Carbon Monoxide Enters Ambient Air
A colorless, odorless gas begins displacing oxygen in breathed air.
- Combustion: CO Source (incomplete burning)
- 0%: Detectability (no smell, no color)
- 400–1000 ppm: Typical fire scene (ambient CO)
- 50 ppm: OSHA 8-hr limit (occupational ceiling)
Why CO is undetectable
No smell, taste, or color warns of rising CO.
Common exposure settings
Fires, faulty furnaces, and closed-garage engines.
Ambient ppm is the starting input for every COHb calculation.
Forensic relevance
Scene ppm anchors the reconstructed exposure timeline.
CO Competes With Oxygen for Hemoglobin
Carbon monoxide binds hemoglobin roughly 240 times tighter than oxygen.
- ~240×: Affinity ratio (M) (CO vs O₂ for Hb)
- 4: Hb binding sites (per tetramer)
- Fast: O₂ unbind rate (seconds-scale)
- Very slow: CO unbind rate (hours-scale)
Haldane affinity constant
CO displaces O₂ far faster than O₂ displaces CO.
Site-by-site displacement
Each of four heme sites can independently trap CO.
Higher ambient ppm raises the odds any given collision binds CO.
Why symptoms lag exposure
Binding is essentially one-directional once CO locks on.
Carboxyhemoglobin Percentage Accumulates
Bound CO sites accumulate toward an exposure-dependent equilibrium level.
- τ≈4h: Equilibration constant (model half-life)
- Saturating: Rise pattern (exponential approach)
- 2: Governing variables (ppm and hours)
- <3%: Baseline (nonsmoker) (normal COHb)
Exponential approach to equilibrium
%COHb climbs fast at first, then levels off over time.
Equilibrium ceiling
Ambient ppm sets the maximum reachable %COHb.
The gauge fill mirrors accumulated bound-CO fraction.
Duration matters as much as concentration
Short high-ppm and long low-ppm exposures can converge.
Concentration-Time %COHb Calculation
A saturation formula converts ppm and exposure hours into %COHb.
- Haldane-based: Model type (saturation curve)
- ppm, hours: Inputs (two sliders)
- %COHb: Output (single value)
- Forensic estimate: Use case (not clinical dosing)
Equilibrium term
COHb_eq scales ppm through the 240× affinity constant.
Time term
A 1−e^(−t/τ) factor scales the approach to equilibrium.
Investigators back-solve exposure time from a measured COHb.
Model limits
Ventilation rate and activity level shift real results.
Interpreting COHb Against Forensic Thresholds
Computed %COHb is classified against incapacitating and lethal bands.
- 30–40%: Incapacitating range (confusion, syncope)
- 50–70%: Lethal range (coma, death)
- 5–10%: Smoker baseline (chronic elevation)
- >50%: Fire-death median (reported autopsy COHb)
Threshold bands
Fixed COHb ranges separate mild from lethal outcomes.
Autopsy interpretation
Postmortem COHb helps confirm death by CO poisoning.
COHb over ~50% is generally considered a lethal finding.
Confounders
Preexisting disease can lower the lethal threshold further.