"It was the worst possible situation--being plunged into the water right at the freezing point."

"Some people die very quickly."

"If you have to swim, even a good swimmer can drown in 5 or 10 minutes. Your arms and legs just get numb."

"But if you have something to hang onto, you can last longer in water like that than people have thought, up to an hour or an hour-and-a-half."

These were among the reactions of experts in the physiology of cold yesterday after an Air Florida jetliner plunged into the icy Potomac here.

Overexposure to freezing or near-freezing air can be devastating to the human body physiologically. Water at the same temperature is 25 times worse, said Moulton Avery, director of the Center for Environmental Physiology here.

"You lose heat from the body at a phenomenal rate of speed, at a minimum of 25 times faster than in air," he explained. "The conductivity of water is so much greater than air's. It's a critical situation.

"The minute you go into the water the shock of immersion is, first, painful and, second, disorienting. You may panic. Panic or not, after 2 to 5 minutes in the water, you involuntarily start breathing at a 5 or 6 times faster rate. This can cause a loss in carbon dioxide, which in turn can cause a reduction in blood flow to the brain, also muscle spasm.

"So within a couple of minutes you've got all these things against you.

"Then, after 20 minutes, your inner body temperature starts to fall. That's when hypothermia begins."

Some people don't last that long, of course. Panic and shock make some drown immediately. Anyone hyperventilating may start swallowing water and drown.

Adults usually last longer than children, women longer than men, fat men or women longer than thin. They simply have more body mass to protect their "inner core": the brain, heart and other vital inner organs.

But hypothermia--cooling of the inner core to around 93.5 degrees Fahrenheit--takes a long time to act, even in water, said one of the world authorities on cold-water survival, Dr. John Hayward of the University of Victoria in British Columbia.

Most studies of volunteer subjects have been conducted in cold water at 40 to 50 degrees. That is dangerous enough. Last year Hayward, who is 45, dared to immerse 10 men, including himself, and 10 women, all lightly clothed, like the persons in the airplane, into 32-degree water--the temperature of the Potomac--for 25 to 40 minutes, until inner body temperatures fell to 95 degrees.

"What we learned," he said, "is that a normal adult, say, anyone up to age 60, is very resistant to inner cooling.

"When you go into the water, there is a very strong vasoconstriction, a shutdown of blood flow to the skin, arms and legs." People get cold, but the inner body stays warm.

"Then you shiver, and that increases the heat inside you. Your inner temperature has to get down to around 85 before you die of hypothermia." Then the heart fails.

Cold water survival, in short, is often possible--for persons who get any chance at all.