselves from potential threats posed by hazardous atmospheres. Remember that standard firefighter protective clothing may be inadequate for protection against some chemicals and that there may be additional hazards at the scene beyond chemical exposure. These hazards include the potential for structural collapse; personnel entrapment while monitoring the air near an overturned vehicle, should the vehicle’s position shift; and tripping hazards that may not be obvious during night operations. Moreover, although self-contained breathing apparatus (SCBA) may offer considerable protection against airborne hazards, remember that air supplies are limited—don’t get so far inside a building that you don’t have time to get out. Explosive atmospheres may require additional precautions, including hoselines to protect personnel, utility control, and proper ventilation techniques.

Vapor density. When attempting to measure the presence of a gas or vapor, you must know the vapor density or weight of the material. Simply holding an atmospheric monitor at waist level may not provide an accurate reading, since gases and vapors that are heavier than air tend to collect in low-lying areas, whereas gases that are lighter than air will rise into the atmosphere. Heavier-than-air gases may also collect in physical depressions, such as storm drains and crawl spaces, whereas a lighter-than-air gas may enter concealed spaces such as drop ceilings and attics.

Vapor densities are usually expressed as a relative numerical value when compared to air, with air assigned a value of 1. A gas or vapor with a density of less than 1 will rise, whereas one with a vapor density of greater than 1 will sink.

The vapor densities for two of the most common fuel gases used today, natural gas and propane, are often confused. Firefighters responding to incidents involving a possible release of these products should remember that natural gas is lighter than air, with a vapor density of about 0.551; but propane is heavier, with a vapor density of approximately 1.56.2 Remember, too, that gasoline vapors, produced almost any time gasoline is handled, are heavier than air, with a vapor density of between approximately 3 and 4.3

Of course, the vapor density of almost any gas or vapor you are likely to encounter can be determined from that product’s material safety data sheet (MSDS). Remember, however, that certain processes, such as cryogenics, in which a material is “super-cooled,” can affect the vapor density of the gas or vapor.

When using an atmospheric monitor, consult the manufacturer’s instructions to determine the device’s response time—the amount of time necessary to obtain results, usually measured in seconds. Suspending a monitor momentarily over a suspected spill or carrying it while walking through an area may not allow the device enough time to provide accurate results.

Sensor damage can result from exposure to various types of atmospheres, including those containing corrosive vapors. Air monitor manufacturers will generally describe which types of environments to avoid as well as any filters that can reduce the potential damage to sensors.


Decision making. As stated earlier, the information available from atmospheric monitors is often nothing more than a simple numerical value, and you are responsible for using this value as the basis of effective decision making by comparing it with a known standard. However, it is important to know which standard to refer to and how to apply the atmospheric monitoring results to that standard. Furthermore, regardless of the standard used, it is important to set the atmospheric monitor’s alarm thresholds (the point at which the monitor will alert the user to a dangerous condition through visual or audible means, or a combination of both) for these values.

For example, when dealing with combustible gases and vapors, concentrations that exceed 10 percent of the lower explosive limit (LEL) are defined as a hazardous atmosphere by OSHA 29 CFR 1910.146 for Permit-Required Confined Spaces. Thus, at this concentration, the atmospheric monitor should alert the user to the potential dangers present.

This same standard also considers a hazardous atmosphere as one in which the “concentration of any substance … which could result in employee exposure is in excess of its dose or permissible exposure limit.” These concentrations are available from various sources, such as MSDSs or technical documents such as the Pocket Guide to Chemical Hazards, published by the National Institute for Occupational Safety and Health (NIOSH). However, remember that exposure limits intended for healthy adult employees in a factory or other workplace setting may not be appropriate for the average citizen, since these values do not take into account young children or older adults, those with compromised respiratory systems, or individuals likely to experience allergic reactions from exposure.

Multiple hazards on-scene. When interpreting results from a multigas atmospheric monitor, remember that the capability of the monitor is limited to the types of sensors installed. These results are usually for explosive atmospheres; oxygen content; and one or more types of toxic gases, depending on which sensors were selected at the time of purchase. However, other components of a toxic atmosphere may be present, and based on circumstances—e.g., a structure fire or chemical spill—these components may go unnoticed because your monitor may not have the appropriate sensor, thereby exposing personnel to the potential for considerable harm.

Another situation involves products that present both fire and inhalation hazards. For example, consider an MSDS for regular unleaded gasoline that lists OSHA’s permissible short-term exposure limit (STEL or 15-minute exposure) for gasoline vapors as 500 ppm and the lower explosive limit (LEL) as 1.4 percent.(3) Next, consider an incident involving a spill of this product and a surrounding atmosphere that, according to the combustible gas sensor of your atmospheric monitor, contains 4 percent of the LEL for gasoline. With a reading of 4 percent of the LEL, which is below the normal alarm threshold for 10 percent of the LEL, you may believe that the atmosphere is relatively safe, but it is safe only from the potential for fire or explosion. Remember that with a reading of 4 percent of the LEL, the concentration of gasoline vapors in the atmosphere is approximately 560 ppm, which is in excess of the permissible STEL exposure limit. Therefore, while the monitor may not provide audible or visual warnings for the presence of an explosive concentration of gasoline vapors, personnel working in that atmosphere still face a potential inhalation hazard.

