Hazmat Survival Tips: Interpreting Readings from Atmospheric Monitors

Beyond the Rule of Thumb
Survival Tip 54

By Steven De Lisi

On a cold evening in January, your engine is dispatched to a report of a carbon monoxide (CO) alarm that has activated in a single-family structure. On arrival, you discover that all three residents have evacuated and are being dealt with by EMS personnel who had arrived earlier. One of the medics asks you to “check out” the interior and determine the level of carbon monoxide present. Using the four-gas atmospheric monitor assigned to your unit, you determine the highest level of CO found inside to be 76 parts per million (ppm). Shortly after you complete your assessment, the media informs you that all residents have refused treatment and that they are eager to go back inside. Based on the readings obtained with your meter, should you allow them to return?

During another incident, you are assigned to monitor near the perimeter of a gasoline spill while awaiting the arrival of a truck equipped with foam applicators and foam concentrate. You report your readings every two minutes to the incident commander. Following the incident, you attend a debrief session during which you discover that the person who made the entries into the atmospheric monitor log changed all of your readings, You question this individual as to why these changes were necessary; he informs you that because you were measuring the concentration of gasoline vapors, it was necessary to “correct” the readings. You seem perplexed and angrily tell him that you provided him with the correct readings and that there was no need for him to “correct” your work.  

General Use of Atmospheric Monitors

The most common type of atmospheric monitor first responders use employs one or more sensors with each sensor intended to detect a specific target gas or perhaps flammable vapors. Many of these devices are commonly referred to as “four-gas meters,” indicating that they have four sensors, typically one each for flammable vapors (combustible gas indicator), oxygen, carbon monoxide, and hydrogen sulfide. The sensors are interchangeable, and some departments instead use those capable of detecting gases such as ammonia or chlorine.

Another atmospheric monitor popular with many hazardous materials teams is a photoionization detector (PID), which uses an ultraviolet light source to detect and measure the presence of various substances in the atmosphere. The PID accomplishes this by exposing the atmosphere to the UV light, which then “ionizes” or removes a negatively charged electron from molecules in the air, which results in the formation of a positively charged “ion.”
Regardless of which device is used, all rely on changes in electric currents on internal circuits to determine the presence and concentration of various gases in the atmosphere. These changes are then displayed as numerical readings in various units of measurement such as percent of lower explosive limit (LEL), percent concentration in air, ppm, or parts per billion (ppb).  
Almost all atmospheric monitors will provide users with audible alarms and flashing lights which activate whenever readings exceed a preset alarm threshold so that you do not have to rely solely on a visual display of the reading. However, be aware that when using atmospheric monitors, you must set the alarm threshold to the desired level (usually the point at which a known standard is exceeded) or, if the alarm threshold is preset at the factory, you must be sure that the preset level is satisfactory.    
As stated in previous columns, it is imperative that those who use atmospheric monitors are well trained to be able to obtain these readings and that this training is documented. Ensuring that the units are properly maintained in accordance with the manufacturer’s guidelines is also important to ensure accurate readings. This maintenance includes periodic calibrations using a gas with known concentrations so any deviations in readings can be detected.
None of the atmospheric monitors discussed here will provide first responders with a text or voice message describing what actions they should take. Instead, effective use of atmospheric monitors is based not only on the ability to obtain accurate and reliable numerical readings but also the ability to interpret these readings as the basis for making sound decisions that impact the health and well-being of firefighters and the public. Whether this involves decisions to evacuate or reoccupy buildings, the level of personal protective equipment worn by first responders, or if a foam blanket applied to a flammable liquid spill is working as intended, first responders will be held accountable for the outcome. Therefore, the interpretation of the numerical readings provided by atmospheric monitors must be correct.  

Interpreting Numerical Readings

The basis for interpreting numerical readings from atmospheric monitors involves comparing the reading to a known standard usually published by a regulatory agency such as the federal Occupational Health and Safety Administration (OSHA) or research organizations such as the National Institute for Occupational Safety and Health (NIOSH). Simply put, the reading will either be equal to, below, or above the standard.

