Death Occurs — The 37.2°C Starting Point
Every PMI estimate starts from one fixed number.
- 37.2°C: Standard core temp (Henssge reference value)
- Rectal: Measurement site (deep core, most stable)
- 8 cm: Probe depth (standard insertion depth)
- ~1 h: Plateau duration (before steady decline)
Why 37.2°C, not 37.0°C
Rectal temperature runs slightly above oral baseline.
The postmortem plateau
Core temperature briefly holds before cooling truly starts.
Why the starting value matters
Every later PMI estimate is anchored to this number.
Cooling Begins — Heat Flows Toward Ambient
Once circulation stops, thermoregulation ends entirely.
- Conduction: Heat transfer mode (plus convection, radiation)
- Minutes: Skin vs core lag (skin cools first)
- Stopped: Circulation status (no active heat production)
- ΔT: Driving force (body minus ambient gradient)
No more internal heat production
Metabolism stops, so only passive heat loss remains.
Surface cools before the core
Skin and extremities lose heat faster than deep tissue.
The gradient drives the rate
A bigger body-ambient gap cools the body faster.
The Sigmoid Cooling Curve
Cooling is not a straight line — it is S-shaped.
- Double-exp: Curve type (sigmoid decay shape)
- Slow: Early phase (plateau resists heat loss)
- Fast: Mid phase (steepest temperature drop)
- Slow: Late phase (approaches ambient asymptote)
The Henssge double-exponential
Two exponential terms combine into one sigmoid curve.
Why not simple linear cooling
Body mass and insulation delay early heat loss.
Reading the curve shape
Slope steepness reveals the current cooling phase.
Ambient Temperature Sets the Pace
Colder surroundings pull heat away much faster.
- Faster: Cold room effect (steeper cooling curve)
- Slower: Warm room effect (flatter cooling curve)
- 0–30°C: Ambient range modeled (slider-controlled)
- Accelerate: Wind & moisture (convective heat loss)
Ambient sets the asymptote
The body cools toward, never past, room temperature.
Rate versus final value
Ambient temperature changes both curve speed and floor.
Real-world correction factors
Clothing, body mass, and airflow adjust the rate constant.
PMI Estimated From the Temperature Gap
Two readings are enough to back-calculate time of death.
- 2: Inputs needed (body temp and ambient temp)
- Curve inversion: Method (solve for elapsed hours)
- ±2.8 h: Typical accuracy (95% confidence interval)
- 0–24 h: Reliable window (best estimation range)
Inverting the cooling curve
Solve the sigmoid equation backward for elapsed hours.
Sources of estimation error
Clothing, body size, and posture widen the confidence range.
Combining with other evidence
Rigor, lividity, and scene findings refine the final estimate.