You pull your engine past the front of the building, where fire is showing out three windows. Truck company members are forcing the front door for an aggressive search. Getting water on the fire is a priority. You run through the procedure in your mind. Lives are on the line; firefighters can die if you fail. You give them tank water, tie into the hydrant, flush and open the hydrant, open the intake, shut down the tank water, and spin up the throttle. You run the procedure flawlessly. The nozzleman goes through his procedures: bleed the line, mask up, listen to the radio, make the push. Your ability to know, train to, and run your procedures makes or breaks the operation. However, at an explosive gas emergency, an all-too-common call, your standard operating procedures (SOPs) and training may not be so clear and decisive.

This article examines important size-up procedures, strategic planning, and tactical steps to add to your explosive gas response SOP. These concepts and tactics will help your first-due units and leaders to develop written and on-scene procedures to conduct an effective size-up and develop on-scene, strategic, and tactical objectives for responses to releases of explosive gases. Although there are many flammable and explosive gases, the focus here is on the two most common—natural gas and propane.

The Training Gap

There is a huge gap in training for explosive gas emergencies, for three reasons. First, procedures/strategies/tactics to prepare you for responses to explosive gases are not taught in recruit firefighter programs and most officer development programs. If they are included, they are an ancillary topic. Second, the training necessary to provide a thorough understanding of air-monitoring tools is not in training curricula. Third, written explosive gas response SOPs are generally weak or nonexistent.


Tactical Size-Up for Explosive Gas Emergencies

Natural Gas Emergency Strategy and Tactics

Training for Natural Gas Emergencies

Tactical Size-Up of Natural Gas Emergencies

Often, you only get awareness training from local utilities. Awareness-level training is not adequate, considering we operate at the operations, technician, and command levels. The reality is, firefighters often only get a 30-minute slide presentation by a utility company rep (to meet the legal requirement), which leads to false confidence that it is all we need to know. It is just enough information to be dangerous, especially when an explosive gas is released and we are responsible for the life safety of civilians and our members. We need detailed procedures, just like the engine company procedures described above.

(1) Responses to explosive gas emergencies can turn deadly in a microsecond. (Photos by authors.)

Fire departments commit an all-too-common error as a result of this training gap. Many times, with few or no SOPs, emergency responders will react and take actions not necessarily out of need but more out of their need to do something. For example, a fire department responded to a struck gas line (service was hit with a backhoe). The ½-inch plastic line was venting into the atmosphere. Gas was not getting into the building, and there was no life hazard. The right thing to do was to resolve any life safety issues, monitor using a combustible gas detector, call the utility, provide as much specific information as possible, and control the potential ignition sources as much as possible.

Responders, feeling the need to act, filled the large excavation with water, thus defeating the natural property of natural gas (rising). This made it much more difficult to shut the flow of natural gas off because the excavation was flooded. Gas technicians had zero visibility of the broken service line. It delayed the repair and restoration of the customer’s natural gas. The excavation was full of muddy water that had to be hand pumped (it is unsafe to use a vacuum truck or internal combustion portable pump to pump out a water/natural gas mixture).

The Mission

Our mission is to determine if this release of explosive gas is a threat to life and, like in a military operation, neutralize the threat swiftly and decisively by following our SOPs for that specific type of gas emergency. Just like in a fire situation, we have tactical options based on the scenario and must use our judgment on which to apply. We can remove the threat by extinguishing the fire (in this case, shutting off the gas) and concurrently or sequentially removing the occupants (evacuation/rescue) from the path of the fire (near the escaping/accumulating gas). It is the utility worker’s or the plumber’s job to find and fix the leak.

Part of our life safety mission is to treat all gas leaks as a threat to life until proven otherwise. This is not to say your first tactical objective after the air brake is on is to immediately evacuate a 300-foot perimeter for a pilot light out on a stove. It is to emphasize the thought process in your head to first think big (major gas emergency), then downgrade as quickly as you can based on your size-up. This can be as fast and as simple as asking the caller about the source of the leak—i.e., a pilot light out (simple) or an underground (UG) gas leak that is migrating into nearby buildings (complex).

Make decisions in conjunction with the utility company if on scene. However, remember that the fire department is in charge (responsible); plan on being in control of the emergency until you and the utility resolve it. Responding with millions of dollars of equipment/staffing and taking no action is not what the public deserves. Follow procedures to neutralize the threat. Most importantly, always consider what your meter is telling you.

Figure 1. Gas Distribution System

Illustration courtesy of the Pipeline and Hazardous Materials Safety Administration.

It is critical that you understand your specific air monitor and its strengths, weaknesses, and idiosyncrasies. Know that a meter calibrated to natural gas (methane) will have lower than actual readings if you are monitoring for propane. This means that in the field you will have falsely low readings for propane releases.

