As the body cools, blood moves to the central organs to maintain core temperatures. This means hypothermic swimmers may get numb fingers and toes, which affects their ability to grasp objects. As the brain receives less oxygen, the swimmer can become confused and disoriented, so someone who's in need of rescue may not follow directions well.
If someone is shivering and has blue lips, but no other problems, they just need to be warmed, Musler says. But don't put them in a warm shower or tub.
"In the field, use blankets and clothing to maintain body temperature. Once they're indoors, you may sit them in a warm room, but not right next to a heat source. Use normal temperature blankets at first, and then perhaps use warmed blankets. Rewarming should be a slow, steady, gradual process."
Give them lots of lukewarm liquids, says Todd Schlifstein, MD.
"Hydration is very important to help get rid of muscle metabolites that are formed when you shiver a lot," says Schlifstein, a clinical instructor at New York University Medical Center.
However, if someone is so cold they can't stand or move their limbs normally, if they don't know who they are or where they are, those are signs of more severe loss of function.
"In that case, it makes sense to go to an emergency room, where trained people know how to deal with this situation," Musler says.
When a person is short of breath and close to passing out, or feels their heart beating irregularly, they too should go to the emergency room, Schlifstein adds.
"Or when you try normal rewarming measures and the person continues to shiver and doesn't improve, take them to the emergency room," he says. "In some cases of hypothermia the body cannot rewarm itself."