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Hydrogen cyanide: Deadly HCN gas in fire smoke threatens responders

Fire services personnel often think of carbon monoxide as the silent killer. More and more, however, research is pointing to HCN or hydrogen cyanide, as a second and equally hazardous threat.

December 6, 2007 
By Carlin Riley and Steve Young

hydrogen_10568Fire services personnel often think of carbon monoxide as the silent killer. More and more, however, research is pointing to HCN or hydrogen cyanide, as a second and equally hazardous threat.


By now, those in the fire service should know that HCN is extremely toxic and has serious effects on the body.

Early in 2006, 27 firefighters in Providence, R.I., were tested for HCN levels after three separate structural fires. Eight of the 27 had elevated levels of HCN and required treatment. One firefighter, who collapsed at the scene and was treated for HCN poisoning, has yet to return to work. In May, the state of Rhode Island announced $435,000 US in federal homeland security money to buy cyanide antidote kits for ambulances and train 4,000 rescue workers how to administer the antidote on the spot. The kits are intended for use following a chemical terrorist attack using cyanide but the antidotes can be used for any victims of cyanide poisoning, including firefighters suffering the effects of smoke inhalation. Most firefighters can probably recall incidents during which they may have been exposed to HCN. As more firefighters become ill or die as a result of an unknown, researchers expect the finger will be pointed at HCN.


"The recognition of cyanide as a major component of fire smoke is one of the most significant discoveries over the past 25 years," says Curtis Varone, deputy assistant chief with the Providence Fire Department and part of the investigating committee into the cyanide poisonings of the Providence firefighters. "It impacts not only our fire ground operations in terms of enhanced wearing of SCBA but implicates staffing levels, crew rotations, incident management and accountability. Add to that the advent of hydroxycobalamine as a cyanide/smoke inhalation antidote and the result is enhanced survivability for firefighters and citizens alike."

According to the report, "there is overwhelming evidence that cyanide is present in fire smoke more commonly and in greater quantities than previously believed due to modern materials such as plastics, rubber, asphalt, and polyacrylonitriles." The report determined that firefighters are routinely exposed to dangerous levels of cyanide at fires without realizing it. Indeed, research is telling the fire service it's time to change the way it operates at any type of smoke event. There have been numerous studies on HCN – where it comes from, how it affects humans, how it affects firefighters and how firefighters can protect themselves from it. HCN is present in many of the products people eat off of, sleep on, drive in and use every day. In a stable state, as part of a composition of materials, HCN is relatively safe. But when the material is heated it becomes a concern. Consider:

  • HCN is 35 times more toxic than CO;
  • HCN is produced when products such as wool, silk, cotton; nylon, plastics, polymers, foam, melamine, polyacrylonitriles and synthetic rubber burn;
  • HCN can enter the body by absorption, inhalation or ingestion and targets the heart and brain;
  • HCN often incapacitates the victim within a short period of time;
  • HCN has a half-life in blood of one hour (more on this below);
  • HCN is highly flammable and most of it will burn away during combustion. (Keep in mind, though, that HCN is produced by heating objects.

Despite burning away during combustion, the heated objects within the fire continue to produce more product as long as their temperature remains elevated.)

Given that HCN is 35 times more toxic than CO, consideration of this deadly gas should be front and centre in firefighters' minds. HCN exists, and has for years, in the smoke that firefighters are exposed to every day. Scientists are just figuring out what the long-term effects will be with repeated exposures. Until the research is definitive, how do firefighters protect against the effects of HCN? Many would suggest that the answer lies on the firefighter's back and that SCBA is a firefighter's best friend when it comes to protecting against HCN. For years, wearing an SCBA was optional or considered taboo.

Nowadays, in most departments, it is mandatory. The question is when to remove the SCBA. Oftentimes, after the flames are out, HCN might still be present but we can't see it or test for it. Should SCBA be worn until the atmosphere is completely free of HCN? The answer is yes but how do firefighters know if or when the air is clean since the four most commonly used gas detectors generally do not have the HCN sensor in them?