Each meter has a numerical correction factor to convert readings from what your calibration gas is to what you are measuring. Some meters have this function built into the software. Read the instructions that came with your meter to find out what these correction factors are, since they vary with each type of gas your meter will detect.

The Size-Up

Size-up of explosive gas emergencies is the key to success (neutralize the life safety threat) and a skill we have not been trained in. It is not a secret where the gas is coming from; it is coming from a transmission line, a distribution main (pipe), a service line, a meter, internal gas piping, or a propane tank or service line. Your mission to protect life is based on your size-up. The goal of your size-up is to find out where the gas may be going.

The first critical size-up step is to answer this question: Is this a minor or major gas emergency? Is it endangering lives or significant property (major) or not (minor)? We get called to gas leaks that are very minor because normal, healthy humans can detect the odorant at less than 1 part per million (ppm). The lower flammable limit is 5% gas or 50,000 ppm.

For this discussion, we will limit ourselves to the natural gas distribution system, which includes piping from the gate station through the gas mains into service lines of a building and the interior gas piping. It is important to realize gas transmission lines that transport natural gas (NG) and other products can present massive releases of explosive gases from pipelines that can range up to 42 inches in diameter with working pressures approaching 1,000 pounds per square inch (psi). If you are responding to a transmission line emergency, it is a major emergency, and you will likely have to handle it as such.

There are six types of natural gas emergencies: a locked building containing gas, a main or service line struck, an inside odor of gas, an outside odor of gas, a building explosion, and an ignited leak. There is a lot of overlap in procedures, such as action levels, life safety priorities, air-monitoring techniques, and instruments used. Also, consider which of your tools, lights, hand lights, fans, and radios are intrinsically safe and won’t be the ignition source. A lot of important size-up details are situation dependent, such as where to monitor, location, and life safety priorities.

(2) Do not fill excavations that damaged gas lines with water.

(3) A simple tabletop with state-of-the-art training aides. The tablet sends notional air-monitoring readings to the firefighters with the four-gas monitor. The yellow chem tape represents the gas odor.

Using the UG NG leak scenario, let’s look at some of the key strategic concepts and tactical objectives that are important for this leak situation. The UG leak scenario is the most complex and the most dangerous, according to utility responders.

Unlike a fire that shows by smoke and flames, you must use aggressive procedures to detect fugitive gas and determine where the gas is going and if it is creating a life safety or large-scale property conservation issue. If you do not have good SOPs, it can be like chasing the invisible man at night, usually without success. Underground migrating gas will take the path of least resistance. It may also leak from gas pipes into the soil, then back into utility chases/pipes or other roundabout routes into nearby buildings.

Use the scenario below as a template for the four key points of your department SOP: policy, procedures, strategies, and tactical objectives. Like all procedures, temper them with your specific conditions, utility procedures, utility response time, fire department equipment, staffing, and air-monitoring capabilities.


You have been dispatched to #3 Leak Street for an outside odor of natural gas. It is a typical suburban street. Dispatch tells you the caller was walking her dog and smelled gas outside in front of her house at #3 Leak Street. For training purposes, the odor site is represented by the yellow chem tape on the board in photo 3. This tabletop training method keeps it simple so trainees can focus on using their new policies, procedures, strategy, and tactics in the SOP they are being trained to execute.

The first part of any size-up is to establish your command post and assign your first-due units. To treat this as a threat to life until proven otherwise, park your vehicles outside the kill box (the area in which you and your members are likely to be killed if the UG gas that has leaked into the nearby building finds an ignition source and explodes). This may be unlikely, but be prepared for the worst and hope for the best.

We have all been to hundreds of gas odor calls like this. How did you handle it? The policy section of your SOP should tell you to treat this as a major emergency until your size-up proves otherwise: Assume this UG leak is migrating into nearby structures, creating a life hazard. This is the most dangerous scenario; best practice is to rule this out first. It could be just a small leak from a nearby residential gas meter outside a home. Your size-up steps should lead you to confirm or deny that your assumption is correct.

You ask the homeowner, “Do you smell gas in your house?” If she says yes, it is likely that gas is migrating into her house, and you lean toward a major emergency and immediate response procedures to protect life. It also provides immediate key information to act on.

Although this verbal “air monitoring” may be an important early clue, you must follow up with a combustible air monitor. I (Focht) was recently at a job that had 8% NG by volume in a house. The fire department was there six hours before and asked if the occupant smelled gas; the reply was “No.” The department never entered the premises, and the building was full of natural gas. The person who said he did not smell the gas had had a stroke and had no sense of smell as a result.

(4) The penetration in the foundation can allow gas to enter from outside UG.

(5) Follow up air monitoring just inside the front door with monitoring in the basement at the point of entry of utility lines into the building.

