Air Monitoring 101

Hazardous materials incidents remain one the most complex calls that the fire service can run. We typically are receiving very limited information and have to use the clues we find on arrival, along with our reconnaissance efforts, to piece together what the problem is. We typically can use our air monitors to assess the severity of the hazardous materials release. 


However, it requires that we prioritize what it is we are looking for and that we have access to air monitors that are capable of detecting the various hazards.

Monitor for Radiation

First, we want to monitor for the presence of radiation. Radiation is the first priority because we have no defense against it. Rather, our only defense against radiation is time, distance and shielding. Recall that there are four types of radiation: Alpha, Beta, Gamma, and Neutron. Alpha particles travel only a few inches — unless they are dispersed — are easily blocked by skin and are mainly inhalation and ingestion hazards.

  • Beta particles travel a few feet in the air, can penetrate the skin by a few millimeters (causing “beta burns”) and can be easily blocked by a thin sheet of metal, plastic or a block of wood.
  • Gamma rays can travel long distances and are only blocked by very dense materials — lead, large amounts of water, etc. Gamma rays destroy cells as they penetrate the body.
  • Neutron radiation is present only when a fission/fusion reaction has occurred (a nuclear device has detonated or inside a nuclear reactor). Neutron radiation normally decays very quickly (sometimes in fractions of a second), which makes our primary concerns Alpha, Beta, and Gamma.

If we park 100 yards away from a tractor trailer that is involved in a motor vehicle collision and our radiation detectors are giving us readings above background levels, we would assume that it is Gamma as it is the only radiation type capable of potentially travelling 100 yards. If there has been an explosion or fire, we may assume that Alpha or Beta particles have been distributed in the atmosphere, until our air monitor tells us otherwise. In the absence of Alpha and Beta particles, we need only worry about Gamma rays. This is where our time, distance and shielding defense comes into play. If our radiation detector reads “25 R/hr,” that would be 25 REM (Roentgen Equivalent Man) per hour. That means we have to be there for an hour to absorb 25 REM. The solution is that we don’t stay there an hour. We do what we need to do (i.e., grab the patient) and get out, using time as our friend.

The inverse square law tells us that if we double the distance from the source, we reduce the amount of radiation by a factor of four. So, if we were reading 25 R/hr at 10 feet, we can move to 20 feet from the source and only be exposed to one-quarter of 25 R/hr (6.25 R/hr). Distance is our friend as well. Shielding is difficult to find as we do not normally travel with very dense materials on our trucks. However, our trucks themselves (filled with water) can provide us some protection.

Consider Corrosivity

Next, we need to consider corrosivity. Why? Because corrosive vapors can destroy our air monitoring equipment and cause them not to detect everything else we are worried about. My department monitors for corrosivity using a very high-tech device. It’s called a broomstick. A piece of pH paper is attached to the end of the broomstick which our entry teams hold out in front of the air monitors. A change in color on the pH paper suggests the presence of a corrosive atmosphere. Normally, a red color change suggests an acid and a green color change suggests a base and are normally associated with a numeric scale of 0 to 14 (with 7 being neutral). Remember that a 0 – Acid is just as corrosive as a 14 – Base, they are just different materials. While we hope that our chemical protective clothing will protect us from both acids and bases, we want to know when we are close to them so we can avoid them as our chemical protective clothing is our last line of defense.

Speaking of avoiding certain environments while wearing chemical protective clothing (PPE), our next monitoring priority is flammability. Since the PPE we are wearing are basically plastic, this is a priority that we need to pay special attention to, unless we don’t mind being shrink-wrapped in our Level A suits if we’re caught in a fire that occurs as a result of a flammable environment. Normally, we use our multi-gas meters to measure for flammability. It is normally calibrated to a specific type of flammable gas, such as methane. That means that every flammable gas that multi-gas meter comes in contact with, it thinks is methane. Methane’s flammable range is from its lower explosive limit of five percent concentration in air to its upper explosive limit of 17 percent concentration in air. Any methane concentrations below five percent are too lean to burn and any methane concentrations above 17 percent are too rich to burn.

To compensate for our meter thinking everything is methane, we have our turnback thresholds of 10 percent indoors and 20 percent outdoors. When our meter reaches 10 percent of the Lower Explosive Limit of Methane (that is 10 percent of the LEL of five percent or 0.5 percent concentration in air), we would turn around if we were inside a building or vessel. If we are outside, we would turn around at 20 percent of the LEL of Methane (which is five percent concentration in air and 20 percent of that [our outdoor turnback] would be one percent concentration in air). These turn backs provide us a margin of safety to prevent us from being caught in a flammable environment.

