Temperature Thresholds for Cold Stress Responses
Cold stress activates sequentially: TRPM8 cold receptors at <25°C skin; BAT thermogenesis below 19°C skin; shivering at ~35.5°C core; hypothermia at <35°C core; ventricular fibrillation risk at <28°C core temperature.
| Measure | Value | Unit | Notes |
|---|---|---|---|
| TRPM8 activation threshold | 8–25 | °C | Cool/cold sensation receptor; threshold varies with prior adaptation |
| TRPA1 (noxious cold) activation | <17 | °C | Painful cold; activates nociceptive pain pathways |
| BAT thermogenesis activation (skin) | <19 | °C skin temperature | NE-driven; below this threshold BAT UCP1 is fully active |
| Pre-shivering thermogenesis threshold | 36.5–37 | °C core | Subtle muscle tone and BAT increase before overt shivering |
| Shivering onset (core) | ~35.5–36 | °C | Overt rhythmic shivering begins; threshold varies ±0.5°C |
| Clinical hypothermia threshold | <35 | °C core | Brown 2012; rectal or esophageal measurement |
| Severe hypothermia threshold | <28 | °C core | Cardiac arrhythmia risk; ventricular fibrillation at <28°C |
| Minimum survivable core temperature | ~13–14 | °C (recorded) | Reported survival cases with aggressive rewarming; extreme medical context |
Cold stress responses activate in a staged, hierarchical manner as temperature falls. Each threshold marks the onset of a new physiological response layer.
The Temperature Response Cascade
| Core/Skin Temp | Physiological Response | Consequence |
|---|---|---|
| Skin <25°C | TRPM8 receptor activation | Cool sensation; gentle sympathetic response |
| Skin <19°C | BAT thermogenesis activation | Non-shivering thermogenesis; increased metabolic rate |
| Skin <17°C | TRPA1 (noxious cold) activation | Pain sensation; urgent withdrawal motivation |
| Core ~36.5°C | Pre-shivering thermogenesis | Subtle muscle tone increase; BAT ramps up |
| Core ~35.5°C | Overt shivering | 2–5× metabolic rate increase |
| Core <35°C | Clinical hypothermia | Cognition impaired; coordination decreasing |
| Core <32°C | Moderate hypothermia | Severe cognitive impairment; lethargy |
| Core <28°C | Severe hypothermia | Cardiac arrhythmia risk; death possible |
| Core <20°C | Profound hypothermia | VF, cardiac standstill; medical emergency |
Skin vs Core Temperature: Two Different Signals
The thermoregulatory system uses both skin temperature (peripheral) and core temperature (central) signals with different weights:
| Signal | Weight | Primary Role |
|---|---|---|
| Core temperature | ~80% | Drives final thermoregulatory response |
| Mean skin temperature | ~20% | Early warning; modulates shivering threshold |
Cold skin can trigger shivering before core temperature drops — an anticipatory protective mechanism. However, core temperature is the dominant determinant of physiological state.
Thresholds in Standard Cold Exposure Protocols
For typical recreational cold exposure:
| Protocol | Skin Temp | Core Temp Change | Zone |
|---|---|---|---|
| Cold shower (20°C water) | ~20°C | Negligible | TRPM8 zone |
| Cold shower (10°C water) | ~15°C | Negligible | TRPA1 + BAT zone |
| Ice bath (12°C, 15 min) | ~12°C | <0.5°C | BAT + mild shivering zone |
| Extended cold water (30+ min) | <10°C | 0.5–1.5°C | Shivering + early hypothermia risk |
| Near-drowning in cold water | <10°C | Progressive drop | Hypothermia emergency |
Standard protocols (ice bath at 12°C for 15 min) operate well below the BAT and shivering thresholds but remain far from the hypothermic range for healthy individuals.
Clinical Implications
Emergency medicine categorizes hypothermia severity using core temperature:
- Mild (32–35°C): Shivering, confusion, tachycardia
- Moderate (28–32°C): Decreased shivering, stupor, bradycardia
- Severe (<28°C): No shivering, cardiac arrhythmia, unconsciousness
- Profound (<20°C): Apparent death; aggressive rewarming may succeed
The 28°C threshold is particularly important: below this temperature, the heart is at risk of ventricular fibrillation from the direct effects of cold on cardiac conduction tissue.