(6) Subsurface structures can provide a clue about gas migrating underground and a ventilation capability to allow gas to escape.

(7) Yellow mark-outs indicate UG gas pipes.

Faced with this situation, your procedures should lead you toward immediate action because your size-up indicates you may have a life safety threat in that home. The first tactical objective is to send a firefighter or an officer to monitor the air in the home for the presence/ absence and level of gas.

If the occupant does not smell gas, you still must confirm that gas entered the building because the odorant may be scrubbed out by passing through the soil. Best practice here is to start at the building closest to the outside odor site, assuming gas could migrate underground first into the nearest building to the outside odor.

Meter procedures. Just inside the front door of #3, your firefighter gets a reading of zero lower explosive limit (LEL) on his four-gas meter. Your SOPs should direct him to also air monitor the basement at the point of entry of the gas and other utilities. It is very likely that gas is migrating into the house through the pipe penetrations or cracks in the foundation walls/floors.

If this reading is zero on the LEL scale, there is no immediate life safety issue. However, the occupant said she smelled gas inside as well. What do your SOPs tell you to do now? You should use a metal oxide-based detector in combination with your four-gas meter to detect low levels of explosive gases. (See “Tactical Size-Up for Explosive Gas Emergencies,” Fire Engineering, April 2021.)

Recall that your typical four-gas meter will not detect lower than 500 ppm at best. Practically, this number can be as high as 3,000 ppm in real-life situations. Your metal oxide-based sensor will detect down to about 10 ppm and much faster and lower than your four-gas. Recall also how important it is to monitor this building. The odorant maybe scrubbed out as it passes through the soil.

The interview with the caller provided a positive clue (“she smelled it inside”) that gas was really entering the building. Of course, the SOPs above will help rule out the odor was drifting in through open windows as well. Your size-up is still trying to determine minor or major emergency. More important, your SOPs still keep protecting life as your first priority. Your SOPs should also remind your members not to turn on lights, ring doorbells, turn on appliances, and so on while searching for explosive gases.

Your firefighter reports that his meter is in alarm at 15% LEL in the basement of #3. He has directed other family members to evacuate the building and walk down the street outside the kill box. Your SOP has set that action level of 10% LEL for mandatory evacuation. Although there is no national standard, the best practices for gas companies are 20% LEL but may vary from utility to utility. This action level makes good sense. Your meter will alarm (low) at 10%, and 20% LEL is often a number your utility will agree with and provides a large margin of safety for civilians and firefighters.

Consider that you get this low level where you are at that moment in time in that section of the building. You do not know if gas has accumulated in dangerous levels in other areas of the building. Your SOP must set an action level, but the number is your call.

At this point, your size-up has provided significant actionable information. Gas is migrating into nearby buildings, creating a hazard. Your assumption appears to be confirmed that there may be an UG leak near #3 and it is migrating into #3 and may be migrating into other buildings nearby.

Your estimate of the kill box now is your best estimate. The current kill box is the area around #3 that people will likely be killed if there is an accumulation of gas and it explodes. Is an explosion likely at this point? Probably not, since the LEL is only 15%. If your loved ones were in that house or nearby, what would you do?

The Emergency Response Guidebook recommends a 300-foot evacuation in all directions for escaping explosive gases. In many urban and suburban communities, this is unachievable. While examination of limits of debris fields from actual building explosions indicates 150 feet is the norm, strive to use 300 feet to ensure a safer perimeter. The policy section of your explosive gas SOP should set the kill box dimension for your department. Again, it’s your call.

You parked your vehicles two houses away from the reported leak site. The caller reported she smelled gas near the curb line directly in front of #3. Unless the wind is blowing it from an outside source, this should increase your confidence in your planning assumption of an UG leak near #3. It could, however, be far from #3 and following a path of least resistance and showing there. The source is not your concern; the life hazard it creates is.

You have called for the utility company, but it is 45 minutes out. This scene is yours to continue to assess and reduce the life hazard if any. What else can you do but wait for the utility? You can do a lot. You just rolled up with a couple million dollars of fire apparatus and a crew that costs taxpayers a lot. Your size-up is not finished, and neither is your life safety mission.

Tactical objectives. So far, the information from your size-up is pointing toward an UG leak. For such leaks, your SOP tells you to continue to monitor nearby buildings for accumulating gas. In this case, houses 1 and 5 are closest to #3 and on the same side of the street. House #2 is directly across from #3. You should start from those areas closest to the reported leak site or where you think the leak may be and work your way away from the leak site. Air monitor these homes just like you did for #3. Continue with this unless you get conflicting information such as a strong odor in a nearby building, which may require immediate attention.

Subsurface structures like sanitary systems, storm pipes, and UG electric boxes may be good, easily accessible air-monitoring sites. You have a major emergency if you are getting high gas readings in the ambient air outside without checking UG activities. If the UG gas leak is significant in both pressure and volume, it may migrate into these sites.

