7.3 Flammability, Reactivity, Toxicity & Physical Hazards

7.3 Flammability, Reactivity, Toxicity & Physical Hazards

To safely execute product control, isolation, and defensive tactics, Hazardous Materials Operations responders must possess a thorough understanding of physical and chemical properties. A material's physical state, vapor pressure, density, flammability limits, and reactivity dictate how far a chemical plume will spread, whether vapors will pool in basements or rise into the atmosphere, and whether water application will extinguish a fire or trigger a violent explosion.

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Flammability & Combustion Parameters

Combustion requires fuel, oxygen, and an ignition source in proper proportions. Responders must master three fundamental flammability metrics:

1. Flammable Range (LEL and UEL)

No gas or vapor-air mixture will burn unless the concentration of fuel falls within its flammable (explosive) range:

• Lower Explosive Limit (LEL): The minimum concentration of vapor or gas in air (expressed as a percentage by volume) below which flame propagation does not occur when exposed to an ignition source. Below the LEL, the mixture is "too lean" to burn.
• Upper Explosive Limit (UEL): The maximum concentration of vapor or gas in air above which flame propagation does not occur. Above the UEL, the mixture is "too rich" to burn (insufficient oxygen).
• Flammable Range: The numerical difference between the LEL and the UEL. Materials with wide flammable ranges present extreme hazards because they remain ignitable across a vast spectrum of concentrations.
  • Gasoline: LEL = 1.4%, UEL = 7.6% (Narrow range: 6.2%).
  • Natural Gas (Methane): LEL = 5.0%, UEL = 15.0% (Range: 10.0%).
  • Acetylene: LEL = 2.5%, UEL = 100.0% (Extremely wide range: 97.5%).
  • Hydrogen: LEL = 4.0%, UEL = 75.0% (Wide range: 71.0%).

Tactical Warning: Ventilation of an over-rich environment (above UEL) can dilute the vapor cloud down into the flammable range, resulting in violent ignition if ignition sources are present.

2. Flash Point vs. Fire Point vs. Autoignition Temperature

• Flash Point: The lowest temperature at which a liquid produces sufficient vapor to form an ignitable mixture with air near the surface of the liquid. At the flash point, exposure to a flame will cause a momentary flash, but burning will not be sustained.
  • Flammable Liquids: Liquids with a flash point below 100°F (37.8°C) (e.g., gasoline flash point is -45°F).
  • Combustible Liquids: Liquids with a flash point at or above 100°F (37.8°C) (e.g., diesel fuel flash point is 125°F).
• Fire Point: The temperature at which a liquid produces sufficient vapors to sustain continuous combustion once ignited. The fire point is typically a few degrees higher than the flash point.
• Autoignition Temperature (AIT): The minimum temperature to which a material must be heated in air to initiate self-sustained combustion without an external spark or flame (e.g., gasoline AIT is ~536°F / 280°C).

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Density Characteristics: Behavior in Air and Water

Understanding density allows responders to predict where spilled chemicals will migrate in ambient environments.

Vapor Density (Behavior in Air)

Vapor density is the relative weight of a gas or vapor compared to an equal volume of dry air, where air is assigned a baseline value of 1.0.

• Vapor Density > 1.0 (Heavier than air): Most hazardous gases and flammable vapors have a vapor density greater than 1.0 (e.g., propane = 1.56, chlorine = 2.47, gasoline vapor = 3.0 to 4.0). These gases sink, hug the ground, flow into low-lying terrain, culverts, catch basins, and basements, and can travel long distances to an ignition source, causing flashback.
• Vapor Density < 1.0 (Lighter than air): Gases with a vapor density less than 1.0 rise and dissipate rapidly in unconfined outdoor spaces. Responders use the mnemonic HAHA MICOPOC or 4H MEDIC ANNA to remember common lighter-than-air gases:
  • Hydrogen ($0.07$)
  • Anhydrous Ammonia ($0.59$)
  • Helium ($0.14$)
  • Acetylene ($0.90$)
  • Methane ($0.55$)
  • Isoprene ($0.90$)
  • Carbon Monoxide ($0.97$)
  • Hydrogen Cyanide ($0.94$)

Specific Gravity (Behavior in Water)

Specific gravity is the ratio of the weight of a liquid or solid to the weight of an equal volume of pure water, where water is assigned a baseline value of 1.0.

• Specific Gravity < 1.0 (Floats on water): Liquids with a specific gravity less than 1.0 float on water (e.g., gasoline = 0.72, diesel = 0.85, crude oil = 0.88). Applying straight water hose streams to burning hydrocarbon liquids will cause the burning fuel to float on top of the runoff, rapidly spreading the fire. Specialized Class B firefighting foam (AFFF or AR-AFFF) is required.
• Specific Gravity > 1.0 (Sinks in water): Liquids with a specific gravity greater than 1.0 sink to the bottom of water bodies (e.g., carbon disulfide = 1.26, sulfuric acid = 1.84, trichloroethylene = 1.46).

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Solubility & Miscibility Tactical Considerations

• Water Solubility / Miscibility: The degree to which a substance will mix uniformly with or dissolve in water.
• Polar Solvents: Water-miscible liquids such as alcohols (ethanol, methanol), acetone, and methyl ethyl ketone (MEK). When polar solvents catch fire, standard Aqueous Film-Forming Foam (AFFF) will be dissolved and destroyed by the alcohol. Responders must use Alcohol-Resistant AFFF (AR-AFFF).
• Hydrocarbons: Non-polar, water-insoluble liquids (gasoline, toluene, benzene) that do not dissolve in water.

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The BLEVE Mechanism (Boiling Liquid Expanding Vapor Explosion)

A BLEVE is one of the most catastrophic events in hazardous materials emergency response. It occurs when a sealed container holding a liquid under pressure above its normal atmospheric boiling point suffers catastrophic structural failure.

Step-by-Step BLEVE Failure Sequence

1. Direct flame impingement strikes the outer shell of a pressure vessel (e.g., propane tank or MC 331 tanker) in the vapor space (above the liquid level).
2. Because liquid inside absorbs thermal energy while vapor does not, the dry metal in the vapor space overheats rapidly, causing the steel to lose its structural tensile strength.
3. Simultaneously, heat transfers into the liquid phase, increasing internal vapor pressure past the opening setting of the pressure relief valve.
4. Overheated metal weakens until it can no longer contain the elevated internal pressure, leading to a violent structural breach.
5. The instantaneous drop in vessel pressure causes the superheated liquid to flash violently into gas, expanding by 270 to 600+ times its liquid volume.
6. The expanding gas cloud ignites instantly, producing a massive fireball, severe blast wave, and high-velocity metal shrapnel.

Critical BLEVE Indicators & Tactical Actions

• Warning Signs: High-pitched screeching from pressure relief valves, glowing/discolored metal shell, flame contact on vapor space.
• Defensive Actions: Apply unmanned master streams supplying at least 500 gpm (1,900 L/min) per point of flame contact directly to the vapor space of the tank. If adequate water cooling cannot be established immediately, evacuate the area instantly.

Physical & Chemical Property Reference Table

The table below details core physical parameters, baseline reference standards, chemical behaviors, and emergency response tactical impacts.