Helen Caldicott worried over it for about a year.

A pediatrician at the Harvard Medical School, she says it finally came down to this:

"I couldn't see the point in keeping these children alive another five to 10 years with meticulous medical care when during that time they could be vaporized.

"It just seemed bad practice."

So Helen Caldicott resigned last year from Harvard to give full time to her other mission: saving the world. Saving the world from itself.

She is president of Physicians for Social Responsibility, a Boston-based organization of doctors determined to make the world see how close it is to committing planetary suicide through nuclear war. She is practicing, she believes, "the ultimate form of preventive medicine."

Helen Caldicott is a zealot. But it is not merely zeal that gives the glint to her pale blue eyes. It is the sight of Armageddon. Of what she calls "The Final Epidemic."

Caldicott is not alone. Begun as a small group of concerned medical specialists, PSR has grown into an international organization of about 10,000. ("We have in common our Hippocratic oath," says Caldicott.) A PSR ad in the New England Journal of Medicine happened to coincide with the Three Mile Island episode. More than 500 doctors signed up on the spot. About 250 join each week, Caldicott says.

PSR is working with other scientists to calculate the impact on people of a nuclear hit. Their studies are scientific and, she says, carefully read by Pentagon officials. Several studies have been published by the New England Journal of Medicine.

If only 10 percent of the existing nuclear missiles were fired, between 70 and 80 percent of the ozone layer would be destroyed. If 10 to 20 percent were fired, the glare would blind all unprotected eyes. People, of course, could protect their eyes with glasses. But animals would be blinded and would inevitably die . . . . The entire ecosystem of the planet could collapse --From a presentation by MIT scientist Kosta Tsipis at a symposium sponsored by Physicians for Social Responsibility.

Helen Caldicott was born and raised in Australia. She credits Nevil Shute's "On the Beach" with radicalizing her -- at age 14 -- to the dangers of nuclear warfare. (The 1957 novel was a chilling account of progressive worldwide destruction by the radiation cloud from a nuclear blast, from the point of view of Australian protagonists.)

In the early '70s, when she was a medical intern, the French were testing bombs on Pacific islands and the fallout was drifting over Australia. She was invited to discuss the medical ramifications on Australian television.

"I talked about strontium-90 concentrating in milk, especially breast milk, and how babies are 20 times more sensitive to radiation than adults, how they could develop leukemia or cancer . . . and every time the French blew up another bomb I was invited back.

"The response was amazing . . . there were spontaneous marches; people stopped buying French perfume; postmen wouldn't deliver French mail; longshoremen wouldn't unload French ships; a man burnt his beret on TV, and finally Australia and New Zealand took France to the International Court of Justice. Now France tests underground."

Two years later huge deposits of uranium were discovered in Australia. This time, because of the balance of payments, she recalls, the radio and TV stations weren't so eager to have her appear. She wanted to take her antinuclear message to trade unions and was warned, "Okay, you can come talk, but we need the jobs and you'll never convince them."

"I would convince them in 10 minutes," she says, rather disdainfully. "I just talked about the effect on their testicles and what radiation does to the genes and the sperm, and I'd talk about nuclear war and what it means to their children and I got the Australian Council of Trade Unions to pass a resolution not to mine, transport or sell uranium." That ban lasted from 1975 until last year. "It was overturned," she says, "because the multinationals put full-page ads in our papers calling us selfish not to export our uranium to an energy-hungry world."

