Safety
Into the cold safely
What's truly dangerous about cold plunging happens in the first few seconds — and it isn't hypothermia, it's the cold shock response. Here's what you need to know before you start.
The first 60 seconds are the dangerous part
In water below about 15 °C, your body triggers an involuntary cold shock response: a reflexive gasp for air, hyperventilation, and a racing heart. This reaction is strongest in the first 30 seconds and subsides after 60–90 seconds. The tricky part: you can't suppress this reflex. If you fall into the water unexpectedly or go in headfirst, the gasp happens underwater — and instead of air, water reaches your lungs. In open water, that's one of the most common causes of drowning. But if you get in slowly, in a controlled way, with your head above water, that same reflex plays out in the air — and the cold shock loses its bite.
A second danger remains, though: even if you get through the shock, you keep cooling down. Over the first few minutes, the cold gradually paralyzes your muscles — this is cold incapacitation. Your arms and legs stop obeying, even a few meters become impossible, long before hypothermia even comes into play. A controlled entry won't protect you from this. So stay where you can stand up or get out instantly at any moment — and never where you'd have to swim back.
The most important rule
Get in slowly, keep breathing calmly and deliberately, and give the shock 30–60 seconds to settle. Never go into open water (lake, river, sea) alone, and always keep your head above water. Do your breathing exercises always on land and away from the water's edge — never in the water. Forceful breathing or breath-holding can trigger a blackout; if that happens in or near the water, you risk drowning. Finish your breathing first, then get in.
Head underwater + breath-hold = double the danger
If you put your head under and hold your breath, two opposing reflexes collide: the cold shock response (which speeds the heart up) and the diving reflex (which slows it down). This simultaneous flooring of the gas and the brake can trigger arrhythmias. In a small lab study of just 12 otherwise healthy subjects, such extra beats occurred with the face immersed and the breath held in about 80 % of the test dives — and in 11 of the 12 participants overall. With normal breathing, they were almost absent (another study found them there in only about 2 % of cases). So these percentages describe individual dives under lab conditions, not how often it happens in everyday life — but the underlying mechanism is real and well documented. For cold plunging, the takeaway is: head above water, don't hold your breath.
Afterdrop: the cold keeps working
After you get out, your core body temperature often keeps falling: this afterdrop happens mainly because the cold from your chilled outer layers keeps moving inward. The strongest feeling of cold (and shivering) therefore often sets in only a few minutes after you get out — frequently around 10 minutes later. So rewarm in a controlled, slow way: a hot shower or hot bath can actually make the effect worse — the sudden dilation of your skin's blood vessels flushes cold blood back to the heart and can drop your blood pressure enough to make you dizzy or to make you faint. And be careful: the pleasant feeling of warmth right after the bath (your skin getting its blood flow back) does not mean that your body is already warmed up.
Who's better off not cold plunging
Contraindications
Cold spikes your pulse and blood pressure abruptly and puts strain on the heart. A cardiologist from Harvard Health explicitly advises against it if you have:
- cardiovascular disease and heart rhythm disorders (especially atrial fibrillation)
- high blood pressure
- circulatory disorders / Raynaud's phenomenon
Likewise, with asthma and other respiratory conditions (the cold shock response can narrow the airways), with epilepsy, and during pregnancy, it's wise to check with a doctor beforehand.
When in doubt — and especially with pre-existing conditions — talk to your doctor first.
The good news: it gets safer quickly
The cold shock reflex flattens out fast — after about five sessions it's already noticeably dampened. That makes getting in more comfortable over time. What stays important: habituation dampens the reflex, not the real danger of staying in too long. Keep respecting the cold. For how to dose it sensibly, see the ice bath guide.
Quick questions
How dangerous is cold plunging?
The biggest danger isn't hypothermia — it's the cold shock response in the first 30–60 seconds and cold incapacitation in the first few minutes. Done in a controlled way, slowly, and with your head above water, the risk is low.
Who shouldn't cold plunge?
If you have cardiovascular disease, heart rhythm disorders, high blood pressure, circulatory disorders or Raynaud's phenomenon, epilepsy, asthma, or you're pregnant, check with a doctor first.
Can I take a hot shower right after an ice bath?
Better not. An immediate hot shower can worsen afterdrop and lead to dizziness or fainting. Rewarm slowly: towel off, warm layers, a warm drink.
Can I cold plunge alone?
Never alone in open water — cold shock and cold incapacitation can leave you unable to act. In your own tub the risk is lower, but caution still applies.
Sources
- Tipton et al. (2017), Experimental Physiology — Cold water immersion: kill or cure?
- Tipton, Kelleher & Golden (1994), Undersea Hyperb Med — Supraventricular arrhythmias following breath-hold submersions in cold water
- Shattock & Tipton (2012), J Physiol — Autonomic conflict
- Lundström et al. (2025), Physiological Reports — Assessment of arrhythmias and heart rate response in healthy adolescents performing face immersion and body submersion in ice-cold water
- Harvard Health — Cold plunges: healthy or harmful for your heart?
- Romet (1988), J Appl Physiol — Mechanism of afterdrop after cold water immersion
- U.S. Masters Swimming — Is it Safe to Swim in Cold Water?
- Outdoor Swimming Society — The Subtle Art of Warming Up