While some of us in the fire service are fortunate enough to have a dedicated Hazardous Materials Response Unit (HMRU) in our department, many agencies throughout the world do not. However, that does not mean we are exempt from responding to these types of incidents. In fact, the communities and citizens we serve expect us to respond to, and mitigate, any emergency that arises, regardless of staffing, training, or equipment limitations. Many career and volunteer departments may have little or no hazardous materials training, but because of low cost and abundant availability, many have some type of atmospheric monitoring equipment.
Some of you may be reading this and thinking, “our jurisdiction doesn’t have hazardous materials to worry about”. Be that as it may, I urge you to reconsider your stance. Do you have natural gas supplied to businesses or residences? Do you have wood-burning fireplaces in your response area? Any restaurants in your community? Most importantly, do you respond to structure fires? If none of these but the last is true, let me break it to you, you have responded to a hazardous materials incident already… You just may not know it.
Many of the substances listed as hazardous materials, or that cause a hazardous environment, can be found throughout our communities in various forms. Vehicle accidents can involve releases of flammable liquids. Restaurants or food processing facilities often use large amounts of Ammonia to keep produce cool. Natural gas and propane can be found in many homes as a heating and cooking fuel. Gas powered equipment, wood-burning fireplaces, and vehicle exhaust all produce Carbon Monoxide. Confined spaces can contain limited oxygen and hydrogen sulfide. And yes, structure fires can produce many types of hazardous substances (aka “products of combustion”). While many of these scenarios involve products with distinct smells and/or obvious signs and symptoms, many of them do not, and therefore are undetectable and present a high hazard to human health. It is for this reason that atmospheric monitoring is so vital to our safety as firefighters, and to the public whom we have sworn to protect.
Single or multi-gas monitors do not tell us everything about an environment, however, we can determine the presence, or absence, of many harmful substances, and thus, interpret the threat associated with them. The great thing about these instruments is that you do not have to be a hazardous materials expert to use them. Many of today’s monitors are simple, needing only one button to activate before using. These can range from single-sensor style, such as CO (Carbon Monoxide), HCN (Hydrogen Cyanide), and H2S (Hydrogen Sulfide), to multi-sensor versions with a combination of LEL, O2, HCN, CO, H2S, CL2, or NH3 sensors installed. The possibilities are many and the applications depend on jurisdictional need and/or budget.
A multi-sensor monitor helps us, as firefighters, to determine if an atmosphere is dangerous. Additionally, they allow us to locate the source of a leak or spill. In such situations, having the capability of measuring several gases at once becomes a distinct advantage. Portable multi-sensor monitors come in a variety of configurations and styles. Most of them offer simultaneously detection of multiple gases and alert the user when the level of gas concentration becomes a concern.
Monitoring & Detection
Flammable Environments. Combustible gas indicators (CGI) are able to detect a wide variety of flammable gases and vapors. This sensor is excellent at recognizing flammable environments such as gasoline spills or natural gas leaks. When displaying readings, the monitor shows a percentage of the Lower Explosive Limit (LEL). What does this mean for us? It tells us that if there is just enough gas to support combustion, it has reached its LEL, but if an abundance of gas is present, it displaces oxygen and reaches its UEL (Upper Explosive Limit). The space between the LEL and UEL is known as the flammable range. The actual LEL level for different gases will vary widely and is measured as a percent by volume (%Vol) in air (Figure 1). For instance, consider the following common gases:
Gasoline LEL 1.4% UEL 7.6%
Propane LEL 2.1% UEL 9.5%
Natural Gas LEL 5% UEL 15%
Acetylene LEL 2.5% UEL 100%
Readings are measured between 0-100% LEL. For example: a 50% LEL reading means the sampled gas mixture contains one half of the amount of gas necessary to support combustion.
