Hydrogen Cyanide: It's Not Just a HazMat Anymore

Aug. 30, 2006
Not much thought has been given to hydrogen cyanide even though it is 35 times more toxic than carbon monoxide.

When we started our fire service careers, one of the first things we learned about was products of combustion. One of the products mentioned was hydrogen cyanide (hereafter referred to as HCN). Not much more was said about it. More attention was paid to carbon monoxide, the "silent killer", and its effects on fire victims that succumbed to smoke inhalation. Not much thought has been given to HCN even though it is 35 times more toxic than carbon monoxide. HCN is a highly toxic gas that is formed when such materials as wool, silk, cotton, nylon, plastics, polymers, foam, melamine and synthetic rubber burn. These products are found in common household items such as mattresses, carpeting, upholstered furniture and blankets. Some materials such as polyacrylonitrile convert half of their mass to HCN while burning. In 2004, 1.838 billion pounds of cyanide was produced in the United States with the majority being used to produce materials for building construction and interior furnishings. HCN may also be found in outside fires in vehicles and dumpsters in addition to vehicle exhaust (something for all Fire Equipment Operators out there to be aware of.) HCN is highly flammable and much of it will burn away during combustion. It continues to be produced during smoldering fires however. Newer construction techniques, in addition to newer materials also contribute to HCN production. More energy efficient construction yields tighter buildings. This leads to more products of combustion being entrained in the fire plume as opposed to leaking to the outside. This lack of ventilation also leads to increased HCN and CO levels. Just as there are no two fires that are exactly the same, this also applies to HCN production. HCN production depends upon the materials burning, oxygen levels and temperature.

  • Time Weighted Average:..... Carbon Monoxide - 35 ppm.....Hydrogen Cyanide - 4.7 ppm
  • IDLH:..... Carbon Monoxide - 1200 ppm.....Hydrogen Cyanide - 50 ppm
  • Lower Explosive Limit:..... Carbon Monoxide - 12.5%.....Hydrogen Cyanide - 5.6%
  • Upper Explosive Limit:..... Carbon Monoxide - 74 %.....Hydrogen Cyanide - 40 %

In the first classes we take either in firefighting or hazardous materials, we learn about the characteristic "bitter-almond" like smell of HCN. DO NOT BE FOOLED BY THIS!!! As First Responders, we should never rely on our sense of smell to identify anything, much less a compound as dangerous as HCN. In order to detect HCN by smell, our bodies must have a particular gene to do so. This gene is absent from 20-to-40-percent of the general population. Even if you do have the gene, you may not be able to detect the odor of HCN due to other odors in the air.

Scientific analysis of several fires within the past 20 years has shown the previously misunderstood role of HCN. The foam wall covering at The Station nightclub in Rhode Island not only contributed to a rapid flame spread, but also contributed significant amounts of HCN to the smoke. A fire in a passenger aircraft in Manchester, England in 1985 killed 54 people. Of these, 47 had possibly lethal cyanide levels while only 11 had possibly fatal levels of CO. Thirty five prison inmates were killed in a 1991 Argentinean prison fire. Lethal cyanide levels in their blood were traced to the burning polyurethane mattresses. The DuPont Plaza Hotel fire in San Juan, Puerto Rico killed 53. Of these, 25 had elevated levels of cyanide with only 3 showing elevated levels of CO. A fire in a disco in Gothenburg, Sweden killed 63 people. Most of the victims succumbed to the fire gases as opposed to the heat of the fire. Experiments conducted by the National Institute of Standards and Technology after The Station nightclub fire in Rhode Island in 2003 suggest that CO and HCN levels rose to lethal levels shortly after the fire started as oxygen levels plummeted. This was due to the foam panels used on the walls as sound insulation. A report by the Swedish National Testing and Research Institute determined that HCN was a factor in the fatalities and that the construction materials could produce large amounts of HCN. It is impossible to accurately predict how someone will be affected by HCN exposure as people process it differently. Pre-existing liver conditions are not able to handle HCN exposure as well as healthier individuals.

