Why ten degrees
is the number
that matters.
Cold therapy works in a narrow window. Drop below and the body fights back, vessels re-open, blood floods in, the cooling effect inverts. Stay above and you've put a damp flannel on a sprain. Kelvin sits, on purpose, in the middle.
Thermo-strip
A narrow band where the
work actually happens.
Cool too aggressively and you trigger a protective rebound. Cool too gently and nothing reaches the structures underneath. Five degrees decide it.
Triggers the Lewis hunting reaction within 5 to 10 minutes: periodic vasodilation defeats the cooling. Ice burn risk rises with direct contact.
Peak cooling benefit. Nerve conduction drops by ~33% at 10°C. Inflammation modulates without ischemic rebound.
Cooling and anti-inflammatory effect drops away. You're cooling skin, not the structures underneath it.
Cool the tissue below 10°C and the body fights back, paradoxically rewarming the very area you're trying to treat.
Sir Thomas Lewis observed it almost a century ago: hold skin too cold for too long and blood vessels swing open in a protective surge of warm blood. It's why a frozen gel pack feels intense for ten minutes, then quietly stops helping.
Heart, 1930 · Royal Society of Medicine
Kelvin holds 10 to 12°C, the band that sits below the cooling threshold (13.6°C) and safely above the Lewis hunting reaction. Therapeutic, all the way through.
~95 years of overlooked scienceWhat going too cold actually does to your tissue.
- 0:02min:sec
Ice burn begins
Direct sub-zero contact freezes the superficial skin layer. This is why every ice-pack label tells you to wrap it in a cloth.
- 0:05min:sec
Deep vasoconstriction
Blood vessels clamp shut to protect the core. Useful, briefly, but the tissue is now under-perfused and starved of the oxygen needed to heal.
- 0:10min:sec
Lewis hunting reaction
The body fights back: vessels suddenly dilate to rewarm the tissue. Cooling flips into the opposite of what you wanted. Inflammation and swelling return, often worse than before.
Sir Thomas Lewis, Heart, 1930
- 0:30min:sec
Nerve injury risk
Prolonged uncontrolled cryotherapy is clinically linked to peripheral nerve palsy. Cold below the optimal therapeutic window stops being treatment and becomes damage.
PMID 1443317
Kelvin never goes below 10°C, so the cascade never starts. No ice burn. No Lewis rebound. No risk of nerve injury from cold that was always too cold to begin with.
Temperature, plotted against time.
Lab measurement, skin-interface thermocouple, 21°C ambient. The ice pack starts colder than it should and rises out of usefulness well before the fifteen-minute mark. Kelvin holds the line.
Latent heat,
not sensible heat.
A standard gel pack stores sensible heat - its temperature climbs steadily as it absorbs energy from your body. Useful for a quarter of an hour, then it's a warm bag.
PCMx™ stores latent heat. It absorbs energy by changing phase - solid to liquid - at a fixed temperature. While that transition is happening, the pack stays at 10-12°C. It does not drift. It does not spike. It simply holds.
When the phase change completes, you put it back in the fridge. The material re-sets. You use it again. There is no consumable. There is no waste.
The reading.
- 01Algafly & George (2007). The effect of cryotherapy on nerve conduction velocity, pain threshold and pain tolerance. BJSM, 41(6). PMID 17261561.
- 02Bleakley C, McDonough S, MacAuley D. (2004). The use of ice in the treatment of acute soft-tissue injury: a systematic review of randomized controlled trials. Am J Sports Med, 32(1):251-261. PMID 14754753.
- 03Lewis T. (1930). Observations upon the reactions of the vessels of the human skin to cold. Heart, 15, 177-208. Archive.
- 04Internal thermal performance testing, Kelvin Therapy Ltd, 2025. Available on request.