It is important for firefighters to understand these readings, as they are susceptible to variation due to correction factors for different gases. For instance, many instruments are calibrated to Methane (Natural Gas) which will then present a true 0-100% scale on the display of the monitor. However, if we are in an environment of Gasoline (Octane) there is a correction factor of 2.7 in order to account for the difference between Methane. Thus, a reading of 10% LEL would “correct” into a reading of 27% LEL. Quite the difference when considering it is a flammable environment. Why is that important to us as firefighters? Remember, the goal is stay under 100% LEL and well below the flammable range. By using 10% LEL as our suggested action level we minimize the risks associated with the environment. We can do this by wearing proper PPE, placing a hose-line for protection, and changing our atmosphere by ventilating. Understanding how to use this sensor and following these steps will help you safely search for spills and leaks in an environment.
Oxygen. While normal atmospheric conditions contain between 20.8% – 21.0% O2, OSHA defines an oxygen deficient atmosphere as containing 19.5% or less, and an oxygen enriched atmosphere as any containing more than 23.5%. How does this apply to us as firefighters? We need O2 to survive in an environment, or we need assistance from a SCBA.
While OSHA recommends wearing an SCBA when O2 drops below 19.5%, I ask you to reconsider. A 0.1% sensor drop in O2 accounts for a 5,000ppm gas movement… Let me explain. When speaking of gaseous environments, we typically refer to them in PPM, or Parts Per Million. O2 is 20.9%, or approximately 1/5th the atmosphere, thus, accounting for 210,000 ppm. Hence, 210,000/20.9 equals roughly 1,000ppm for 0.1%. However, you must figure in the remaining 4/5th of atmospheric gas displacement, thus multiplying 1,000 by 5 and equaling 5,000. If you are not thoroughly lost, you now realize that an atmosphere of 19.5% O2 has roughly 70,000ppm of displacement. This is crucial for firefighters, especially if your monitor does not provide any other explanation for the displacement. Many toxic gases are deadly in the 100-200ppm range. Hence, I urge you to consider an SCBA much sooner than 19.5% O2.
On the other side of the scale, oxygen enriched atmospheres present their own dangers. Any concentration over 23.5% is considered to be an increased flammability risk. Enriched environments can also influence the combustible gas indicator (CGI) readings, as it utilizes O2 to accurately measure the LEL.
Toxic Gases. Carbon Monoxide responses are a routine incident for many fire departments. The CO sensor helps us determine presence of CO, because it is odorless. Nor would we want to use our sense of smell, due to its toxicity. What does that mean for us? Always were our SCBA. Remember that the majority of gases are heavier than air, thus requiring monitoring near ground level for accuracy. When responding to these incidents remove occupants to the outside, especially if they show signs of CO poisoning, and call for EMS. Pay close attention to gas appliances, furnaces, fireplaces, and running vehicles in garages, as these can produce CO by incomplete combustion (Figure 2). It is also a great idea to attach monitors to medical bags when responding to unknown medical emergencies, or unexplained sicknesses, so that we don’t run into toxic environments.
Hydrogen Cyanide sensors are also important for firefighters. This toxin is suspected to have been a contributing factor in many LODDs, because it mimics the signs and symptoms of a heart-attack and CO poisoning. However, there is very little that can be done to reverse its effects, especially in high doses. 270ppm can cause death in 6-8 minutes. Firefighters will encounter this hazard at structure fires and chemical suicides. Do not use signs and symptoms to diagnosis, or as a measure of safety. You must wear your SCBA and use a monitor to keep your crew and citizens out of harm.
Hydrogen Sulfide is a highly poisonous gas that is produced naturally from decaying organic matter and is also released from sewage, hot-springs, and natural gas. This comes into play quickly for rescue operations in confined space areas, as H2S settles to low-lying areas. Use of monitoring, SCBA, and ventilation are musts in these types of incidents. Firefighters may be clued into the presence of this toxin if by-standers, or patients, state they have smelled “rotten eggs”.
When responding to incidents, always assume you have an unknown substance until you can confirm otherwise. Do not trust outside information or your sense of smell to identify hazards. SCBA should be used, without question, until an atmosphere is proven safe. Retrieve baseline readings before entering a suspected environment and watch the monitor closely for changes. Move slowly through an environment to give the monitor and sensors time to measure and display information. Once the source or hazard is located, and isolated, the environment can be ventilated and made safe once again (Figure 3).
For more information, go to www.littlerock.gov