Cold-water immersion may offer health benefits -- and also presents
risks
[October 07, 2025]
By STEPHEN WADE
Dr. Mark Harper recalls his first cold-water swim in the south of
England 20 years ago. It was August, but the initial jolt from the
plunge took his breath away.
The shock to his system lasted a minute or two until he was
“recombobulated and able to think about something other than the cold,"
Harper says. A surprise sensation soon replaced his discomfort.
“I remember getting out of the water the first time and feeling so
good," Harper, an anesthesiologist who has since researched the
potential risks and rewards of taking a nippy dunk. "I wasn't expecting
that.”
Claims about the benefits of cold-water immersion date back centuries.
Thomas Jefferson, the principal author of the Declaration of
Independence and the third American president, wrote toward the end of
his life about using a cold foot bath daily for 60 years. He also owned
a book published in 1706 on the history of cold-water bathing.
While evidence is building around the positive health effects of
swimming in chilly water, bathing in ice or taking cold showers,
scientific confirmation is still lacking.
But Harper, who became a regular cold-water swimmer after his
initiation, said there are strong signs that people can derive mental
health benefits from the activity. He mentioned a positive effect on
depression and general well-being.
“For now, we have a very strong base, but not hard evidence, that
cold-water immersion is effective for mental health," Harper told The
Associated Press.
Boosting mental health
Harper said his own early experiences with cold-water swimming piqued
his professional curiosity. As a physician, he wondered if the brief
bodily shock had clinical uses for treating depression. He cited the
biological phenomenon of hormesis, in which a stressor introduced at a
low dose creates a positive response.
Muscles and bones put under stress —for example, with weight-bearing
exercise — also respond and grow.
Harper acknowledged that the positive effects he's observed may be the
result of the placebo effect, a phenomenon in which people who are given
a medicine they think will help them report responding to the treatment
even if it had no active ingredients.
In the case of cold-water immersion, the benefits may come from meeting
people, the exercise itself, or simply accepting a challenge and
accomplishing it, which improves self-confidence, he said.
“Personally, I think it’s all of those things, and the cold has an
additive effect,” Harper said. “I think we have a good physiological
basis for that. The basic science tells us the cold has a very strong
effect on the body.
“What we’re talking about is an intervention, when it's used clinically,
that produces beneficial effects on mental health. So in a way it
doesn’t matter which aspect is generating the positive effects.”
Harper cited what he said was a common reaction among first-timers who
have taken the plunge.
They often say “the fact that I’ve done this means I can do anything,”
he said. “It’s quite a confidence builder and it’s something people
stick to because they enjoy it so much.”
Know the risks when you start
Dr. Mike Tipton, a physiologist at the University of Portsmouth in
England who has studied extreme environments, authored a paper along
with Harper and two more researchers several years ago. The title
highlighted the potential advantages and dangers of stimulating the body
with a sudden drop in temperature: “Cold Water Immersion: Kill or Cure.”
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Revelers enter the cold water during the annual Polar Bear Plunge on
New Year's Day, Jan. 1, 2025, in New York. (AP Photo/Andres Kudacki,
file)
 “Like other environmental
constituents such as pressure, heat and oxygen, cold water can be
either good or bad, threat or treatment depending on circumstance,”
they wrote after reviewing the research then available on the topic.
One of their findings: The activity presents dangers. Deaths from
cold-water immersion are not uncommon. The shock of frigid water can
cause people to hyperventilate and drown. But Tipton told The AP the
risks can be managed with common sense and precautions.
How cold, how long?
You do not need to go to extremes to get a potential payoff.
Entering water at a temperature of 20-15 degrees Celsius (68-59F)
and staying for only a few minutes is sufficient.
Tipton explained if the water is colder than that, you may get more
benefit — but limit your exposure.
“If you ask me what protocol you would follow if you thought this
was going to be doing you good, I wouldn't go in water much below
12C (54F), and I wouldn’t stay in for much more than two minutes,”
Tipton said.
Tipton and Harper both said that any benefits from cold-water
immersion come from the body's cold-shock response: the sudden fall
of skin temperature, the release of the stress hormones, the release
of endorphins, and the anti-inflammatory effect.
Tipton cautioned against long spells in an ice bath.
“Sitting in water for as long as I can fills me with horror,” Tipton
said. "This could destroy small nerves, blood vessels in extremities
and could result in amputation the same way frostbite can."
Harper, who has swum as long as four-hour stretches in open water,
advised vigilance.
“People think it’s got to be super cold, super long and the longer
the better,” Harper said, "and that’s wrong.”
Be safe. You're a tropical animal
Tipton said he's not trying to be the “fun police” but suggested
caution and a medical screening if you take the plunge. He said
humans are considered “tropical animals” who need to adapt to the
cold and are better suited to warm conditions.
A human’s resting body temperature is about 36.5 to 37.5 degrees
Celsius (97.7 to 99.5F). Because of that, cold-water immersion is
stressful and carries risks for the cardiovascular and respiratory
systems. Not to mention the risk of drowning.
“We don’t want to stop people doing it, but we want to make sure
they do it in a way that maximizes the benefits and minimizes the
risks,” Tipton said.
He suggested swimming in a life-guarded area, or going with
experienced outdoor swimmers. He also suggested knowing the body of
water you're swimming in. Hazards, among others, include tides,
temperature, depth and pollution.
“Understand that taking a tropical animal and putting it in cold
water is probably the greatest stress that most people will
experience in their life,” Tipton said.
He suggested entering the water in a controlled fashion instead of
jumping in.
"Be sensible about it,” he said. “Incremental is the key.”
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