What is the afterdrop in cold plunge?

The afterdrop is the continued drop in core body temperature for 10-15 minutes AFTER exiting cold water. Caused by cold blood returning from extremities as blood vessels dilate. This is why you feel colder 5 minutes post-plunge than you did in the water. Don't rush to a hot shower - let your body rewarm naturally.

What is brown fat and how does cold plunge activate it?

Brown adipose tissue (brown fat) is specialized fat that burns calories to generate heat. Unlike white fat (which stores energy), brown fat consumes energy. Adults have small amounts in neck, shoulders, upper back. Cold exposure is one of the most effective known activators of brown fat. With 3+ months of regular cold exposure, brown fat volume and activity increase measurably.

What is the Søberg Principle?

Dr. Susanna Søberg's research finding that ending on cold (not warm) maximizes brown fat activation and metabolic benefits. When you end on cold, your body must work to rewarm itself, activating brown fat and increasing metabolic rate for hours. Always end contrast therapy on cold - even 30 seconds of cold at the end provides the benefit.

What is cold shock response?

Involuntary physiological response to cold water immersion: gasp reflex, hyperventilation, tachycardia (rapid heart rate), blood pressure spike. Peaks in the first 60 seconds, subsides by 2-3 minutes. This is the moment most cold plunge accidents happen - never submerge your head during entry. The technique to manage cold shock is slow nasal exhales.

Cold Plunge Glossary: Complete Terminology Guide

The complete cold plunge glossary

Confused by cold plunge terminology? This glossary defines every term you'll encounter in our guides and the broader cold exposure community.

A

Afterdrop: The continued drop in core body temperature for 10-15 minutes AFTER exiting cold water. Caused by cold blood returning from extremities as blood vessels dilate. Why you feel colder 5 minutes post-plunge than you did in the water.
Algaecide: Chemical or mineral treatment that kills algae in plunge water. PoolRX is a popular mineral-based option ($79).
Auto-defrost: A freezer feature that periodically warms the interior to prevent ice buildup. AVOID for chest freezer plunge conversions — the defrost cycle warms your water.

B

Beta-blockers: Medications that lower heart rate. Contraindicated for cold plunge — they mask the cold shock heart rate response, making it dangerous.
Biofilm: Slippery layer of bacteria that forms on submerged surfaces. Prevented by ozone, chlorine, and regular brushing of tub walls.
Brown fat (brown adipose tissue): Specialized fat that burns calories to generate heat. Activated by cold exposure. Adults have small amounts in neck, shoulders, upper back.
Bulkhead fitting: Plumbing fitting that creates a waterproof port through the wall of your plunge tub. Used for chiller plumbing connections. 3/4" PVC bulkhead fittings are standard.

C

Chiller: Compressor-based appliance that cools water. The most expensive single component in a DIY cold plunge. See our chiller buyer's guide.
Chloramines: Compounds formed when chlorine reacts with sweat, body oils, or urine. Source of "pool smell" and skin/eye irritation.
Cold shock response: Involuntary physiological response to cold water immersion: gasp reflex, hyperventilation, tachycardia, blood pressure spike. Peaks in first 60 seconds, subsides by 2-3 minutes.
Compressor delay: Minimum off-time enforced by temperature controller before chiller can restart. Prevents short-cycling that kills compressors. Standard setting: 3 minutes.
Contrast therapy: Alternating hot and cold exposure (sauna + cold plunge) in a single session. Multiplies the benefits of either alone. See our contrast therapy guide.
Cryotherapy: Cold exposure using nitrogen vapor at -200°F to -300°F for 2-3 minutes. Different mechanism than cold plunge. See our comparison.

D

Deload week: Planned reduction in training or cold plunge intensity (50% reduction) for one week, typically every 8-12 weeks. Allows nervous system to fully recover.
DOMS (Delayed-Onset Muscle Soreness): Muscle soreness that peaks 24-72 hours after intense exercise. Cold plunge reduces DOMS by 30-50% vs passive recovery.
Drain plug: Threaded plug at the bottom of a plunge tub, used for draining water. Standard 1" NPT fits garden hose adapter.

E

EMF (Electromagnetic Field): Radiation emitted by electrical devices, including sauna heating elements. Quality saunas (HigherDose) keep EMF under 2 milligauss at body surface.
Endothelial function: Health of the inner lining of blood vessels. Improved by cold exposure. Key cardiovascular health marker.
Epsom salt: Magnesium sulfate, used in warm baths for magnesium absorption and muscle relaxation. Amazon Basics Epsom Salt ($14) is a budget option.

F

Filter pump: Pump that circulates plunge water through a filter cartridge. Removes skin cells, hair, and debris. Intex C1500 ($89) is the DIY standard.
Flow-mediated dilation (FMD): Measure of blood vessel flexibility. Improved by regular cold exposure. Key cardiovascular biomarker.
Food-safe: Material certification meaning safe for human contact. Critical for plunge vessels, plumbing, and sealants. Use food-safe silicone instead of pipe dope.
Foam roller: Cylindrical foam tool for myofascial release. Pairs well with cold plunge for recovery. Amazon Basics Foam Roller ($19).

