When the Airbus A320 took off from New York City's LaGuardia Airport yesterday, the air temperature outside was well below freezing—around 20 degrees Fahrenheit (–6.7 degrees Celsius). The 150 passengers on board no doubt assumed they would spend the next hour and a half in the cushioned seats of a cozy, warm airplane cabin en route to Charlotte, N.C. Little did they know that just minutes after takeoff they would instead be bobbing on the frigid waters of the Hudson River off Manhattan's west side.

Just minutes after Capt. Chesley Sullenberger orchestrated a near-perfect emergency water landing (after a collision with a flock of Canada geese reportedly knocked out both engines), water began seeping into the plane. Two passengers treated for hypothermia at nearby Saint Luke's–Roosevelt Hospital emergency room said that the water was waist-high almost immediately, according to Gabe Wilson, associate medical director of the hospital's emergency medicine department. According to media reports, some of the passengers were submerged up their necks in water once they had evacuated the plane and awaited rescue.

"They were all shaking from both the [cold] temperature and stress," says Wilson, who treated 11 of the plane's passengers for hypothermia, a potentially fatal condition that occurs when the body cannot generate enough heat to compensate for the warmth it loses.

Many of the symptoms of hypothermia resemble those of a drunken stupor: sleepiness, clumsiness, confusion and even slurred speech. Doctors also check for shivering, a weak pulse, low blood pressure, and a body temperature below 96 degrees F (35.5 degrees C). (Normal body temperature is 98.6 degrees F, or 37 degrees C.)

Fortunately, none of the passengers that he treated had body temperatures below 95 degrees F (35 degrees C), Wilson says, adding that all they needed to warm up were Bair Huggers, special blankets hooked to a heater which send warming air currents over the body.

But what if the passengers had not been rescued so fast? What would have happened if they had spent hours wading or swimming through the Hudson, or in any cold water, awaiting rescue? We asked Christopher McStay, an emergency room doctor at New York City's Bellevue Hospital Center about the potential consequences and treatments for hypothermia.

[An edited transcript of the interview follows.]

How long can a person survive in water that is 41 degrees F like the Hudson was when the plane went down?

When you first go into extremely cold water there is this weird response called a cold shock response. People start to hyperventilate immediately. For one to three minutes you breathe very fast and deep, uncontrollably. If you go underwater, you could swallow water and die. …I can't tell you how often this occurs but it's certainly a very real phenomenon. Once that response goes away, you're fine…for awhile.

Generally, a person can survive in 41-degree F (5-degree C) water for 10, 15 or 20 minutes before the muscles get weak, you lose coordination and strength, which happens because the blood moves away from the extremities and toward the center, or core, of the body.

There are many factors that determine how fast a person submerged in water cools. People who are obese, who have a lot of soft tissue that provides a lot of insulation, are likely to last longer than lanky people, because the body fat provides insulation. Another factor is how much of the body is actually underwater. (Water conducts heat away from the body much faster than air does, even if the water temperature is 20 degrees higher than the air temperature. So, the more the body is submerged, the faster its heat will be drained, according to Craig Heller, a Stanford University physiologist). If you have a flotation device that you can pull yourself on top of, you are much better off.

How cold does the water have to be to put a person at risk for hypothermia?
Even water temperatures as high as 75 and 80 degrees F (24 and 27 degrees C) can be dangerous, but it would most likely take much longer than 15 minutes to become debilitated. There is no set time for when hypothermia will set in, but generally the colder the water, the faster it happens.

So if you find yourself submerged in icy-cold water, what should you do?
If you have a flotation device, you should get on top of that device and hug yourself to keep as much of your body away from the water as possible. If you keep your arms and legs in tight, close to the core of the body, you keep your limbs from being exposed to the cooling water. If you do not have a flotation device, get out of the water as fast as you possibly can.

What is the difference between frostbite and hypothermia?

Frostbite is actually the freezing of tissue [such as skin, muscle and nerve tissue]. Suppose you're on top of Mount Everest and you're bundled up; your core temperature is 98.6 degrees F. If you take off your gloves, you have exposed that area and it may get frostbite. That's not hypothermia. Hypothermia is a drop in the core temperature of the body.

When are you in danger of getting frostbite and were these survivors at risk?

For frostbite to occur, the tissue actually has to freeze—meaning a dip to 32 degrees F (0 degree C) or lower. The parts of the body submerged in water are not in danger of becoming frostbitten, because the water temperature (41 degrees F) is not freezing. However, the parts of the body exposed to air are at risk because the air temperature is 20 degrees F (–7 degrees C), which is below freezing.

Can you die from frostbite?
You can certainly die from frostbite, but that is exceptionally rare. Usually when people die from frostbite, it's from some complication down the road such as gangrene, the decay and death of tissue which occurs when it does not receive enough blood or becomes infected.

What do you do to treat hypothermia?
If your body temperature is above 95 degrees F and you're healthy, your body will warm itself up and you generally don't need treatment.

If your body is 90 degrees to 95 degrees F (32 to 35 degrees C) and you look okay, we'll do things like put a warming blanket around you. If your temperature drops much lower, we might give you an IV with warm fluids, insert a breathing tube to supply the lungs with warm air, and insert tubes through the mouth and urethra to put hot saline into the stomach and bladder, respectively. Heating from the inside (by introducing these fluids) helps warm the body's core tissues faster than heating the body from the outside (by using blankets or putting a person in a warm environment, for instance).

If a patient comes into the emergency room with a body temperature between 70 and 80 degrees F (21 and 27 degrees C), they often appear dead—or are dead. Cardiac arrest often occurs in this temperature range. Even if it appears someone has passed away, it is still important to warm them (using the techniques described above), because with this degree of hypothermia the heart can slow to a point at which doctors cannot even detect it. Thus, they could make the mistake of presuming someone dead who is actually still alive.

For these unconscious patients, we also do cardiopulmonary resuscitation (CPR) on them and often use a cardiopulmonary bypass (heart–lung) machine that will actually oxygenate the blood and provide a pulse for them.