Hiroshima and Nagasaki were single events with effects decaying over time; today we are faced with the possibility of multiple events--a thermonuclear explosion at 10 a.m. and another at 4 p.m. At the time of Hiroshima, there was one nuclear power and the world's total arsenal comprised two or three weapons; today there are at least six nuclear powers and the total arsenal is--conservatively--in excess of 50,000 warheads . . . But most important, Hiroshima and Nagasaki were isolated, limited disasters. They could, in time, be saved and reconstructed with help from outside . . . in any full-scale contemporary nuclear exchange, however, THERE WILL BE NO "OUTSIDE" THAT WE CAN RELY UPON. From the presentation of H. Jack Geiger, M.D., professor of community medicine, City College of New York

"We have here," says Caldicott, "a terminally ill planet infected with lethal macrobes (as opposed to microbes) which are metastasizing rapidly. The prognosis is grim. Many say we'll be lucky to survive the next 10 years, much less the next 20. The etiology is psychiatric -- it's not the bombs, it's the people -- and for the first time war is anachronistic. We can't fight. It's either survival or annihilation."

That is Helen Caldicott's position and that of Physicians for Social Responsibility. It is something she says each time she shows a 38-minute film of the symposium sponsored by PSR and the Council for a Livable World. She is on a road tour with the film and, with her pleasant smile, in her maroon silk dress and elegant string of expensive pearls, she stands in chilling contrast to the film's grim and compellingly believable depiction of the planet's "terminal event."

It is no accident.

Caldicott's mission is to shatter the "psychic numbing" she sees in America. "Why," she says, "only about one percent of the people I talk to in this country can even tell me what a strategic weapon is."

Images from the film: mushroom clouds; a seal; a dead tiger; verite' Japanese footage from Hiroshima and Nagasaki, mercifully in black-and-white, of victims burnt to horrible travesties of human form, of a vast wasteland on which nothing stood. And of San Francisco on a sunny morning with Californians going about their daily business.

Staying with her medical analogy, Caldicott likens Americans today to caged rats confronted with imminent, unavoidable catastrophe. In what psychologists call "the displacement effect," they simply choose to ignore the danger and go off to do something quite irrelevant.

"Many people," she says, "say, 'Oh yes, a nuclear war will kill me,' but that's like saying, 'One day I'm going to die.' They haven't taken it on, and they don't until they're mortally ill.

"But we're all practicing psychic numbing, as well as displacement activity . . . In England they describe this American mode as one of 'manic denial.' Americans are denying so hard, they're into gourmet foods, Jacuzzis, hot tubs, new china for the White House. Some say it's the sort of situation that existed in Germany before the Second World War.

"We as physicians break through this psychic numbing every day. The first stage of grief is shock and disbelief, almost no feeling. Then comes depression so profound one may wish one had cancer instead. Then profound anger followed by bargaining with God and eventual adjustment to reality."

Children, she notes, are not numb to the nuclear danger. In a series of independently conducted surveys, thousands of adolescents--in Boston, Houston and elsewhere--have indicated that they see for themselves . . . no future.

Helen Caldicott and her husband, a pediatric radiologist at Harvard, have three children. She is 43. "As a mother," she says, "I am determined that those children grow up and die of natural causes."

Helen Caldicott and her colleagues are showing their film wherever anyone will stop long enough to see it. Many of the presentations are also contained in a book, "The Final Epidemic, Physicians and Scientists on Nuclear War." Caldicott wrote an earlier book, "Nuclear Madness. What You Can Do." A PSR symposium will be held in Washington May 11. Symposiums are also scheduled around the country during Ground Zero Week, April 18-24.

PSR has been less effective in making its case to the government.

Whenever its members have been scheduled to testify, "the senators are too busy," Caldicott says bitterly.

Helen Caldicott used to be an atheist. "Until a few years ago," she says. "Now I believe there's a God. I'm nonsectarian. But for me God is life. It's the DNA molecule. It's the universe . . . I pray and meditate and there is a higher force in me that gives me strength. And it really tells me the right thing to do . . .

"The most fulfilling way to live is to face and understand one's death during life because that makes life so much more precious . . . My children are so precious. To smell a rose is a profound experience. To look at the beauty of this country, the beauty of the world, is just extraordinary. To look at a baby, to know what that means.

"I wake up every morning and I thank God that the planet is still here . . ."