Would it be safe to operate while our meter read 80 percent LEL? If we were absolutely sure we were dealing with Methane, we would likely still be in an environment that is too lean to burn. However, our 10 percent and 20 percent turn backs are designed to protect us from other gases, especially those that are much more flammable than methane.

Next, we want to monitor for oxygen. While that may not be important for us as we are likely wearing our own oxygen in the form of an SCBA, it is important for unprotected individuals — those without their own air supplies — and can tell us something about the environment. Normal air has 20.9 percent oxygen in it, not 21 percent as is commonly claimed. Our multi-gas meters normally have an oxygen sensor that is calibrated to 20.9 percent. This 20.9 percent would be the equivalent of 209,000 parts per million (or ppm) – one million parts per million would be 100 percent. The movement of our Oxygen sensor from 20.9 percent to 20.8 percent suggests that 1,000 parts per million of something displaced enough oxygen to make the oxygen sensor reading drop.

If we consider the level that Chlorine is immediately dangerous to life and health is 10 ppm, a movement of our oxygen sensor from 20.9 percent to 20.8 percent by a Chlorine release would suggest we are already 100 times higher than the level that is immediately dangerous to us. We also have to remember that our Oxygen sensor is only measuring approximately one-fifth (20 percent) of the environment, which means there is likely five times whatever we are reading present. In other words, our 20.9 percent to 20.8 percent suggests 1,000 parts per million but is only one-fifth of the environment. So, there is likely five times our 1,000 parts per million detected by our oxygen sensor, or 5,000 parts per million present. When it comes to extremely toxic substances, 5,000 ppm is a lot. Increases in our oxygen levels above 20.9 percent also suggest the presence of an oxidizer. Oxygen rich environments are often much more dangerous due to their explosive nature and the fact that many materials behave differently in environments above ambient oxygen levels. Both oxygen deficient and oxygen enriched environments can also cause our flammability/LEL sensor to be unreliable.

Monitor for Toxicity

Finally, we want to monitor for toxicity. There are a lot of monitors out there that can aid in measuring the level of toxicity for a substance. Unfortunately, this requires that we have an idea of what the substance is that we are measuring, so we use the right monitor. For example, using a Chlorine Draeger tube will not help us identify the level of toxicity present if we are looking at an Anhydrous Ammonia leak. Identifying what we think the released material is and using the correct monitor is sometimes difficult. We have to use our other monitoring priorities and any clues we can assemble at the scene to help guide us for this initiative. For example, if we think we are dealing with hydrochloric acid and just as we are deploying a Draeger chip system for hydrochloric acid, our entry team calls us and says the pH paper is solid green, that should give us a moment of pause. Green pH paper suggests we are dealing with an alkali (or base), not an acid.

Perhaps we have some more research to do or we need to gather more clues before we proceed with attempting to measure the current level of toxicity. The gold standard for identifying unknown materials is gas chromatography/mass spectroscopy (GC/MS). If a portable GC/MS is available, it will tell you exactly what the material is, or at the very least, what the material is like. A GC/MS may tell you a material is Benzene or Xylene, but both are aromatic hydrocarbons and require similar mitigation strategies as they are both components of gasoline. A GC/MS is yet another tool that can help you determine the best way to protect the responders, the public and the environment.

Hazardous materials incidents are not getting any easier. We frequently encounter transporters that have no idea what they are transporting. They often have multiple sets of shipping papers that further complicate our material identification. Air monitoring is a complex endeavor that must be prosecuted quickly and accurately in order to begin managing any hazardous materials incident. Regardless of the type of monitors implemented, the priorities are the same. Monitor for radiation first and always. Then check for corrosivity, if for no other reason, to protect your multi-gas meters. Then check for flammability so we do not add fires to our already complex incident. Finally, we want to measure oxygen levels and toxicity. Use all of the available information to determine what the material is and implement the best possible mitigation strategies based on the full assessment.

Be safe and do good.

Dr. David A. Greene has over 27 years of experience in the fire service and is currently the deputy chief with Colleton County (S.C.) Fire-Rescue. He holds a PhD in Fire and Emergency Management Administration from Oklahoma State University and an MBA degree from the University of South Carolina. He is a certified Executive Fire Officer through the National Fire Academy, holds the Chief Fire Officer Designation from the Center for Public Safety Excellence, holds Member Grade in the Institution of Fire Engineers, is an adjunct instructor for the South Carolina Fire Academy and is a Nationally Registered Paramedic. He can be reached at

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