G

GFCI (Ground Fault Circuit Interrupter): Electrical safety device that trips in 1/40 second when it detects current leakage. MANDATORY for all cold plunge electrical. Use GFCI adapter ($18) if your outlet isn't GFCI.

H

Heat shock proteins (HSPs): Family of proteins that help repair damaged cellular proteins. Activated by heat stress (sauna) and the rewarming process after cold exposure. Associated with anti-aging effects.
HRV (Heart Rate Variability): Variation in time between heartbeats. Higher HRV = better autonomic nervous system health. Tracked by wearables (Whoop, Garmin, Oura). Should trend up with regular cold plunge.
Huberman, Andrew: Stanford neuroscientist whose podcast has popularized cold exposure. His research on norepinephrine and dopamine from cold exposure is frequently cited.
Hypothermia: Condition where core body temperature drops below 95°F. Mild: shivering, cold extremities. Moderate: slurred speech, clumsy movements. Severe: unconsciousness. NEVER push to hypothermia in cold plunge.

I

Inkbird ITC-308: $35 dual-stage temperature controller. The de facto standard for DIY cold plunge temperature control. See our review.
Infrared sauna: Sauna that uses infrared radiation (120-150°F air temp) to heat body directly, vs traditional saunas that heat air (170-200°F). Deeper tissue penetration.
Insulation: Material that reduces heat transfer. Critical for plunge tubs — uninsulated tubs lose 4-6°F/day, insulated lose 1-2°F. Use 2" XPS foam board.

M

Minimum effective dose: Smallest amount of cold exposure that produces measurable benefit. Per Søberg research: 11 minutes per week, spread across 2-4 sessions.

N

Norepinephrine: Neurotransmitter responsible for alertness and focus. Cold exposure triggers 200-300% increase, sustained for 2-4 hours. The "focus boost" from cold plunge.

O

Ozone (O₃): Gas injected into plunge water to oxidize bacteria and contaminants without chemical residue. Preferred primary sanitizer for cold plunge. Coospider 300 mg/h ($45) is DIY standard.

P

pH: Measure of water acidity (0-14, 7 is neutral). Cold plunge target: 7.2-7.8. Outside this range, sanitizers work poorly and skin/eye irritation occurs.
Pond liner: Food-grade PVC liner used in chest freezer plunge conversions. PolyGuard 20-mil PVC ($189).
PVC spa hose: Reinforced flexible hose for plumbing chiller to tub. Food-grade, won't collapse under suction. 3/4" flexible PVC spa hose ($28). NEVER use garden hose.

R

Raynaud's syndrome: Condition where blood vessels in extremities over-constrict in cold. Contraindicated for cold plunge — can cause tissue damage from reduced blood flow.
Recovery: Process of returning to baseline after training stress. Cold plunge accelerates recovery by 30-50% vs passive recovery.

S

Sauna blanket: Sleeping-bag-shaped infrared sauna. Most portable home sauna option. HigherDose ($899) is premium tier.
Søberg Principle: Dr. Susanna Søberg's research finding: ending on cold (not warm) maximizes brown fat activation. Always end contrast therapy on cold.
Stock tank: Livestock watering vessel, repurposed as DIY cold plunge tub. Rubbermaid 50-gal ($130) is the most popular.
Sympathetic nervous system: "Fight or flight" branch of autonomic nervous system. Activated by cold exposure (cold shock response). Balanced by parasympathetic activation (slow nasal breathing).

T

TDS (Total Dissolved Solids): Measure of all dissolved substances in water. Test with TDS meter. Above 1500 ppm = time to drain and refill.
Thermoregulation: Body's system for maintaining core temperature. Cold exposure trains this system. Children and older adults have reduced thermoregulation capacity.
Trichlor: Slow-dissolving chlorine tablet used for maintaining chlorine residual in plunge water. Standard pool chemical.

V

Vasoconstriction: Narrowing of blood vessels. Caused by cold exposure. Pushes blood toward core, reduces blood flow to extremities.
Vasodilation: Widening of blood vessels. Caused by heat exposure or rewarming after cold. Returns blood to extremities.
Vagal tone: Activity of the vagus nerve, which controls parasympathetic activation. Higher vagal tone = better stress resilience. Improved by cold exposure.
Venturi injector: Plumbing fitting that creates suction as water flows through a constriction. Used to inject ozone gas into plunge water.

W

Wim Hof method: Practice combining cold exposure, breathwork, and meditation. Popularized by Wim Hof ("The Iceman"). See our breathwork guide for simplified version.

Common abbreviations

Abbreviation	Meaning
CBT	Cold Bath Therapy (cold plunge)
CWI	Cold Water Immersion
FMD	Flow-Mediated Dilation
GFCI	Ground Fault Circuit Interrupter
HP	Horsepower (chiller size)
HRV	Heart Rate Variability
HSP	Heat Shock Protein
NPT	National Pipe Thread (plumbing standard)
PVC	Polyvinyl Chloride (plastic pipe)
RX	PoolRX mineral algaecide
SAD	Seasonal Affective Disorder
TDS	Total Dissolved Solids
WBC	Whole-Body Cryotherapy
XPS	Extruded Polystyrene (foam insulation)

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