• All fire smoke is toxic. Inhaling it is one of the dangers of fire, and measuring it is important for fire safety.
• NIST research showed that the lethality of most smoke can be estimated by measuring just a few key gases.
• NIST also found that reducing ignition risk and limiting fire size can significantly decrease life loss from smoke inhalation.

Even if you stay away from the heat in a fire, the smoke can poison you before you have time to escape. People have known for a long time that fire smoke is toxic, but figuring out what makes it deadly was a real challenge. That’s partly because the chemistry of smoke is so complicated. Smoke from a typical fire contains hundreds of gases interspersed with tiny liquid droplets and solid particles.  

This smoke mixture is different depending on what’s burning, and research at NIST and elsewhere found that difference matters — not all smoke is equally life-threatening.  

This insight led to two important practical questions for fire research:  

1. Of the hundreds of gases in fire smoke, do just a few of them determine how lethal the smoke is?  
2. Could we save lives by regulating commercial products based solely on the toxicity of the smoke they make when they burn?

Estimation of Smoke Lethality

To address the first question, NIST researchers burned a variety of common materials in a lab and measured the smoke that was lethal to laboratory rats. They also worked with colleagues at the Southwest Research Institute to expose rats to individual toxic gases and mixtures of these gases, identifying the atmospheres that were lethal.  

Comparing these two sets of data, NIST researchers found that the lethality of the smoke from each of the materials could be explained by just six gases: carbon monoxide, carbon dioxide, hydrogen cyanide, hydrogen chloride, hydrogen bromide and oxygen. It might seem strange for oxygen to be on the list of lethal gases, but a lack of oxygen can be deadly. So, unlike the other items on this list, with oxygen, they measured how much the gas decreased.

Not only did they identify the gases, but also they created an equation to estimate the lethality of smoke from a burning material just by knowing the concentrations of those six gases. NIST scientists named this the N-gas equation. It works remarkably well at estimating the smoke lethality of most materials, with only rare exceptions discovered so far.  

This was a breakthrough because it greatly simplified the measurement process. The old way of measuring toxicity was to burn a material, expose lab rats to the smoke and observe whether or not they died.  

Now a laboratory only needs to burn the material in a specially designed box, measure the concentrations of a few gases in the smoke, and plug those numbers into the N-gas equation. This procedure replaced routine animal testing for most materials.  

This approach became the basis of the first and only standardized toxic potency measurement method in the United States, NFPA 269, and the comparable ASTM E1678.

These standardized methods enabled smoke toxicity tests for more types of materials and led to a deeper understanding of smoke lethality. Ultimately, this research answered an important question about fire research — making regulations for materials based on their potential to make toxic smoke would not save many lives during a fire.

Reducing the Hazard From Smoke Inhalation  

The main goal for fire toxicity research in the mid-1900s was to identify and ban materials whose smoke was “supertoxic,” i.e., lethal from just a few breaths. It turns out, these materials are rare. In laboratory measurements, the lethal toxic potency of common materials does not differ greatly.

Far greater success at saving lives has been achieved by mandating products that burn less vigorously and produce less smoke in the first place. For example, preventing ignition of upholstered chairs by cigarettes keeps smoke from being generated at all. Capping the burning rate of a mattress at 200 kilowatts reduces the mass of smoke by at least 90%. In both examples, the potential for burn injuries and extensive property damage is reduced far more than what might be achieved by just limiting the toxicity of the smoke.

Additional Reading:

NIST report: Toxic Potency Measurement for Fire Hazard Analysis, 1991.  

ASTM International standard: ASTM E1678, Standard Test Method for Measuring Smoke Toxicity for Use in Fire Hazard Analysis, 2024.

NFPA standard: NFPA 269, Developing Toxic Potency Data for Use in Fire Hazard Modeling, 2022.