Of particular interest to the fire service are the events in Providence, RI, in March of 2006. Twenty seven firefighters were tested for HCN exposure following several fires. These fires were not at chemical facilities, nor were they the result of a terrorist attack. The fires occurred in two residences and a restaurant. Eight firefighters had elevated HCN levels and two firefighters HCN levels were that they required a cyanide antidote kit (further discussed below).

HCN poisoning can only be accurately diagnosed in a hospital or laboratory setting through a whole blood cyanide test. Very few hospitals in the country perform cyanide testing in-house and only eight laboratories in the entire country perform a whole blood cyanide test. In these cases, it may take a week or longer to get the test results back. Hospitals may routinely test for CO, but rarely, if ever check for HCN unless specifically requested to do so. As with all medical calls, the initial diagnosis may be made based on the initial signs and symptoms as noticed by the first responder. Since we are talking about HCN being present at structure fires, it is essential that ALL firefighters become aware of the signs and symptoms of HCN exposure. This is entirely too important to be left to our brothers and sisters in EMS because "we didn't join the fire department to do medical calls." HCN can enter the body by absorption, inhalation or ingestion and targets the heart and brain, often incapacitating the victim within a short period of time.

The following are all signs and symptoms of HCN exposure.

  • Weakness/Dizziness
  • Flushing of the Skin/Pink or Red Skin Color
  • Rapid Breathing
  • Anxiety/Excitement
  • Perspiration
  • Vertigo
  • Headache
  • Drowsiness
  • Difficulty Breathing/Respiratory Arrest
  • Prostration
  • Tremors
  • Irregular/Rapid Heartbeat
  • Convulsions
  • Stupor
  • Paralysis
  • Coma

Take a look at the list above. How many of the symptoms are similar to CO poisoning? How many times have you experienced a headache after conducting a heavy fire attack followed by extensive overhaul? Was it because of CO or HCN? Think about that for a minute...

It is extremely difficult to detect HCN in the field as very few instruments can detect it at all or are set up to detect it in the first place. While HCN sensors are available for four-gas meters, they must be special ordered as they are not part of the stock sensor package on any monitor. The HazMatCadPlus detects HCN, but does not give a definitive number like a four-gas meter will. The M256 kit used for WMD incidents detects HCN, but can't be used in areas where smoke or burning debris is present and doesn't give a definitive concentration. The M256 also takes 10-15 minutes to give a reading and can only be used one time. Draeger and Sensidyne tubes both detect HCN, but a new tube must be used for each sample, making these impractical for monitoring during structure fires.

Because it is so difficult to detect HCN in the field given the monitoring capabilities of most fire departments, we must fall back on recognizing signs and symptoms so this information can be relayed to the emergency department at the receiving hospital. The antidote kit for HCN poisoning contains amyl nitrite, sodium nitrite and sodium thiosulfate and is known as the Lilly Kit, Taylor Kit or Pasadena Kit. Check with your EMS provider and local hospitals and you will probably be quite surprised. Most hospitals only carry one kit in their inventory. The majority of ALS transport units (approximately 70-to-80-percent) don't carry these kits or these drugs in their inventories. Some of you may be thinking that this isn't something to worry about. What happens if there is a terrorist act in your community involving HCN? What happens if you are faced with a situation like the Providence Fire Department was faced with back in March where several of their members were exposed to HCN during normal fire suppression activities?

While the antidote kit does work, it is not without its problems and therefore must only be administered in a hospital setting as opposed to in the field. Side effects may include a severe drop in blood pressure, vomiting, headache and a further decrease in the blood's ability to carry oxygen. These side effects may make the cure worse than the exposure. The latter is particularly dangerous in cases where the victim has suffered a significant CO exposure due to smoke inhalation. Because of these complications, the antidote kit is only administered after HCN poisoning has been confirmed by laboratory testing or there are strong indicators that the victim was indeed exposed to HCN.

So what can we do to protect ourselves against HCN poisoning? The answer may be simple. Wear your SCBA all the time, including during overhaul. Be aware of what's burning around you. Know the signs and symptoms of HCN poisoning. Finally, consider monitoring for HCN at fires. The Greensboro, NC, Fire Department is considering this possibly. The results of which will be included in a future article here at Firehouse.com.

Stay safe.


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