Ambient Toxic Gas Concentration Measurement
Forensic reconstruction starts with the ambient gas concentration in parts per million.
- ppm: Unit of measure (parts per million by volume)
- 1,200 ppm: Typical CO IDLH (immediately dangerous to life)
- 50 ppm: Typical HCN IDLH (immediately dangerous to life)
- Sensor logs: Detection method (scene or device recovery)
Where concentration data comes from
Fire investigators pull sensor logs, HVAC data, or reconstruction models.
Why concentration alone is not dose
Concentration is only half the toxicological equation.
Gas behaves as a uniform field
The model assumes even mixing throughout the exposure space.
Inhalation Begins at Recorded Concentration
The victim starts breathing contaminated air, initiating the toxicological clock.
- 12–20: Resting breath rate (breaths per minute)
- ~6 L/min: Minute ventilation (at rest, adult)
- ×3–5: Exertion multiplier (panic or physical exertion)
- t = 0: Clock start (first contaminated breath)
Breathing rate drives uptake
Faster breathing pulls in more toxic gas per minute.
Alveolar gas exchange
Gas crosses into blood through thin alveolar membranes.
Exposure clock starts ticking
Every second of inhalation adds to the toxic load.
Concentration × Time Toxic Load Accumulates
Haber's Law treats toxic load as the product of concentration and exposure time.
- C × T = k: Haber's Law form (constant toxic effect)
- ppm·min: Units (concentration-time product)
- n = 1: Model assumption (linear C-T relationship)
- Forensic dose: Use case (reconstructing lethality)
The Haber's Law formula
Toxic load equals concentration multiplied by exposure minutes.
Why the product matters
Low concentration for long, or high for short, can equal.
Limits of the linear model
Real gases deviate with an exponent n other than one.
Comparing Accumulated Dose to Toxic Thresholds
The accumulated C×T value is checked against known sublethal, incapacitating, and lethal bands.
- 30,000: Sublethal ceiling (ppm·min C×T)
- 90,000: Incapacitating ceiling (ppm·min C×T)
- >90,000: Lethal zone (ppm·min C×T)
- AEGL / LC50: Reference source (published toxicology data)
Reference toxicology databases
AEGL and LC50 tables anchor each threshold band.
Threshold bands are gas-specific
Each toxic gas carries its own C×T threshold set.
Comparing measured against reference
The live C×T bar is plotted against these fixed bands.
Classifying the Exposure — Sublethal to Lethal
The final forensic step assigns a classification label from the accumulated toxic load.
- Recoverable: Sublethal outcome (below 30,000 ppm·min)
- Impaired: Incapacitating outcome (30,000–90,000 ppm·min)
- Fatal risk: Lethal outcome (above 90,000 ppm·min)
- Cause of death: Forensic use (toxicology report input)
Sublethal classification
Victim likely recovers with no lasting toxic injury.
Incapacitating classification
Victim loses coordination or consciousness, escape unlikely.
Lethal classification
Accumulated dose is consistent with fatal poisoning.