The tactical objective of monitoring these sites provides two important pieces of information. First, the leak is migrating from its origin and moving into nearby pipes. Second, it emphasizes that water, sewer, gas, electric, and information lines all can provide direct routes to allow gas to go directly into nearby buildings through pipes, electric or information line chases, or loose backfill. This reinforces your planning assumption of major emergency until proven otherwise.

Your SOP should tell first-due units that removing the covers of these structures will provide a path of least resistance for escaping gas. This may lessen the hazard of gas entering buildings. Fire department units should not remove electric covers and should place cones around open storm and sanitary manholes. Coordinate these actions with local utilities and the Department of Public Works. Despite negative readings in UG facilities, gas can still migrate through soil and cracks in rocks back into nearby buildings, creating a hazard.

Your battalion chief (BC) arrives on scene and wants to take command but wants a CAN (Conditions, Actions, Needs) report first. Here is what you tell him:

Conditions: “It appears to be an UG leak that has migrated into the storm, sanitary, and maybe UG electrical systems. We have 50% LEL in storm drains and sanitary sewers in front of #2 and 75% LEL near houses #5 and #7 Leak Street.”

Actions: “I have interviewed the caller and done air monitoring in #3, 15% LEL at the point of entry, and evacuated residents to the corner of Veterans and Hero Streets.”

Needs: “We need additional units to get air monitoring into nearby homes #1, 2, 5, and 7 to determine if and how far the gas may be migrating UG. We may want to evacuate the closest houses if readings increase.

The BC asks if there are any mark-outs or signs of recent construction in the area. You respond that there are no mark-outs or recent patches to the roadway or sidewalk. You tell him you looked for bar holes made by previous visits from gas technicians and there are none, indicating the utility may not have been called to this leak site previously.

Next steps. If the air monitoring by second-due units shows there is no gas entering the nearby buildings, it looks like this is a minor emergency but still worthy of continuous monitoring of nearby structures and subsurface structures. You can reduce the size of your kill box if you overestimated and maybe collapse your perimeter that the police are holding for you (vehicles and pedestrians). You may consider opening windows and doors in #3 to release any gas that may still be accumulating there. Monitoring every 10 to 15 minutes will keep you apprised of any increases in danger (life safety).

While waiting for the utility, you talk more with neighbors and the caller now that you have determined this is not a major emergency. You ask more detailed questions about any landscaping or other projects. The caller tells you, “Yes, we just put in a new mailbox on the post this afternoon at the end of the driveway.”

The utility tech confirms through bar holing and his drawings that it is very likely the service line to the house was damaged, causing the leak. He excavates the area near the mailbox and finds a small hole in the plastic service line (usually a half to one inch) and repairs it.

After-action review. If you had become complacent at this call, you may have been content to sit in the rig until the utility tech arrived, thinking, “Gas is lighter than air, it is escaping, so there is no hazard.” During that time, gas could have been accumulating in #3 through the sandy backfill of the utility trench for water, sewer, electric, and gas that ran from the street back to the foundation. The frost layer on the ground was preventing much of the leaking gas at 60 psi street pressure from escaping up through the soil. Gas could have taken the path of least resistance back into the house or nearby buildings. Your complacency and possibly your career would have ended abruptly if the house exploded.

This turned out to be a minor emergency. You quickly and successfully found that out because you completed your size-up SOPs. You classified it initially as a possible threat to life safety until proven otherwise. It did not take too long to rule out a major emergency because you and your crew ran your SOPs and were confident in your air-monitoring tools.

JERRY KNAPP is a 44-year firefighter/EMT with the West Haverstraw (NY) Fire Department and the author of House Fires (Fire Engineering). He is a training officer with the Rockland County (NY) Fire Training Center and chief of the hazmat team and a technical panel member for the Underwriters Laboratories research on fire attack at residential fires. He is the author of the Fire Attack chapter in Fire Engineering’s Handbook for Firefighter I and II and has written numerous articles for Fire Engineering.

BRIAN FOCHT, CFPS, CFEI, is a 36-veteran of the fire service; deputy chief of the Willow Grove (PA) Fire Company; and emergency response supervisor and past senior training specialist with PECO, assigned to gas training at the PECO Fire Academy. He is a Pro Board level 3 fire instructor and teaches at several academies and venues across the country. He is a Pennsylvania Department of Health paramedic and instructor. He is the 2016 recipient of the Garry Briese IAFC Safety Performance Award, the Thomas Jefferson Community Volunteer Award, the PECO Cherry Cooper Award, the 2017 Liberty Museum Valor Award for Community Betterment, and the 2019 Pennsylvania Department of Health Instructor of the Year Award.