When is it safe to take the SCBA off? It is known that HCN is probably present before flames can be seen and after the flames are extinguished. Studies have shown that some materials, mostly plastics, give off toxic levels of HCN when they are heated. Although HCN is highly toxic it is also highly flammable and most is burned off during fire conditions. Unfortunately, HCN is again produced after the flame is out and the materials continue to off-gas or smoulder. In other words, HCN may be present even if smoke is not visible, thereby exposing firefighters to potentially toxic atmospheres depending on the level of personal protective equipment worn, as well as other variables, such as wind direction. Therefore, should additional personal protective equipment be considered for vehicle operators, command-post personnel, EMS and police? The only way for firefighters to protect against this unknown is to wear the SCBA until they have left the area.

Departments have started sending their hazmat units to car fires and structure fires. Considering the byproducts of today's combustion, this makes good sense. One of the main byproducts of fire is smoke. Smoke contains particulate matter and heated gases, such as hydrochloric acid, sulfur dioxide, ammonia and carbon dioxide, and toxins, including hydrogen sulfide and hydrogen cyanide. For firefighters to protect themselves, they must try to change the old culture of smoke eating and make it acceptable to wear an SCBA throughout an incident.

Some have proposed that that every fire should be considered a hazmat incident. Those who promote this school of thought don't suggest that firefighters wear class A suits to fight structure fires but are trying to raise awareness of that other silent killer.

Consider this: With a half-life in blood of one hour, if a firefighter were exposed to HCN, the detectable levels could be just half of the original exposure levels by the time the firefighter arrived at hospital and even lower by the time the blood work was completed.

This makes it extremely difficult for doctors to diagnose HCN exposure. Many of the symptoms of HCN poisoning mimic that of CO poisoning. More importantly, studies have shown that many hospitals are not equipped to deal with HCN poisoning, so it's critical that the fire service work to raise awareness of the issue.

In 2001 (the last available statistics), a Canadian Medical Association publication titled Availability of antidotes at acute care hospitals in Ontario, reported that of 179 hospitals surveyed, only one had all 10 of the antidotes on the study list. One of the antidotes on the study list was sodium trisulfate, the antidote for cyanide. Unfortunately, the report did not mention the number of hospitals that stocked this product. However, due to the high cost of this antidote, it can be assumed that not many Canadian hospitals stock it.

Picture this scenario: A firefighter has finished at an incident and experienced symptoms of HCN poisoning. He is taken to hospital, where the symptoms are mistaken for CO poisoning. Blood tests later reveal HCN in the bloodstream but enough time has passed that the HCN levels have dropped considerably (especially given the fact that testing for HCN takes up to two hours). Would this firefighter receive the proper dose of antidote? Does the hospital even have the antidote?

Below are some common symptoms associated with HCN poisoning.

  • Weakness/dizziness
  • Flushing of the skin/pink or red skin colour
  • Rapid breathing
  • Anxiety/excitement
  • Perspiration
  • Vertigo
  • Headache
  • Drowsiness
  • Ventricular fibrillation/tachycardia
  • Prostration
  • Tremors
  • Irregular or rapid heart beat
  • Convulsions
  • Stupor
  • Paralysis
  • Coma
  • Difficulty breathing/respiratory arrest
  • Heart attack

When we compare the symptoms of cyanide poisoning with the symptoms most firefighters' experience after physical exertion on fire scenes the similarities can be frightening. There are several groups in Canada and the U.S. working on further research into the effects of cyanide on firefighters, including how the cyanide is produced and how it moves around the fire scene.

Still, the best defence against the effects of cyanide poisoning is at every fire station – personal protective equipment (mainly SCBA) – along with policies, procedures and guidelines to dictate its use and company officers to enforce this.

Carlin Riley is a captain with the City of Kitchener and co-owner of Ontario Fire Training. Steve Young is a 15-year veteran of emergency services and has spent the last five years with the Kitchener Fire Department. Both are members of the Level 2 regional hazmat team.

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