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.