It is important, though, to select the appropriate standard. As an example, when dealing with CO, the NIOSH Guide to Chemical Hazards states that 35 ppm is the OSHA time-weighted average concentration for workers during any eight-hour work shift over a 40-hour week. However, when dealing with incidents involving CO exposure in the home, a different exposure level may be desirable, such as one for 30 ppm issued by the Consumer Product Safety Commission. A copy of a document entitled “Responding to Residential Carbon Monoxide Incidents–Guidelines for Fire and Other Emergency Response Personnel” is available at http://www.cpsc.gov/LIBRARY/FOIA/FOIA04/os/Resident.pdf. This document provides first responders with the rationale for using 30 ppm as opposed to 35 ppm for a “safe” exposure threshold and provides valuable recommendations for dealing with incidents involving known or suspected CO exposures in residential occupancies.
When interpreting the reading on a combustible gas indicator, the readings represent the percentage of LEL. The important thing to remember is that the LEL is stated as a percentage of concentration of a gas in the atmosphere while the meter reading is displayed as a percentage of that concentration.
Because methane has an LEL of 5 percent concentration in air, a reading of “10” indicates that the concentration is only 10 percent (or 1/10th) of the 5 percent concentration in air that would be needed to reach the LEL. Likewise, a reading of “5” indicates that the concentration is only 5 percent (or 1/20th) of the 5 percent concentration needed to reach the LEL.
Regardless of the LEL value, a reading of 10 percent or more of that value is cause for concern, as stated in OSHA Standard 29 CFR 1910.146, “Permit-Required Confined Spaces,” which considers 10 percent or more of any LEL to be a “hazardous atmosphere.”  

To help explain this better, refer to Figure 1, which includes a chart based on a gas with an LEL of 20 percent. The chart is read left to right. As with the readings discussed earlier for methane, for this particular gas, a reading of 5 percent LEL is 1/20th of the concentration in air that would be needed to reach the LEL (1/20th of 20 percent LEL is equal to 1 percent concentration). Likewise, a reading of 25 percent LEL is equal to 1/4th of the LEL concentration of 20 percent, or a 5 percent concentration of the gas in the atmosphere.  


Use of Correction Factors

Another concern with using a combustible gas indicator is that readings provided by the monitor are accurate only when attempting to measure the same gas as used during calibration procedures. Since the sensitivity of combustible gas sensors varies with exposure to different types of atmospheres, any attempt to measure the concentration of gases other than that used during calibration will result in a reading that is likely greater or less than the actual concentration. Remedying this situation willrequire the use of a correction factor or relative response curve specific to the gas or vapor measured to obtain more accurate results.  

To better understand this, consider an analogy to time zones, where a clock set to Eastern Standard Time in New York is accurate only in that time zone. To use that same clock in California, an adjustment would be necessary: You would have to subtract three hours from the time displayed.
Although some atmospheric monitors are capable of performing these adjustments internally based on the appropriate relative response or correction factor, it may be necessary with some instruments to compare the displayed reading to a chart or graph, which is normally provided by the manufacturer, and then manually calculate the actual reading.
For example, if a combustible gas indicator of an atmospheric monitor is calibrated with methane, readings when the instrument is used to detect methane will be “correct.” However, if this same instrument is used to detect the presence of propane gas, it is necessary to use a “correction factor.”  
For example, if the correction factor for propane is “1.5,” all readings obtained with the atmospheric monitor when measuring propane must then be multiplied by 1.5. Therefore, a reading of 1 percent LEL is actually 1.5 percent of the LEL, whereas a reading of 6 percent LEL is actually 9 percent of the LEL, determined by multiplying 6 percent x 1.5. Likewise, when using an atmospheric monitor calibrated to methane to measure the concentration of gasoline vapors, calculations using a correction factor will be necessary to adjust the readings accordingly. 

Zero Readings and Interference Gases

When you consider the potential for problems associated with “zero” readings and the fact that most atmospheres contain numerous contaminants and that some of these can “interfere” with the readings displayed, the process of interpreting readings from atmospheric monitors becomes more complex.  