Finally, remember that if the oxygen content in a room is below normal concentrations, something else must be present in the atmosphere (unless you’re working in a potential vacuum), and that something could be dangerous!

Potential for inaccurate readings. When using an atmospheric monitor to determine the concentration of combustible gases and vapors, a common mistake is failing to recognize that the monitor readings are accurate only when attempting to measure the same gas as that used during calibration. Since the sensitivity of combustible gas sensors varies in different types of atmospheres, any attempt to measure the concentration of gases or vapors other than that used during calibration will give a reading that is likely to be higher or lower than the actual concentration. This situation requires using a correction factor or relative response curve specific to the gas or vapor measured to obtain correct results.

To better understand this issue, consider an analogy to time zones. A clock set to Eastern Standard Time is accurate only in that time zone. To use that same clock in California, it is necessary to adjust that time by subtracting three hours from the time displayed.

Although some atmospheric monitors can perform these adjustments internally based on the appropriate relative response or correction factor, with some monitors, the user may need to compare the displayed reading to a chart or graph (normally provided by the manufacturer) and then calculate the actual reading.

Also remember that although an oxygen content that is below 19.5 percent is defined by OSHA 29 CFR 1910.146 as a hazardous atmosphere that presents the potential for asphyxiation, another concern is that low oxygen levels can interfere with the performance of combustible gas sensors. In addition, oxygen concentrations of more than 23.5 percent (also defined as a hazardous atmosphere in the same standard) present a greater potential for fire or explosion and the possibility once again for inaccurate readings when using a combustible gas sensor.

When using monitors to detect the presence of toxic atmospheres, there is always the potential for cross-sensitivity, a situation in which some gases and vapors other than those you are attempting to measure interfere with electrochemical sensor operation. This can result in false positive readings or an otherwise inaccurate response. Most manufacturers can provide some indication of potential problems from cross-sensitivity and of the types of atmospheres to avoid.

“Zero” readings. When interpreting a reading of “zero” from any sensor, remember that this does not necessarily mean there is no hazardous gas or vapor present in the atmosphere. Instead, it may simply mean that the gas or vapor is present but at a concentration below the capability, or detectable limits, of the monitor. As stated earlier, it may also mean that other gases are present, some of which are not detectable with the atmospheric monitor in use, or that there are substances present that can interfere with the monitor’s accuracy.

Finally, when faced with a zero reading, resist the urge to say, “There was no reading.” Although most personnel on scene would understand a zero reading and no reading to mean the same thing, from a legal perspective, the statement, “There was no reading” could be interpreted as the monitor was not operating properly, if at all.

Decision making is always a major firefighter responsibility during an emergency operation, and here atmospheric monitors can play an important role. But always remember that although atmospheric monitors can assist you in making the right decision, your failure to understand the capabilities and limitations of each device can quickly prove you wrong—dead wrong!


1. Material safety data sheet for natural gas, Praxair Technology, Inc., May 1999.

2. Material safety data sheet for propane, BOC Gases, April 5, 2001.

3. Material safety data sheet for regular unleaded gasoline, Chevron, December 31, 2002.

STEVEN M. DE LISI is an assistant chief with the Virginia Air National Guard Fire and Rescue in Henrico County and a 22-year veteran of the fire service. He has served as a company officer with the Newport News (VA) Fire Department and as a regional training manager for the Virginia Department of Fire Programs. He is a certified hazardous materials specialist and previously served with the Virginia Department of Emergency Management in the Technological Hazards Division. De Lisi has an associate’s degree in police science and a bachelor’s degree in governmental administration and is pursuing a master’s degree in public safety leadership.

Atmospheric Monitor Checklist

When purchasing an atmospheric monitor, consider the following:

  • What types of hazardous atmospheres are you most likely to encounter?
  • What type of power source is used, and is there an alternative power source? Can you substitute standard batteries for rechargeable batteries? Should you purchase a spare rechargeable battery pack?
  • What are the manufacturer’s recommendations for recharging batteries?

When calibrating and repairing an atmospheric monitor, consider the following:

  • Sensors: How do you replace them? What is the average life expectancy, cost, delivery time, and shelf life?
  • Calibration gas: What type and in what concentration? Is the calibration gas consistent with the type of hazardous atmospheres you are likely to encounter? What are the cost and shelf life?
  • Calibration and repairs: Who is qualified to perform these operations, and what type of records should be maintained?

When using an atmospheric monitor, consider the following:

  • How should personnel protect themselves from the atmosphere to be monitored?
  • What is the vapor density of the gas or vapor to be measured? Is it heavier or lighter than air?
  • What is the response time of the atmospheric monitor? How long does it take to display results?
  • Will the atmosphere in which the monitor will be used damage the sensors?

When interpreting atmospheric monitor results, consider the following:

  • What are the alarm thresholds? What standards are they based on?
  • Are there other hazards at the scene that may not be detectable with the atmospheric monitor in use?
  • When using the atmospheric monitor to detect the presence of a combustible atmosphere, do you have access to a relative response curve or correction factor?
  • Could the oxygen level in the monitored area affect the readings?
  • When using the atmospheric monitor to detect the presence of toxic atmospheres, is there the possibility of cross-sensitivity?
  • What is the lowest reading that can be obtained with the atmospheric monitor? Could a reading of “zero” actually be below the detectable limits of the device?

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