When dealing with interference gases, instruction manuals included with atmospheric monitors will generally provide a list of known gases and vapors with which users should be concerned. If first responders believe that any of the interference gases could be present, they should then be suspect of any readings provided by the atmospheric monitor. There are also some atmospheres, especially those containing corrosive vapors that can damage sensors and that could possibly result in an erroneous reading that could prove deadly.  
When interpreting a reading of “zero” from any sensor, remember this reading does not necessarily mean that there is no hazardous gas or vapor present in the atmosphere. It may simply mean that the gas or vapor is present but at a concentration below the capability or below the detectable limits of the monitor.
For example, if the lowest reading capable with an atmospheric monitor is 1 ppm, a concentration of .4 ppm might then be displayed as 0 ppm. In this situation, there is a hazardous atmosphere present, yet the reading is below the detectable limits of the monitor. Therefore, interpreting 0 ppm as a “zero” reading, meaning there is no material present is incorrect, whereas stating that 0 ppm is “below detectable limits” provides for a more accurate and defensible interpretation. Recording a reading of 0 ppm or 0 percent LEL as “below detectable limits” states that that there may be some of the material present but your instrument is not sensitive enough to detect it.  
When using a PID, first responders must be aware of a specific aspect of chemicals referred to as the “ionization potential (IP).” As stated earlier, PIDs operate by removing an electron from the molecule of a chemical. IP is a numerical value measured in electron volts (eV) that makes reference to the strength of attraction for these electrons; the higher the IP, the greater the strength by which electrons are held and the greater the power from the ultraviolet light that will be required to remove the electron to “ionize” the molecule.
The fact is that the most popular ultraviolet lamp found with PIDs has a power of 10.7 eV. As a result, a PID using this lamp will not be able to ionize chemicals with an IP greater than 10.7 eV so that a zero reading may not mean that a particular chemical is not present in the atmosphere, but rather that the lamp from the PID simply cannot ionize it. Values for various IPs can be found in documents such as material safety data sheets and the NIOSH Guide to Chemical Hazards.
As was stated earlier, a “zero” reading 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 ability of the monitor to provide accurate readings. This can prove especially dangerous when first responders are called to investigate a report of a suspicious odor in a building. With all the occupants evacuated, first responders will take their “four-gas” atmospheric monitor and PID and scan the building. If they receive “zero” readings, they too often declare the building safe and allow persons to reenter. However, the reality is that the building is likely only “safe” from the gases for which the four-gas atmospheric monitor has sensors installed and when using a PID for those materials with an IP that is less than the strength of the ultraviolet lamp in the PID.     
Finally, when obtaining a “zero” reading, first responders should always resist the urge to state, “There was no reading.” Remember that from a legal perspective, the statement “There was no reading” could be interpreted as the monitor was not operating properly, if at all.
Thirty years ago, atmospheric monitors were the sole domain of hazardous materials teams, and even then only a select few were allowed to use them. Since then, vast technological improvements have simplified the operation of atmospheric monitors and reduced their cost to the point that today they are found on many types of fire apparatus. However, one thing that has not changed is the need to interpret the numerical readings provided by these instruments and to do so in a manner that provides first responders with the basis for making informed and defensible decisions. Remember that as long as the atmospheric monitor performs as intended by the manufacturer, you assume all liability for your decisions. In this job, you never want to be dead wrong.  
Questions or comments on this or any other monthly Hazardous Materials Survival Tip may be directed to Steven De Lisi at HazMatSurvivalTip@comcast.net.
Steven M. De Lisi recently retired from the fire service following a 27-year career that included serving as the deputy chief for the Virginia Air Guard Fire Rescue and a division chief for the Virginia Department of Fire Programs (VDFP). De Lisi is a hazardous materials specialist and as an adjunct instructor for VDFP. He continues to conduct hazardous materials awareness and operations-level training for fire suppression and EMS personnel. He began his career in hazardous materials response in 1982 as a member of the hazmat team with the Newport News (VA) Fire Department. Since then he has also served as a hazardous materials officer for the Virginia Department of Emergency Management. In that capacity, he provided on-scene assistance to first responders dealing with hazardous materials incidents in a region that included more than 20 local jurisdictions. De Lisi holds a master’s degree in public safety leadership and is the author of the textbook Hazardous Material Incidents: Surviving the Initial Response (Fire Engineering, 2006)

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